metal interfaces

Photochemistry at adsorbate / metal interfaces

X.-L. Zhou, X.-Y. Zhu and J.M. White Department of Chemistry, University of Texas, Austin, TX 78712, USA

NORTH-HOLLAND

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X.-L. Zhou, X.- Y. Zhu and J.M. White

Contents 1. Introduction 1.1. Orientation 1.2. Historical perspective 2. Methods and important factors 2.1. Experimental methods 2.1.1. Light sources 2.1.2. Detection techniques 2.1.3. Cross sections 2.2. The light and excitation pathways 2.2.1. W a v e l e n g t h / p h o t o n energy 2.2.1.1. Direct excitation 2.2.1.2. Substrate excitation 2.2.1.3. Thermal excitation 2.2.2. Intensity 2.2.2.1. Equilibrium bulk substrate heating 2.2.2.2. Transient surface heating 2.2.2.3. Local and resonant surface heating 2.2.2.4. Excitation by hot carriers 2.2.2.5. Direct processes: single- and multi-photon 2.2.3. Polarization and angle of incidence 2.2.3.1. Classical optics 2.2.3.2. Microscopic calculation 2.2.3.3. Photolysis cross sections 2.2.3.3.1. Substrate excitation 2.2.3.3.2. Direct excitation, > C 2 symmetry 2.2.3.3.3. Direct excitation, C 1 symmetry 2.3. The substrate 2.3.1. Electronic structure 2.3.1.1. Jellium model 2.3.1.2. Band theory 2.3.2. Elementary excitations 2.3.2.1. Single-particle electronic excitation 2.3.2.2. Plasmons 2.3.2.3. Phonons 2.4. The adsorbate 2.4.1. Gas and condensed phase photochemistry 2.4.2. Adsorbate-substrate complex/analogies with organometallics 2.5. Dynamics 2.5.1. Experimental information - TOF, L I F and R E M P I 2.5.2. Interpretation - potential energy surfaces (PES) 2.5.2.1. M G R and Antoniewicz models 2.5.2.2. Theoretical calculations 2.5.3. Lifetime and quenching 3. Experimental evidence for various processes 3.1. Processes driven by direct adsorbate excitation 3.2. Processes driven by photoemitted electrons

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Processes driven by subvacuum level excitation 3.4. Processes associated with plasmon excitation 3.5. Processes associated with phonon excitation 3.6. Processes associated with other kinds of excitation 4. Literature review 4.1. Halogen-containing molecules 4.1.1. CH2I 2 4.1.2. Alkyl halides on P t ( l l l ) and C / P t ( l l l ) 4.1.3. Alkyl halides on A g ( l l l ) 4.1.4. Alkyl halides on N i ( l l l ) and B r / N i ( l l l ) 4.1.5. Alkyl halides on other metal surfaces 4.1.6. Alkyl halides on semiconductor and insulator surfaces 4.1.7. Phosgene on A g ( l l l ) and P t ( l l l ) 4.2. Oxygen 4.3. Water 4.4. Carbon monoxide 4.5. Nitric oxide 4.6. Nitrogen dioxide 4.7. Ketene 4.8. Azomethane 4.9. SO 2 and HzS 4.10. Carbonyl sulfide 4.11. Metal carbonyls 4.12. Metal alkyls 4.13. Aromatics 4.14. Reactions among coadsorbed species 4.15. Photoinactive adsorbate-metal systems 5. Summary and prospects

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Acknowledgement References

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3.3.

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Surface Science Reports 13 (1991) 73-220 North-Holland

Photochemistry at adsorbate/metal interfaces X.-L. Zhou, X.-Y. Zhu and J.M. White Department of Chemistry, Unioersity of Texas, Austin, TX 78712, USA Manuscript received in final form 16 April 1991

Photochemistry at adsorbate/metal interfaces is a relatively new subject in the surface science community and one which provides excellent opportunities for studying adsorbate-surface interactions, including energy and charge transfer, dynamics and kinetics. In this article we review the accumulated understanding of the major issues and opportunities for developing and utilizing this kind of surface chemistry. After some brief introductory remarks on the orientation of this review and an historical perspective of photon-driven chemistry at adsorbate/metal interfaces, the fundamental concepts entering the description of photon-adsorbate-surface systems are presented. Following that, experimental evidence for various kinds of surface photochemical mechanisms is summarized. Thereafter, a detailed review of the current surface photochemistry literature is presented. While photochemistry at adsorbate/metal interfaces is the focus of this review, photochemistry at insulator and semiconductor interfaces is also described briefly to provide comparisons. In several cases, we note the limitations of our present understanding, the places where key experiments are missing and doable and, in our view, the directions and opportunities for future research.

0167-5729/91/$51.80 © 1991 - Elsevier Science Publishers B.V. (North-Holland)

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1. Introduction The interaction of light with matter has long been, and continues to be, actively investigated by the scientific community. While many areas are mature, the development of new tools and ideas continues to open new terrain. The past few years have seen the active investigation of photon-driven chemical processes, e.g., making and breaking bonds, at a d s o r b a t e / m e t a l interfaces. In this paper, we undertake the review of the present literature, through October 1990, on this subject. The adsorbate-metal photosystem can be related to two other actively researched, but more mature, areas - a d s o r b a t e / i n s u l a t o r and a d s o r b a t e / s e m i c o n d u c t o r interfaces. We discuss these two topics only briefly and do not attempt a systematic literature review. There are a number of reviews that connect, some directly and some indirectly, with the topic of interest here. An early review by Lichtman and Shapiro [1] described the status of photodesorption up to 1977. In 1980, Koel et al. published a paper [2] describing what was known about photoeffects on chemical reactions over transition metals. Some aspects of these two reviews are described below, but there was little work indicating the importance of anything other than surface heating. In 1983, Chuang published an extensive review [3] of research involving laser-induced gas-surface interactions and, as part of that review, he discussed electronic excitation and reactions of surface species. Covering a broader area than we undertake here, Chuang pointed out that, compared to insulator and semiconductor substrates, the number of adsorbate-metal systems with demonstrated non-thermal effects of illumination was small indeed. More recently, King and Cavanagh [4] reviewed the literature through January 1988 dealing with the chemical dynamics of molecular desorption from solid surfaces. The literature covered treats extensive work on photon-driven admolecule desorption, but includes only two papers (one a study of CH3Br on N i ( l l l ) [5b] and the other treating C H 2 I 2 on Ag [6a]) describing intra-adsorbate bond-cleavage reactions. Since 1988, such processes have been reported for a rather large number of systems, and it is on these that we focus in this review. While short reviews have already appeared [7,8], along with a more extensive overview focused on photochemistry in the adsorbed state [9], the study of photochemistry at a d s o r b a t e / m e t a l interfaces is in its infancy, and new observations appear with high frequency. These exciting developments are stimulating further research and are helping us transform useful speculation into interpretations and conclusions based on stronger and stronger evidence. In limiting the scope of this review to primarily photon-driven bond breaking within molecules adsorbed at metal interfaces, we thereby neglect a huge and important body of related work. This includes, in addition to photoeffects at a d s o r b a t e / s e m i c o n d u c t o r and a d s o r b a t e / i n s u l a t o r interfaces, such important research areas as photoelectrochemistry, photon-assisted etching of electronic materials, mode-specific chemistry driven by infrared absorption, desorption induced by electronic transitions (DIET) and considerable literature on photon-driven non-thermal molecular desorption. A number of excellent reviews describe progress in one or more of these areas. Among these, the description of fundamental mechanisms of desorption and fragmentation induced by electronic transitions at surfaces by Avouris and Walkup [10] is particularly useful. There have been numerous reviews of photon-driven processes with emphasis on electronic materials [12-15]. Among the more recent, those by Letokhov [14], Bauerle [15] and Herman [310] are helpful. As noted above, photochemistry at a d s o r b a t e / s e m i c o n d u c t o r interfaces has been widely studied from an electrochemical perspective, a research area generally described as photoelectrochemistry. There are numerous reviews [11] on this subject as well as studies of g a s / s u r f a c e analogs [11]. We do not attempt a comparison of that well-developed field with the work

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reported here. For the most part it has relied on wide-bandgap compound semiconductors (e.g. TiO 2 and CdS), with and without metal particles, for activating desired reactions. In this research, much attention has been given to conversion of solar energy to chemical energy. There is in this literature a theme that will be central to the discussion of a d s o r b a t e - m e t a l systems; namely, substrate excitation followed by charge and energy transfer to the adsorbate. The surface science of these systems is, at best, poorly understood. Perhaps even more closely related to the subject matter of this review is the extensive work involving desorption induced by electronic transitions, or D I E T [110]. As the terminology indicates, this work has emphasized the desorption of fragments, particularly ions, driven by photons and electrons (occasionally heavier particles). For the most part, this work involves higher incident energies than the UV light used in surface photochemistry. Such photons and electrons are capable of ionizing valence a n d / o r core levels of atoms at surfaces and ejecting ions therefrom. Clearly, the two worlds are connectable, in principle, through experiments that measure chemical events at wavelengths covering both core and valence level excitations. As we shall see, many of the fundamental mechanistic ideas developed to interpret D I E T experiments carry over directly to the description of photochemistry at a d s o r b a t e / m e t a l interfaces. The details are different because different excited states, from which the chemistry proceeds, are prepared. A very helpful review of the common issues has recently appeared [10]. Two D I E T subfields provide the most fruitful connections between surface photochemistry and D I E T processes - electron-stimulated desorption (ESD) and photon-stimulated desorption (PSD) [306]. Both have focussed on the physics of desorption, historically mainly on ion desorption. While surface chemical processes, e.g. mechanisms and reaction cross sections, have not been widely studied in either ESD or PSD, many of the same excitation pathways are relevant for the physics and the chemistry. 1.1. Orientation

To begin our discussion and orient our thinking, we consider, hypothetically, the behavior of two kinds of isolated and electronically excited gas-phase molecules. Assume that two isolated molecules, A and B, both form electronically excited states in the presence of 5 eV photons. But assume that the subsequent relaxation channels are very different; for A, which could be benzene, relaxation is mainly through photon emission, fluorescence, and phosphorescence, whereas for B, which could be methyl bromide, there is no photon emission but nearly 100% dissociation of the C - B r bond. Now suppose that the excited A and B molecules are both brought close (i.e., within one or two molecular diameters, < 1 nm) to a metal surface [16]. Qualitatively, we expect the fluorescence and phosphorescence of A to be completely inhibited (quenched); B may continue to dissociate, but perhaps with lower probability. Why? As discussed below, time scales are the central issue. The quenching process has been thoroughly discussed. For the kinds of excitations hypothesized above, it is the result of coupling between the excited molecule and the metal [10]. This coupling takes the form of induced-dipole/induced-dipole coupling, electron-hole pair formation in the metal and charge transfer mediated processes. Because characteristic fluorescence and phosphorescence times a r e 10 -9 S or longer, the couplings that lead to quenching have a relatively long time (compared to bond stretching times of order 10 -13 s) to operate. In part, this explains the effectiveness of the quenching. On the other hand, characteristic direct dissociation times along repulsive potential energy curves are much shorter, as short as 10-13 to 10 -~4 s [5c,d,17]; thus, quenching must operate on a much faster time scale to inhibit bond cleavage. Depending upon the resonant or non-resonant character of the relaxation process,

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theory predicts characteristic quenching times relatively close to this, lying between 10-15 and 10 -13 s [18,19]. Experimentally, it is also clear that, in many cases, bond cleavage and quenching within adsorbed molecules operate on about the same time scale, the details depending upon the adsorbate-metal system involved. This discussion illustrates a major point - we are dealing with time- and distance-dependent processes for which characteristic values must be evaluated and compared for clearer understanding. Metals are efficient quenchers of optical emission because the quenching time scale (10 -13 to 10 -14 s) is so much shorter than typical photon emission time scales ( 1 0 - 9 S). As a result, roughly only 1 molecule in 104-105 will emit a photon; the others will be quenched. On the other hand, when direct rapid bond cleavage competes with quenching, each assumed to operate on a 10-14 s time scale, we expect 50% in each channel. Consequently, the widely held notion that metals are very strong quenchers must be qualified. Leaving the above hypothetical processes, in experiments designed to examine the influence of the metal substrate on the excited states of dissociating molecules, the molecule typically must be adsorbed before it is excited. This is particularly true for molecules undergoing direct dissociation along repulsive excited-state potential energy curves because the metal, to have any influence in the dissociation, must respond before the bond breaks (i.e., subpicosecond time scale); thus, the molecule must be within a few tenths of a nm of the surface when it is excited. The adsorption requirement introduces numerous complexities that are associated with photon excitations, of one kind or another, within the metal a n d / o r the adsorbate-metal complex. The multiplicity of interrelationships thus introduced is one of the reasons for the interest in this field and is one of the themes of this review. These adsorbate-metal couplings can be described generically in terms of induced charge and energy transfer processes initiated by photons interacting with metal-based electronic states. Thus, and as discussed below for m a n y cases, there are three possibilities for photon absorption: (1) by the adsorbate, (2) by the a d s o r b a t e substrate complex, and (3) by the substrate. When considering the possibility of direct adsorbate or adsorbate-substrate excitation, we want to ask what kind of optical absorption cross section would be required to obtain a measurable reaction yield, say 10% of a monolayer, of retained surface species in a reasonable experimental time, say 10 2 S. Assuming 1014 to 1013 adsorbate species per cm 2, and an incident and monochromatic photon flux of 1017 p h o t o n s / c m 2- s, 10% of a monolayer would react for a cross section of 10 -2° cm 2 and a quantum yield of unity (flux x cross section × time). If quenching decreases the rate, a higher optical absorption cross section will be required to give 10% yield. Optical cross sections of 10 -20 cm 2 are typical of relatively weak gas-phase absorbers (for example, CHaBr has this cross section at 250 nm = 4.96 eV [20]). Very strong absorbers have cross sections as high a s 10 -16 c m 2. The point is that, in the absence of overwhelming quenching, one could reasonably expect to absorb enough photons in an adsorbed monolayer to produce easily measurable accumulations of surface species. Another estimate is useful: What cross section is required to detect a photodesorbing species during illumination? Suppose we can measure a flux of presumed thermal (300 K) species corresponding to a pressure rise of 1 x 10-11 Torr (from the kinetic theory of gases, flux = 0.25 × number density × characteristic speed). If the characteristic speed is 105 c m / s and 1 cm 2 is irradiated, then we need to desorb roughly 1013 molecules/s (this is a lower bound since it makes the unrealistic assumption that the desorbing species are all directed within the detector solid angle). This would be realized for a monolayer coverage of 1015 m o l e c u l e s / c m 2, an incident flux of 1017 p h o t o n s / c m 2- s, and a cross section of 10-19 cm2/molecule. Compared to typical optical absorption cross sections, this is certainly realizable provided quenching does not dominate.

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1.2. Historical perspective With the above orientation in mind, we turn to a brief overview of early attempts to find and investigate photon-driven non-thermal chemistry at adsorbate/metal interfaces. In 1980, the research into UV-VIS photoeffects on reactions over transition metals was reviewed in a short article [2]; there was very little evidence for non-thermal chemistry, and the best evidence was available only on relatively poorly characterized systems. In one of the first surface science investigations of UV photon-driven processes at adsorbate/metal interfaces, Lange and Riemersma [21], some thirty years ago, published a very interesting paper on the desorption of CO from Ni under the influence of illumination. They measured the yield of CO desorbed per incident photon in the range 250-500 nm and found that it rose by a factor of twenty in passing from 460 to 330 nm, but then dropped as the wavelength decreased to 254 nm. The presence of this local maximum was taken to indicate a non-thermal quantum effect. Similar work, interpreted in the same way, followed [22]. However, a few years later, McAllister and White [23], using a more carefully prepared surface and taking care to control the absorbed power at every wavelength, showed that the CO yield from Ni was independent of wavelength above 300 nm, provided the absorbed power was held constant. The earlier data were reinterpreted in terms of power changes in the incident light as the wavelength was changed. Very recent work [24] shows that oxidized nickel, because of its semiconducting properties, is much more photo-active than clean nickel. We suspect that the early work was not entirely free of oxide [21]. Non-thermal (i.e., quantum) effects, if they exist in the clean Ni system [23], must have cross-sections below 10 22 c m 2. This work agrees qualitatively with later work on Ni [25], Fe [22] and W [26-28]. Below 300 nm, the situation appears to be different: rapidly rising CO yields have been observed and cannot be understood in terms of strictly thermal effects [26,29]. For example, at 250 nm on W, Kronauer and Menzel found a yield of 4 × 10 7 CO's per incident photon, which they attribute to a quantum effect on the basis of carefully compensated photon-induced temperature increases. These results raise a major question, which we will deal with in discussing more recent data - how can one distinguish between thermal and quantum effects? Another system of great interest, and one that may have properties similar to those in recent work on better characterized surfaces [24], was studied by Lichtman and co-workers [30]; here, the photon-driven desorption of CO and CO 2 from stainless steel dropped by three orders of magnitude in passing from 190 to 300 nm. At 190 rim, 1 molecule desorbed for every 1000 incident photons - a remarkable yield that they attributed to the presence of a surface layer of semiconducting chromium oxide. Until recently [7], examples of photo-assisted reactions between coadsorbed molecules on metal surfaces were few in number. There are a few scattered early reports concerning benzaldehyde on Cu [31], acetone [32], and ethylene on Ni and Pt [33], but the most extensively studied system involves CO oxidation over Pd. Baddour and Modell [34] reported that the rate of CO oxidation on Pd wire could be photoenhanced ten-fold at 420 K using 6.1 Torr CO, 14.7 Torr 02, and 740 Torr He. Subsequently, Chen, Close and White [35] examined this system at low (10 -5 Torr) and high (20 Torr) pressures. At low pressures, there was no detectable photoenhancement; but at 20 Torr, and depending on the temperature, the rate of CO oxidation was enhanced by as much as a factor of 20. This was attributed [2] either to photodissociation of O 2 on sites that are not normally active for this reaction, or to a photocatalyzed reaction between weakly held O z and adsorbed CO. Recent and very interesting U H V experiments by Ho and coworkers [36] point to the latter explanation. It is against this backdrop that new developments began to emerge rapidly about five years

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ago. Led by advances in surface chemical science, by laser-based excitation and detection schemes, and by the utilization of the tools of molecular dynamics, surface photochemistry is now a robust subdiscipline of surface chemistry. We would like this paper to serve as a useful introduction to photon-driven chemical processes for researchers familiar with surface chemistry and physics. For that reason, section 2 contains an overview of the important factors that enter into the experimental and interpretive aspects of this field. For the most part, these are simply reminders of familiar topics in physics and chemistry, but with specific application to the m e t a l - a d s o r b a t e - l i g h t problem. Section 3 presents experimental evidence for specific kinds of processes and, in some cases, compares the results with data from related areas. Section 4 reviews the available literature and is organized into sections by adsorbate type. Section 5 presents our view of the current status of research in this area and makes a number of suggestions for future research.

2. Methods and important factors As is commonly the case, one of the long-range goals of this basic research enterprise is to understand more than what happens when an a d s o r b a t e / m e t a l interface is irradiated by light. We want to understand why and how it happens. To achieve this, experimental surface photochemists must identify and vary, in a controlled way, all the key variables in the system. To the extent that we are able to do so, we will move closer to a full understanding of this complex and fascinating problem. With this in mind, we identify and briefly describe the salient factors in a m e t a l - a d s o r b a t e - l i g h t system and discuss their roles in the underlying chemistry and physics. To set the stage for this discussion, we overview briefly some of the experimental methods one can use to probe these systems. Then we will review, beginning with the light itself, the qualitative aspects of the key variables and their roles in the mechanism of surface photochemistry. After discussing the light, we will discuss the substrate, the adsorbate, and finally the dynamics of the desorbing products.

2.1. Experimental methods An experimentalist faces two general choices before doing a surface photochemical experiment: selection of the UV light source to initiate a surface photochemical reaction and selection of an analytical technique to detect the reaction products. In the following, we will briefly describe commonly used light sources and detection techniques. Finally, we will discuss briefly how cross sections for surface photochemical reactions are obtained.

2.1.1. Light sources Two kinds of UV light sources, CW arc lamps and pulsed lasers, are commonly employed in surface photochemical experiments. A CW light source provides low-power UV light with wide tunability. For example, a high-pressure Hg-arc lamp, one of the widely used CW light sources, outputs a broad spectrum of UV light with wavelengths > 230 nm. The emission peaks are at 254, 289, 295, 314, 335, 360, 400, 435, 545, and 575 nm. It also has a broad background that spans the visible and near-IR region. Photon energy selection is usually achieved by the use of bandpass filters, cut-off filters, or a low-resolution monochromator. A typical set of UVmercury-line bandpass filters transmits UV light centered at 254, 280, 289, 313, 334, and 365 nm with F W H M ' s of - 10 nm. Narrow bandpass filters centered at other wavelengths are also available. At the peak, these interference filters transmit around 20% of the incident light. In

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experiments where higher photon flux is required, cut-off filters are employed. The transmittance of a cut-off filter is high, approaching 100% above the cut-off wavelength, and low, approaching 0% below the cut-off wavelength. The transition usually occurs over a 10 to 100 nm interval, depending on the combination of materials used in the filter. When focused onto roughly one square centimeter, a 100 W high-pressure Hg-arc lamp usually gives a power density on the order of 1 to 10 W / c m 2. When narrow-width band-pass filters are used, the power density is usually on the order of 10 to 100 m W / c m 2. To avoid thermal effects, the power density on the surface is usually kept below 100 m W / c m 2. Another widely used CW light source is a Xe-arc lamp, which gives more uniformly distributed emission than the Hg arc, and, thus, broader tunability. When equipped with a monochromator, a 500 W Xe-arc lamp can provide continuously tunable light between 250 and 700 nm with a bandwidth of - 10 nm and a power density - 10 m W / c m 2. The second class of light sources is pulsed lasers and, for surface photochemical studies, the most widely used pulsed-source is an excimer laser. When filled with different gas mixtures, it outputs laser light at 157 (F2), 193 (ArF), 222 (KrC1), 248 (KrF), 308 (XeC1), and 351 nm (XeF). The pulse energy from an excimer laser is usually on the order of 102 m J / p u l s e . Compared to a CW light source, the most important advantage of an excimer laser is its time resolution. The pulse widths range from 10 to 30 ns, a time scale which allows one to address numerous questions of dynamics. For example, the kinetic energy distribution of the gas-phase products from a surface photochemical reaction can be obtained from a combination of pulsed laser and time-of-flight mass spectroscopy. The time resolution of a pulsed laser also results in a very high transient power density, on the order of 102 M W / c m 2. This is either an advantage or a disadvantage depending on the purpose of the experiment. In surface photochemical experiments where thermal chemistry is a side-effect, we need to keep the pulse energy low ( - 1 m J / c m 2. pulse) so that the transient surface temperature rise is below the threshold of thermal reaction. On the other hand, when thermal effects are desired, such as in laser-induced thermal desorption (LITD) studies, a high power density is required to achieve transient surface heating rates of as high as 101° K / s . UV light is introduced into the vacuum chamber through optical grade windows. C o m m o n l y used materials include UV-grade quartz, UV-grade sapphire, and CaF 2. The transmittances of these windows are above 80% for wavelengths longer than 190 nm. However, these materials absorb in the vacuum UV region toward 100 nm and LiF windows are recommended for wavelengths between 100 and 190 nm. While it is usually not a serious problem in post-irradiation surface analysis experiments, light intensity either not striking the surface or reflected from it may result in significant background signals when gas-phase products are detected during illumination, e.g., by a conventional QMS during a photochemical experiment. Careful control experiments are necessary to resolve background contributions to the signals of interest. 2,1.2. D e t e c t i o n t e c h n i q u e s

In terms of detecting the products of a surface photochemical reaction, two approaches are usually employed - detection after irradiation and detection during irradiation. The first detects the products retained on the surface and utilizes conventional surface analysis techniques, such as X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption (TPD). We will not go into the details of these conventional surface analysis techniques. While pulsed lasers are used, the favored light source for this kind of surface photochemistry experiment is a Hg- or Xe-arc lamp; the wavelength range, the tunability, the intensity, the cost and the easy accommodation

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to existing surface science instrumentation are all attractive. Since the time resolution of most surface analysis techniques is not comparable to the time scales of the dynamic surface photochemical reactions, there is little or no advantage in using pulsed laser light. To address dynamics questions, e.g. the translational energy of a gas-phase product from a surface photochemical reaction or the internal energy distribution of this product, requires another approach, i.e., the time-resolved detection during irradiation of desorbing photolysis products. Nanosecond time resolution can be obtained with the excimer lasers described above. Typical detection modes are time-of-flight mass spectrometry (TOF-MS) for velocity and angular distributions, and either laser-induced fluorescence (LIF) or multiphoton ionization, typically resonance-enhanced (REMPI), for internal energy distributions. Detailed descriptions of these experimental methods and the data analyses associated with them are beyond the scope of this paper but can be found in several excellent reviews [4,93-101] and the references cited in them. For TOF, the flight time of a desorbing species over a fixed flight distance is detected. The kinetic energy can be presented by a mean velocity, (v t. . . . ) , of a modified Maxwell-Boltzmann distribution [4,101]. It can also be presented in the form of a mean translational energy ~ E t . . . . ) (meV), which is equal to (½mvZ), or a mean translational temperature, ( E t. . . . ) / 2 k (kelvin). For L I F and REMPI, the species being detected is excited by a probe laser after it desorbs. The excited state either spontaneously fluoresces and the resulting photons are detected (LIF) or it absorbs one or more additional photons, perhaps from a different laser field, undergoes an autoionizing transition, and is detected as an ion (REMPI). With narrow-band excitation, both L I F and R E M P I are capable of exciting and detecting single internal vibration-rotation states. Since the L I F and R E M P I signals are directly proportional to the number of molecules in a particular quantum state within the probe volume, the signal intensities for both techniques reflect internal state distributions of photodesorbing species. L I F gives relative populations in various states directly. For R E M P I to provide these populations, ionization cross sections of the photodesorbing species must be known. In fact, these are known for only a few molecules [102]. Therefore, the R E M P I signal from a dynamics experiment is usually compared with one from a thermally known population distribution of the same species. In m a n y cases this disadvantage of R E M P I is outweighed by its sensitivity, which is 2 - 3 orders of magnitude higher than LIF. T O F - M S only provides information concerning the translational energy or velocity distribution of desorbing molecules and fragments, averaged over the internal states that are populated. If L I F or R E M P I is used in a T O F detection mode, the translational energy distribution of a single internal state can be obtained.

2.1.3. Cross sections The probability of a certain kind of an event, e.g., any kind of photon-driven reaction, is generally expressed as a cross section. We expect cross sections to depend on all the key variables of the system - wavelength, adsorbate coverage, surface structures, surface compositions, etc. From experimental data describing photon-driven rates, the cross section of a surface photochemical reaction is often obtained from the slope of a semi-logarithmic plot of adsorbate concentration versus the incident photon fluence. This procedure assumes that a first-order, or pseudo-first-order, kinetics is valid. There are some important approximations often being made which we now examine briefy. To begin, we take a simple case, seldom fully realized, but instructive. Assume we have a single surface photodissociation reaction, A - B ( a ) + S + h~, ~ A(a) + B(a),

(1)

where A - B represents an adsorbed molecule, A and B are generally polyatomic fragments, and

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S is an open substrate site. It is important to note that the rate for reaction (1) does not generally depend linearly on all the concentrations on the left side, because it is not a single elementary reaction step. It can be separated into at least two steps, excitation and dissociation: A - B ( a ) + hv --> A - B * (a),

(2)

A - B * (a) + S ~ A(a) + B(a).

(3)

A steady-state approximation, a notion which must be examined on a case-by-case basis, for the concentration of A - B * gives d [ A - B ] / d n oh = -- o [ A - B ] ,

(4)

where n ph is the photon fluence ( p h o t o n s / c m 2) and o the cross section (cm2/photon). Neglecting any coverage dependence in o, which is frequently inappropriate to do, we have In{ [ A - B I / [ A - B I °} = - Onph,

(5)

where [A-B] ° is the initial adsorbate coverage. Thus, a semi-logarithmic plot of [A-B] versus n ph gives a simple, effective first-order reaction cross section for the total loss of A-B(a). Many times A - B * (a) reacts to form more than one set of products. Eq. (5) is still useful and gives the total reaction cross section. But to get individual cross sections for the formation of each set of products, the time dependence of the formation of those products, not simply the disappearance of the reactant, must be measured. Similarly, for a surface photochemical reaction involving a gas-phase product, a semi-logarithmic plot of the product partial pressure versus n ph gives the cross section, provided the pumping speed is much higher than the photodesorption rate. Depending on the accuracies of power measurements ( + 5 % - 2 0 % ) and analysis techniques (perhaps as high as 10%), a typical percentage error for a measured surface photochemical cross section is on the order of +_20%. Complications arise when the coverage-dependence of o is not negligible; a variety of factors may be involved, e.g., inter-molecular quenching, site-blocking effects which change the branching ratio between dissociation and quenching, and, in the case of photoinduced dissociative electron attachment, changing local potentials with the accumulation of products. In these cases, In[A-B] will not, except for small variations in concentration, depend linearly o n n ph , and the phenomenological o will depend on the extent of pliotolysis or initial coverage. Another important factor must be kept in mind. To date, most cross-section calculations in surface photochemical studies have neglected the interference between incident and reflected light at the surface. In the case of direct intra-adsorbate excitation, for more realistic cross sections, we need to replace the incident light intensity in a cross-section calculation by the actual surface light intensity, which can be obtained from classic-electrodynamical calculations (see section 2.2.3). However, when substrate excitation is responsible (see below), the photochemical cross section is an overall cross section which is related to light-absorption, hot electron (hole) transport and attachment to the adsorbate, and relaxation.

2.2. The light and excitation pathways Having briefly introduced some of the experimental methodologies, we now turn to a more detailed discussion of fundamental factors - in this section, the light itself and the excitation pathways available for driving surface chemistry using photons. The theories of light-surface interactions have been extensively studied [37,38], particularly in the field of photoemission

Photochemistry at adsorbate / metal interfaces

85

[39]. We are in no position here to discuss these theories in any detail. Rather, we discuss those properties of electromagnetic radiation that are central to a description of photon-driven processes at a d s o r b a t e / m e t a l interfaces. We review some fundamental properties of light itself, but focus on light-surface interactions and the mechanisms by which photons drive surface chemistry. We show how different excitation mechanisms respond to variations of the key properties of the light. Three excitation mechanisms are discussed - (1) direct photon absorption by the adsorbate or adsorbate-substrate complex, (2) substrate-based energetic electron/hole mediated excitation, and (3) thermal excitation. Beginning with light itself, recall that, in classical electrodynamics, light is described as an electromagnetic plane wave with orthogonal electric and magnetic field vectors perpendicular to the propagation direction. For light propagating in the positive z direction in a non-absorbing medium, the instantaneous value of the electric field at time t is given by the time-dependent Maxwell's equation as E = E ° exp[i(o~t -

2~rz/~)],

(6)

where E and E ° are the instantaneous and maximum electric fields, respectively; w and 7~ are the angular frequency and wavelength of the light. The wave-particle duality is stressed in quantum theory, according to which particles of light consist of photons whose energy is given by = h~',

(7)

where h is Plank's constant and ~, = c/)~, the frequency of light. Without loss of generality, we will assume that light is monochromatic and that it can be characterized by the amplitude (E0) and energy (h~,). Certainly, the photon energy is a central variable; it sets limits on energetically allowed processes. Polarization is another important variable associated with light. In light-surface interactions, two forms are important - s- and p-polarization. In s-polarized light, the electric field vector is perpendicular to the plane of incidence (defined by two lines - the surface normal and the direction taken by the incident light). In contrast, for p-polarization, the electric field vector is parallel to the plane of incidence. As we will discuss in more detail later, the use of polarized light is an important means of identifying the mechanism by which photon absorption initiates chemical events involving adsorbates - e.g., surface photochemistry. Intensity, I, is also very important and can be defined as the energy falling on a unit area in unit time. For low-intensity sources that do not produce measurable non-linear optical effects, the fractional decrease in the intensity, or the number of photons absorbed by an absorbing medium, is directly proportional to the number of absorbing atoms or molecules in the light path and to the probability of a transition between the initial and final quantum states. In a form familiar to chemists, this proportionality leads to the famous B e e r - L a m b e r t law:

1,/10 =

exp( -

otoCL),

(8)

where, I t and I 0 are the light intensities after and before passing through an absorbing medium; C is the concentration of absorbing molecules (atoms) in m o l e s / v o l u m e ; L is the sample thickness through which the light passes; a 0 is the optical absorption coefficient in units of area/tool. When divided by Avogadro's constant, a 0 becomes the effective molecular absorption cross section (cm2/molecule), a concept more familiar to physicists. Light can interact with molecules in various ways. It can interact with the electric dipole of the molecule and induce rotational, vibrational, and electronic transitions; it can also interact with higher multipoles, e.g., the electric quadrupole, magnetic dipole and magnetic quadrupole of the molecule. In photochemistry, we are concerned with chemical changes that follow in the

86

X . - L Zhou, X.- Y. Zhu and J.M. White

wake of those photon-electric dipole interactions that produce electronic transitions in a molecular system. Ultraviolet photons (a few eV) are generally chosen because intense sources are available in the laboratory and because many molecular systems have electronic transitions, particularly the lowest energy ones, in this region. Surface chemistry driven by core level excitations has yet to be systematically explored; with the advent of more intense synchrotron sources, such experiments are becoming viable [40]. As far as the surface-adsorbate-light system is concerned, the significant variables are (1) wavelength/photon energy, (2) intensity, and (3) polarization of the light, all of which can be varied in the experiment. We will explore these key factors in some depth.

2.2.1. Wavelength/photon energy As noted above, the photon energy sets an energy limit on what is possible. Beyond that, the wave character and the wavelength dependence of surface photochemistry provide much insight. Before discussing the wavelength as a key variable, we need to define what kinds of systems we want to consider. Suppose we have a metal surface covered with an arbitrary, but known, amount of adsorbate. When this surface is irradiated by UV light, at least three conceptually different bond-breaking pathways initiated by photon absorption can operate, separately or in concert: (i) in direct excitation, the light is absorbed within the adsorbate or an adsorbate-substrate complex and leads to non-thermal chemical changes; (ii) in substrate excitation, the photon absorption occurs in the substrate and the resulting excited electrons or holes induce non-thermal adsorbate chemistry; and (iii) in either direct or substrate excitation, relaxation and randomization of the initial excitation energy results in surface heating and causes thermal chemistry. Varying the wavelength and polarization of the incident light can help in resolving these three different pathways. Moreover, as discussed in section 2.2.2, the incident power dependence is also helpful.

2.2.1.1. Direct excitation. If the initial excitation takes place strictly within the adsorbate (i.e., we can neglect the influence of the substrate on both the initial and final states) the wavelength response should follow that of the gas-phase or condensed-phase adsorbate molecule. In a single particle formulation, the excitation rate (R ~f) between the initial (~i) and final state (q~f) is given by Fermi's Golden rule: Rif

=

(~re/imhv) I(dpf I E - ~t I q,i) I ~ 6 (AEif

- -

hi)),

(9)

where E is the electric field vector; /~ is the transition dipole moment vector; AEit is the energy difference between the initial and final states; he is the photon energy; e and m are the charge (magnitude) and mass of electron. The 8-function enforces the resonance requirement, that is AEif = hp. One good example for a weakly held adsorbate is the photolysis of Mn(CO)6 on S i ( l l l ) , graphite, C u ( l l l ) and A g ( l l l ) [7,41] where, as discussed in section 4.11 (see fig. 57), the wavelength dependence correlates qualitatively with that in the gas phase; in both the gas phase and the adsorbed phase there is a resonance at 290 nm. Generally, however, we cannot neglect the substrate. The presence of a metal surface induces perturbations in the electronic structure of the ground- and the excited-state adsorbate, even when the adsorbate is weakly held. Transitions involving these levels are broadened by as little as 0.1 eV and as much as 1 eV [10,39,42a,43,44] and are shifted, both blue-shifted and red-shifted, from their gas-phase energies. It must also be noted that the single particle picture described by eq. (9) has limited applicability when the molecule is on a metal surface. For example, we expect the positive hole created in the instantaneous transition process to be screened to some extent by the electron "sea" in the metal, i.e., an image potential stabilizes the

Photochemistry at adsorbate / metal interfaces

87

system. As a result, the wavelength response can be red-shifted from the adiabatic process predicted by eq. (9), where, in a frozen orbital approximation, the energies and populations remain unchanged for all orbitals except the two directly involved. All these many-body perturbation effects, along with eq. (9), determine the wavelength response of an adsorbate on a metal surface. In addition to perturbing the electronic levels within the adsorbate, adsorption on a metal surface can introduce new electronic states that may be involved in UV absorption. Because metal character is mixed into the orbitals comprising them, excitations involving these new electronic states will certainly lead to wavelength responses quite different from those in the gas or condensed phase adsorbate. The wavelength dependence of this kind of excitation within the so-called "adsorbate-substrate" complex is also described, within the limitations outlined above, by eq. (9); the mixed adsorbate-substrate complex orbitals, initial a n d / o r final, are involved. For example, in the widely-used Blyholder [45] molecular orbital picture, the interaction of CO with transition metals is described by the overlap of the L U M O in isolated CO (i.e., 2~r*) with metal d-orbitals, resulting in two broadened molecular orbitals. These are denoted as 2~r~' and 2~r*, and have bonding and antibonding characteristics with respect to the admolecule-substrate chemisorption bond. The transition between these two orbitals has an experimentally determined threshold of - 5 eV and can lead to desorption [46]; as expected, electron-stimulated desorption (ESD) studies show an electron energy threshold of - 5 eV for CO desorption [10,47]. One of the important aspects of photochemistry at a d s o r b a t e / metal interfaces is the possibility that the incident photons indirectly drive the surface chemistry; the photon absorbs in the metal, exciting electrons and holes which, after transport, promote chemical events at the surface. Increasingly, evidence has accumulated which demonstrates the importance of hot electrons and holes in surface photochemistry; this is discussed extensively in section 3. Here, we will examine the fundamental factors involved in substrate excitation by photons and see how these factors are related to their wavelength/photon energy. To start, we borrow the classical three-step model widely used to describe photoemission [39,48]. We consider the substrate-mediated surface photochemical process as comprised of three distinct and separable (in time) steps: (i) the optical excitation of electrons in the bulk substrate; (ii) the migration of excited electrons and holes through the crystal lattice to the surface; and (iii) the attachment of these excited electrons or holes to the adsorbate to form an excited state. The first two are identical to the photoemission model, while the last step in photoemission, the escape of electrons from the surface field, is replaced here by the attachment of excited electrons or holes to the adsorbate. The rate of the last step, attachment, is determined by the energies of the adsorbate (or adsorbate-substrate) orbitals, the barrier for tunneling or attachment, and the energy distribution of hot carriers (electrons or holes). The energy distribution of hot carriers, a result of steps (i) and (ii), is related to the energy of the absorbed photons. Within a simplified one-electron theory of photoemission [49], the transition rate inside a bulk metal is also determined by eq. (9), where the initial and final states lie within occupied and unoccupied parts, respectively, of the metal band structure. Thus, the energy distribution of nascent (as formed, before any relaxation and scattering) photogenerated electrons will reflect the band structure, and the upper energy limit for an excited electron lies above the Fermi energy (~F) by an amount equal to the photon energy (hp). In step (ii), which involves many-body scattering processes, the lifetime of excited electrons is short, on the scale of 10 -aS s or so [50]. Since a typical group velocity is of order 108 c m / s [49], only those electrons emerging between 1 0 - 7 and 1 0 - 6 c m 2.2.1.2. Substrate excitation.

88

X.-L. Zhou, X.-Y. Zhu and J.M. White

from the surface will retain their original energy. Experimentally, this is usually expressed as an electron attenuation length, which is a measure of how far an electron will travel before undergoing a scattering event that removes it from those electrons being detected. On their way to the surface, the majority will lose energy through multiple scattering processes, and, while the upper energy limit remains unchanged, only a small fraction of the nascent electrons will appear with this energy. From the perspective of an adsorbate or adsorbate-substrate complex with an empty orbital located at E v above the Fermi level, ~v, the photon energy must be greater than or equal to (EF -- {F) in order for electron attachment to occur. For example, an elegant experiment shows that hot electrons drive the photodesorption (single photon process) of N O on Pt(111) and that photon energies greater than 1 eV are required [42b,51]. The position of the 2"~ * empty orbital in N O is about 1 eV above the Fermi level. Another example, SO 2 on A g ( l l l ) , is presented, with fig. 54, in section 4.9. Between 300 and 360 nm the photodesorption rate of SO 2 faithfully tracks the absorptivity of Ag. On semiconductors, Si and GaAs, the wavelength dependence of the photon-driven N O desorption follows the substrate optical absorption [52-56]. 2.2.1.3. Thermal excitation. What happens to those hot carriers that either do not arrive at the surface or do not attach to the adsorbate-substrate complex? Those with sufficient energy may be transmitted through the interface (photoemitted); those that are not photoemitted will eventually be thermalized and the temperature in the region will increase. Since the UV photon penetration depth in most metals is on the order of 10 - 6 t o 10 -5 cm (several hundred A) and scattering lengths are slightly shorter ( - 10 - 6 c m ) , the majority of the created hot carriers will not reach the surface without undergoing inelastic scattering. In a complicated, and so far poorly understood, many-body scattering process, the energy in the nascent electrons (holes) will be lost through electron-electron (hole-electron) collisions, electron-plasmon collisions, electron-hole recombination associated with Auger processes, and electron-phonon collisions. After - 10-12 s [57], the excitation energy is ultimately shared with the atoms in the solid and is transformed into lattice phonons (heat). Since thermal activation generally leads to chemistry, it is important to determine the extent to which thermal effects contribute to the observed photon-driven chemistry. If a surface photochemical process is the result of surface heating, its rate is then a function of the energy deposited into the substrate. The only wavelength requirement, and one easily met throughout the visible and ultraviolet region, is absorption by the metal. Examples of this kind of surface photon-driven chemistry can be found, e.g., in laser-induced thermal desorption (LITD) studies, which will be discussed in more detail later. 2.2.2. Intensity It is well known that both the chemistry and physics of photon-initiated processes can become, at high values typical of pulsed and tightly focused laser sources, non-linearly dependent on the light intensity: e.g., single photon versus multi-photon excitation. For the light-adsorbate-substrate systems being considered here, intensity, particularly variations in the observed chemistry with incident intensity, is a central issue. At least five possible photon-driven surface chemical reaction pathways, each with a characteristic intensity response (i.e., rate cc In), should be considered: (i) equilibrium bulk substrate heating; (ii) transient surface heating; (iii) local surface heating; (iv) excitation by hot carriers; and (v) direct excitation, both single and multiphoton. 2.2.2.1. Equilibrium bulk substrate heating. Metals strongly absorb light throughout the region from the I R to the vacuum UV. The energy deposited can raise the substrate temperature and

Photochemistry at adsorbate / metal interfaces

89

induce thermal chemistry. This effect can easily be distinguished from other pathways since it must, by definition, be identical to the effect of resistive heating, and, except for the wavelength dependence in reflectivity, must be independent of wavelength. The substrate temperature will always increase monotonically with light intensity, but the dependence is non-linear. In principle, the temperature rise in a metal sample can be calculated from standard thermal conduction equations, assuming the following parameters are known: the photon flux, the optical constants, the specific heat, the density, the thermal conductance, and the extent of thermal contact with the environment. However, the last parameter is generally unknown, and assumptions have to be made to obtain simple estimates. Fortunately, the bulk temperature rise in a metal sample can be directly measured quite accurately, e.g., using thermocouples. Since thermally activated chemical reactions typically depend exponentially on ( - E / k T ) , we expect to see a strongly non-linear dependence of the measured rate on incident intensity. If the photoeffect is exclusively due to substrate heating, the dependence of the reaction yield on light intensity can be modeled using simple thermal kinetic equations and the measured temporal temperature profile. It is worth noting that, under low-intensity continuous (CW) illumination, the spatial temperature gradient in a metal sample is negligible, because metals are good thermal conductors.

2.2.2.2. Transient surface heating. The situation is very different when pulsed laser fight is employed; transient heating of the near-surface region becomes important. This effect has been of considerable interest, e.g., in laser-induced thermal desorption (LITD) studies [58,59]. As discussed earlier, initial excitation in the substrate rapidly decays into heat on a time scale of 10 -12 s. Depending on the laser intensity and pulse duration (10 -8 s for an excimer laser), transient surface temperatures can rise from several kelvins to several thousand kelvins. The spatial and temporal temperature profile in the near-surface region can be calculated numerically from the heat conduction equation (Laplace diffusion equation) [60]: a T i l t = x x7 2T.

(10)

x is the thermal diffusivity (x = K / p C , where K is the thermal conductivity; p, the density; and C the heat capacity). In calculations, the following assumptions are usually made to obtain suitable boundary conditions: (1) since the photon penetration depth in a metal (10 -6 cm) is usually much less than the distance energy can diffuse within the pulse duration time (for Pt, 2 x 10 -5 cm in 10 -8 s), the thermal energy is assumed to be deposited initially at the boundary between the vacuum and the solid (z = 0); and (2) compared to the diameter of the laser beam, the lateral diffusion of thermal energy is negligible within the pulse duration time. With these assumptions, the temporal profile of the surface temperature jump (AT) is given by the following equation [58,59]:

A T = ~(KpCTr)-l/2Iofo'F(t - T)~- - ' / 2 d¢,

(11)

where ~ is the absorptivity at the laser wavelength; I 0 is the laser intensity, assumed spatially uniform; and F(t) is the temporal profile of the laser pulse. As an example of the application of this equation, a maximum surface temperature jump of 800 K can be achieved for a Pt surface irradiated by a 50 M W / c m 2 laser pulse with a F W H M of 30 ns [59]. Since the peak temperature is obtained on the time scale of the laser pulse, the heating rate is on the order of 10 ~° K / s . With such a high temperature jump and heating rate, many thermal reactions, particularly thermal desorption, can occur. If a surface photon-driven process, under pulsed-laser irradiation, is due exclusively to such transient surface heating, the

90

X . - L Zhou, X.- Y. Zhu and J.M. White

dependence of reaction yield on laser intensity will be non-linear. For a reaction with a kinetic b a r r i e r , Ea, if the initial surface temperature is low, the rate ( r ) at the peak temperature is: r ec e x p ( - E a / k T p ) cx e x p ( - E a / k A T ) cX e x p ( - a / I 0 ) , where a is a constant that reflects the properties contained within eq. (11), and the activation energy, E,. This relation between the rate and the laser intensity is valid for cases where only a small fraction of the initial concentration is depleted, a condition typical in surface photochemistry. For discussion of other laser heating conditions, see ref. [307]. 2.2.2.3. Local and resonant surface heating. In addition to the two thermal effects discussed above, highly localized surface heating must also be considered. Quenching of electronically excited states on a metal surface is expected to be very fast ( < 10-~2 s) [10]. On such a time scale, energy in the excited adsorbate or adsorbate-substrate complex can be converted to surface phonons (heating). Since the time scale for thermal energy to diffuse into the metal a distance of 10 - 7 c m ( d = ( 2 r t ) 1/2) is on the order of 10 -12 s, which is not too short for a chemical reaction, it is possible that local randomization of the excitation energy might induce a surface reaction characterizable by a local temperature. Although this possibility has not been tested or observed for U V / m e t a l systems, and it is difficult to get an independent measurement of the local surface temperature, there is evidence for localized heating, mediated by resonant vibrational excitations, that leads to desorption [61]. For example, in a study of IR-induced desorption of pyridine from KC1, Chuang and co-workers suggest that the photon energy resonantly absorbed by one adsorbate molecule is rapidly shared with its neighbors, and desorption occurs rather randomly from among this group of molecules [61]. They also found that the desorption yield depends not linearly, but nearly exponentially, as expected, on light intensity. If similar localized processes were to play a role in photochemistry on metals, we would expect a similar dependence on light intensity. 2.2.2.4. Excitation by hot carriers. Turning to the intensity dependence expected when hot carriers drive the surface chemistry, we note that the power law becomes much more complicated. In systems where hot carrier mediated processes dominate, power laws with fractional order [41], first order [42b,51], and higher orders [62] have been reported. For processes requiring only one carrier, the photolysis rate is proportional to the concentration of hot carriers reaching the surface. Therefore, the dependence of the hot carrier concentration on light intensity must be considered. As discussed earlier, the lifetime of nascent excited electrons (holes) is on the order of 10 -15 s. Full thermalization takes many collisions but in roughly 10-~2 s, these hot carriers fully thermalize via a variety of scattering processes. Within a single particle approximation, as described by the adiabatic model in eq. (9), the concentration of nascent hot carriers is proportional to the number of absorbed photons (incident light intensity). The interaction of hot carriers with the electrons in the conduction band dominates [50,63], and we can neglect scattering among excited electrons. Screening by the conduction electrons also assures the infrequency of excited electron-hole pair recombination, at least for low light intensities (e.g., CW sources of the variety typically used in surface photochemistry). With these assumptions, the mean free path of hot carriers is independent of their concentration; thus, the hot carrier concentration reaching the surface is proportional to that of nascent hot carriers. This leads to a pseudo-first-order power law, i.e., the reaction rate is proportional to incident light intensity. For most of the literature reviewed here, the first-order conditions are applicable. This crude cameo is not valid when high intensity or extremely short-pulsed laser light is employed (e.g., > 10 M W / c m 2 ) . Under these conditions, the interactions among excited hot

Photochemistry at adsorbate / metal interfaces

91

carriers, especially the Auger-related electron-hole recombination process, can no longer be neglected. In addition, multicarrier excitation of the adsorbate might operate. All these may result in complicated power laws. For example, in a recent study of photodesorption of N O from P d ( l l l ) with subpicosecond (200 fs) laser pulses (2.0 eV), Prybyla et al. [62] found a power law with n = 3.3 (see fig. 49 in section 4.5). This is due to the highly non-equilibrium conditions on this time scale and the possible involvement of multicarrier-adsorbate excitation processes (e.g., 3 hot electrons).

2.2.2.5. Direct processes: single- and multi-photon. In a direct, single-photon, electronic excitation of the adsorbate or adsorbate-substrate complex, denoted as R, the reaction rate ( r ) is given by the following equation, which is first-order in photon intensity: r = Fo[R], (12) where F is the photon flux (intensity); o is the cross section; and [R] is the surface concentration of R. This equation, first-order in both F and JR], is generally valid as long as the light intensity is low, so that multi-photon processes are negligible. For example, based on the wavelength response, the UV photodissociation of Mo(CO)6 on Cu, Ag, and Si can be attributed to direct electronic excitation of the adsorbate, and the initial photolysis yield (rate) is proportional to light intensity [7]. Only a few studies have shown the importance of direct, intra-adsorbate electronic excitation via multiphoton absorption at surfaces. Harris and co-workers [64] and Moskovits and co-workers [65] reported that the laser photodissociation of aromatic compounds on rough silver surfaces is second-order in the photon intensity. They attributed these observations to two-photon electronic excitation processes. Under what conditions should we expect a significant contribution from multi-photon processes? Suppose we have a monolayer of adsorbate on a metal surface, with a typical concentration of 1 x 1015 molecules/cm 2. Absorption of one photon results in an excited state with a typical lifetime (tL) of < 10-13 S. If the thin adsorbate layer absorbs 1% of the incident pulsed excimer laser light (1 J / c m 2, 6.4 eV, or 1 x 1018 photons/cm2), then during the laser pulse (t D = 10 ns) the concentration of excited state molecules is roughly 1 x 1011 molecules/cm 2 (fluence per pulse x fraction absorbed x lifetime/pulse length). If we assume the same possibility for a second photon absorption, the number of excited molecules from two-photon absorption is only 104 molecules/cm 2, negligibly small. Since the concentration produced by two-photon excitation will scale roughly with (tL/tO) 2, the laser pulse duration has to be comparable to the lifetime of excited state molecules on the surface, i.e., in the picosecond or subpicosecond region. In this context it is important to note that multi-photon processes can be initiated by focusing laser pulses with energies like those assumed above to give power densities above 10 3 M W / c m 2. 2.2.3. Polarization and angle of incidence Having described the important wavelength and intensity considerations associated with the incident light, we turn to its polarization. The feature distinguishing surface photochemistry from other kinds of photochemistry is the presence of the surface. As we have discussed above, during irradiation, the surface can behave as an absorber, a source of hot carriers and a source of thermal energy. In addition, the surface serves as a reflector. The reflected light wave interferes with the incident light wave, and the resulting combination varies predictably with the polarization and incidence angle of the source. The dependences of the surface electric field intensities and substrate absorbance on polarization and the angle of incidence can be obtained from classical electrodynamics [66]. As suggested by Hasselbrink et al., these polarization and

92

X.-L. Zhou, X.-Y. Zhu andJ.M. White

angle-of-incidence dependences can be used to identify the initial excitation mechanism in a surface photochemical reaction [154,70].

2.2.3.1. Classical optics.

Fig. 1 illustrates the interaction of polarized light with a surface. The incoming light ( + ) is partially reflected ( - ) and partially refracted (t). The direction of the incoming light and direction of the surface normal define the plane of incidence. The angle between the surface normal and the light propagation direction is defined as the angle of incidence, 3'. In classical optical theory, a light wave traveling in an absorbing medium is described by

wt - (2~rnl/X)s" r] ) E 0 e x p ( i [ w t - (21rnl/)~)s. r ] ) ,

E = E 0 exp(i[ =

e x p [ - (2qTkl/~k)$"

¥] (13)

where nl, the index of refraction of the medium, is equal to c/v, the ratio of the velocity of light in vacuum to that in the medium; k 1 > 0, the extinction coefficient; nl, the complex refractive index of the absorbing medium, is defined as n 1 = n I - ikl; s is a unit vector in the light propagation direction; and r is a position vector referenced to an arbitrary coordinate system. This is a modification of eq. (6). The intensity of light, I, is proportional to the mean-square electric field intensity. F r o m eq. (13), we have I = I 0 exp[ - 4(¢rk/X)

s" r].

(14)

This is simply a variation of the B e e r - L a m b e r t law in eq. (8). For a surface problem in fig. 1, the incoming light is reflected and refracted. If we consider the reflectivity, R, eq. (14) becomes I = I0(1 - R ) exp[ -

(4~rk/X)s" r].

(15)

The amount of reflection and refraction at each incidence angle can be derived from Maxwell's equations, the boundary conditions of which require continuity of certain field components at boundaries, namely, the tangential components of the electric and magnetic fields and the normal components of the displacement and induced fields [67]. Solving Maxwell's equations with these boundary conditions, the Fresnel coefficients of reflection at the interface can be derived [66]."

r,=Ep/Ep

= [ - h i z cos

3"+no(n~-n ~ sinZ3")l/z]/[n~

cos

3"+no(nZ-n~

• z

~x/21

sm 3')

],

(16)

rs= E : / E + =

In 2 cos

3"-(n~-n~

sin

3")'/2]/[n2

cos3, +

z

(n~-n2o sinZy)l/z],

(17)

x

t Fig. 1. Schematic diagram illustrating the interaction of polarized light with a surface. The incoming light ( + ) is partially reflected ( - ) and partially refracted (t), For p-polarization, the electric field is in the plane of incidence (defined by direction of light incidence and the surface normal); for s-polarization, it is perpendicular to the plane of incidence.

Photochemistry at adsorbate / metal interfaces

93

1.5'

-~ E ~

PI(111)-280 nm

2 ......... ./<,E z >

1.2

=Ya o9 ,-°¢~ •' ~ .oo

0.6

.o o

0,3

m

o.o ....... 1 0

.,.

/"

An '" / \ ................ .................. 2 ,," ............................................~<.. "-.

"",

30 60 Angle of incidence (degree)

90

Fig. 2. Angular dependence of metal absorbance (Ap, for p-polarization and A, for s-polarization) and electric field intensity at the interface for s- and p-polarized light on P t ( l l l ) at 280 nm. The curves were calculated from Fresnel's equations and the Pt metal optical constants were from ref. [151]. For s-polarization, the electric field intensity has only one component which is in the plane of the surface ((E~)). For p-polarization, it has two components, one in the surface plane ( ( E 2 ) ) and one perpendicular to the surface plane (< E 2 >).

where rp and rs are the amplitude ratios of reflected electric fields (E~-, E~-) to incident electric fields (E~-, E~+) for p- and s-polarization, respectively; n 0 (n o = 1) and n 1 (n 1 = n 1 - ikl) are the complex refractive indices of vacuum and metal, respectively; and y is the angle of incidence. In polar form, the reflection coefficients become rj = _R_j1/2 exp[iS(l))],

(18)

where j denotes p or s; Rj is the reflectivity; and 8 ( r j ) , the reflection phase shift, is equal to t a n - l { I m ( r j ) / R e ( ~ ) } , where Re and Im are the real and imaginary parts of rj. The absorption coefficient ( A j ) is given by, (19)

A j = 1 - R j = 1 - [rjl z

The total electric field at the surface is the sum of the incident and reflected fields, whose intensities are given by the following equations [66]: (r~z) = (1 + R p - 2 R ~ / 2 cos 8p)(cosEy)((E;)2), + 2

(20)

(Ey2) = (1 + R s + 2R1~/2 cos 8s)((E ~ ) ),

(21)

( E 2) = (1 + Rp + 2Rip/2 cos 8p)(Sin23,)((E~ -)2).

(22)

Assuming unit amplitudes for the incident electric fields, the calculated electric field components ((E~), (EYE), ( E ) ) ) and the substrate absorbance (Ap and A,) at 280 rim, based on the optical constants for Pt at various wavelengths [68], are displayed in fig. 2. For s-polarization, both the metal absorbance (As) and surface electric field intensity ((E~)) decrease monotonically to zero with increasing incidence angle. For p-polarization, (Ex2) decreases monotonically to zero, whereas ( E ) ) is zero at normal incidence, maximizes at 56 ° and returns to zero at larger angles. The metal absorbance, Ap, is equal to As at normal incidence but rises slowly to about 65 ° , and then falls sharply to zero. These different dependences on the angle of incidence are useful in distinguishing between different excitation mechanisms in surface photochemistry.

94

X.-L. Zhou, X.- Y. Zhu and J.M. White

For a monolayer-covered metal surface, the adsorbate layer is optically thin, so we can neglect the reflection and refraction of light by the adsorbate layer. This approximation is valid since the thickness of the adsorbate layer (d ~ a few A) is much smaller than the wavelength of the incident light (X > 10 3 ,~). Thus, the contribution of d / X to the phase shift in reflection/ refraction is negligible [66]. However, for multilayer-covered metal surfaces, the thickness of the adsorbate layer cannot be neglected. A more complicated three-layer model, which takes account of the thickness and the complex refractive index of this condensed layer, can be used to calculate the electric field in each layer. We will not treat this problem explicitly here. Interested readers should see ref. [66].

2.2.3.2. Microscopic calculation. In the above classical treatment, the dielectric constant is represented by a step-function across the interface, changing from n o in the vacuum to n~ in the metal. On a microscopic scale, such a description is not physically sensible, since it also results in a step-function for the electric field normal to the surface. In principle, local electric fields on the surface can be calculated, but this has been done only on jellium surfaces [38]. When an external electric field is applied to a jellium surface, defined in section 2.3.1.1, the mobile jellium electrons respond to this perturbation; the result is a charge density distortion with respect to the ground-state distribution (see fig. 4, which will be discussed later). The induced charge distribution is a source of induced fields, and fig. 3 shows the calculated fields [38]. Several important features are obvious in this figure. Let's first look at the in-phase segment. When the photon frequency, to, is lower than the bulk plasmon frequency, top, the in-phase parts of the fields are essentially rounded versions of the classical step-function. When to is higher than top, fields of the bulk plasmon dominate. The oscillation of fields inside the bulk reflects the tendency of the jellium to screen out the external field. For those fields oscillating at 90 ° out-of-phase with the incident wave, the intensity of the field at the surface shows a sharp peak, which signals the presence of an efficient energy loss. Such a jellium surface absorbs energy by exciting electron-hole pairs. The above local field calculation has found strong support from surface photoemission studies. For photoemission from electronic states localized in the surface region on free metal surfaces, AI(001) and Be(0001), the light-wavelength dependence in the measured cross section agrees satisfactorily with that predicted by the above calculation [69c]. As far as surface photochemistry is concerned, one can conclude from fig. 3 that the classical optical theory is a good approximation for electric fields on the vacuum side, - 1 ,~ above the surface. This should be useful for weakly held species, where mixing of the adsorbate and substrate wavefunctions is negligible. If excitation occurs within the adsorbate, we can rely on the electric fields on the vacuum side, where classical theory holds. However, for strongly bonded species, or in systems where excitation within the adsorbate-substrate complex is important, the responsible electric field intensity might be that within the transition region and classical theory would fail. One simple adjustment to the classical theory is to introduce an effective dielectric constant, neff, to describe the interface, of ill-defined thickness, and force it to change smoothly from n o to ha. In this approximation, the intensity of the effective normal electric field is ( E 2 ) / I neff[ 2. Since a microscopic calculation is not available for transitionmetal surfaces, we will apply the classical theory in the following. Possible deviation from the classical theory will be included in neff, a free parameter in experimental fittings. 2.2.3.3. Photolysis cross sections. The dependence of photolysis cross sections on the angle of incidence will be derived for three cases, depending on the optical excitation mechanism [70]:

Photochemistry at adsorbate / metal interfaces

95

j,i

U

/t% - 0.95

::',,,

wlw~

- ao

6

/o

io

;o

,;

-~

~o

= 101

~o

;o

7(A) Fig. 3. In-phase (solid curves) and out-of-phase (dashed curves) contributions to the normal component of the electric field near a jellium surface (Feibelman, 1982). The positive z-axis points into the bulk. After Feibelman [38]. (1) substrate excitation; (2) direct excitation with > C 2 rotational surface symmetry; and (3) direct excitation with C1 rotational surface symmetry.

2.2.3.3.1. Substrate excitation.

As we have discussed earlier, if photochemistry is initiated by hot carriers, the photolysis cross section is then proportional to the concentration of hot carriers reaching the surface. Within a single particle approximation, as described by the adiabatic model in eq. (9), the concentration of nascent hot carriers, [h z ], created at a distance z f r o m the surface is proportional to the light intensity at this distance: [hz] = Io~,Aj(7)a e -"z,

(23)

where I 0 is the incident light intensity; ~'a is a proportionality constant related to eq. (9); A j ( ' / ) is the metal absorbance given by eq. (19); a is a function of the angle of incidence. This equation is a variation of eq. (15). The presence of a in the proportionality constant assures that an integration of [hz] over the whole range of z yields I0~'aAj('y). At normal incidence, a 0, the optical absorption coefficient (not the same quantity as in eq. (8)), is equal to 47rkl/A (k 1,

96

X.-L Zhou, X.- Y. Zhu and J.M. White

the optical extinction coefficient of the metal; ?~, the light wavelength). In the first approximation, we can assume that the interaction of hot carriers with the electron "sea" in the metal conduction band dominates [50,63], and that the mean free path of hot carriers is independent of their concentration; thus, the hot carrier concentration reaching the surface is proportional to that of nascent hot carriers:

[heff/]/[h z ] = e-z/A,

(24)

where [hCu ] is the effective concentration of hot carriers reaching the surface; A is the effective mean free path of all the hot carriers. We have neglected the dependence on hot carrier energy in this equation. Therefore, A represents an energy-integrated effective mean free path. Integration of [ h e f t ] o v e r the possible range of z gives the probability of photolysis: Oj = ~2f0°° [ h eff] dz = ~l~'2Aj(y)/[1 +

1/(aA)]

(25)

where oj is the photolysis cross section and ~2 is a constant related to the energy of hot carriers. A, depending on the energy of hot carriers, is on the order of 10 -6 cm [63]. Since, in the UV region, the optical absorption coefficient, a o, is on the order of 106 cm -1 for metals, the dependence of [1 + 1/(e~A)]-1 on the angle of incidence cannot be neglected. The dependence of a on the angle of incidence is given by the following equation [70], a 2 ( y ) / a 2 = { - n 2 + k 2 +n~ sin y + [ ( - n 2 + k~ + n 2 sin2?)2 + 4k~n2] '/2~)/(2k,)" 2, (26) If we assume that aoA = 1, the dependence of [1 + 1 / ( a A ) ] -1 on the angle of incidence can be calculated. It can be easily shown that, for most metals, [1 + 1 / ( a A ) ] - 1 changes by no more than 10% between 0 o and 90 0. Compared to Aj (~,), the angular dependence in [1 + 1 / ( a A ) ] - 1 is negligible. Therefore, the dependence of oj on the angle of incidence is dominated by A j ( y ) in eq. (19).

2.2.3.3.2. Direct excitation, > C e symmetry. If the surface photochemical process is a result of direct optical absorption of the adsorbate, or the adsorbate-substrate complex, the photolysis cross section is proportional to the transition rate given by eq. (9). In simplified form, the photolysis cross section is given by: oj = 0.5~ I(~" E) 12,

(27)

where ~3 is a proportionality constant; /t is the transition dipole vector; and E is the electric field vector. Let us consider an adsorbed molecule with a unit transition dipole oriented with a polar angle, O, with respect to the Z axis in fig. 1 and an azimuthal angle, q~, with respect to the X axis in fig. 1. For s-polarization, 1 0 t - E ) 12= (Ey2) sin20 sin2¢;

(28)

and for p-polarization, I(/t. E ) 12 = ( E 2) sin20 cos2,/, + ( E 2) cosZO + ( E x • Ez) sin O cos 0 cos ¢,

(29)

where 0 ° _< ¢ _< 360 ° and 0 ° < O _< 180 °. In the above derivation, we have used the following definition for the mean square electric field strength [66]: = ( [ R e ( E ) ] z) = 0.51 E012

(30)

Photochemistry at adsorbate / metal interfaces

97

The electric field strengths (intensities) are given by eqs. (20)-(22), but the mixed term, ( E x • E:), is not obtainable. Fortunately, most substrate surfaces have higher than C1 rotational symmetry and an integration of the matrix elements over all azimuthal angles cancels the mixed term. For example, a fcc (111) surface has three-fold rotational symmetry, and we expect the oriented transition dipoles of the adsorbate or adsorbate-substrate complex to be equally distributed with at least three equivalent azimuthal directions, i.e., 40, (40 + 120°), and (40 - 120° )- Integration of the mixed term in eq. (26) gives: ( E x • Ez) sin 8 cos O[cos 40 + cos(40 + 120 ° ) + cos(40 - 1 2 0 ° ) ] / 3 = 0. For surfaces with > C 2 symmetry, the cross sections are given by: op = ~'3[cos240 sin20o~Ex2) + cos280(Ef)] op = ~'3[0.5 sin2O0(r~) + cos200(e2)] o~ = ~3 sin240 sin200(Ey2) os = ~'30.5 sin2Oo(g2y)

(C2),

(c3, C4, C6, and Coo),

(C2),

(C3, C4, C6, and Coo),

(31) (32) (33) (34)

where % and 00 are the coordinates of a fixed dipole, 0 ° < ~ < 180 ° and 0 ° < 0o < 180 °. For randomly distributed dipoles (all angles equally weighted), integration over a spherical surface gives


(36)

Here, the cross section is simply proportional to the total electric field intensity. 2.2.3.3.3. Direct excitation, C 1 symmetry. In some cases, a surface may possess C 1 rotational symmetry, e.g., a stepped Pt(210), so that the mixed term in eq. (29) does not average to zero. To derive photolysis cross sections, we must directly calculate the electric field intensities in the # direction. If we project the electric field onto the direction of #, eq. (27) becomes: oj = 0.5~"3 I(E~) 12

(37)

For s-polarization, o s ----- 0 . 5 ~ 3

[(Ey sin 0 sin 4 ) I 2

= ~3((E~+) 2) sin20 sin24(1 + R s + 2Rls/2 cos 8s);

(38)

and for p-polarization, Op = 0.5~"3 I(Ex sin 0 cos 4 + E~ cos 0) 12

~---~3~(E;)2)(c12 4-c2Rp q- 2clc2Rlp/2 cos 8p),

(39)

where c 1 = (sin y cos 8 + cos y sin 0 cos 4), c2 = (sin 3' cos 8 - cos y sin 8 cos 4). 2.3. The substrate From section 2, we can see the significant role played by the substrate in light-adsorbatesurface interactions. In this section, we will discuss the key properties of metal substrates and see how these can help guide our choice of metal surfaces for a photochemical experiment. The

X.-L Zhou, X.- Y. Zhu and J.M. White

98

key properties, important to our discussion of surface photochemistry, include electronic structures and elementary excitation processes (phonons, plasmons, etc.). For a more comprehensive account of the physics of solid substrates, interested readers are referred to several excellent books [39,69].

2.3.1. Electronicstructure In solid-state physics, the electronic structure of metal substrates has been described mainly by two models: the free-electron jellium model and band theory. Both are important to our consideration of the light-adsorbate-surface interaction. For example, the optical properties of a metal surface are usually discussed within the framework of the jellium model. However, descriptions of electronic excitations in the substrate and the coupling between the adsorbate and the surface require an understanding of band structures. Therefore, we briefly discuss both, beginning with the Hamiltonian describing the substrate electronic structure: p?

"~=

i=1

2m

N

~ ~

R i=1

Ze 2

1

Ir~-RI + 2

~ i,j

e~ Ir,

rjl'

(40)

where N is the total number of electrons; R is the set of vectors representing the positions of all the atoms in the crystal structure; and r is the position vector for electrons. As in many quantum chemical problems, the electron-electron repulsion term prevents us from obtaining a closed solution to eq. (40), and approximations have to be made. One popular approach is the local density approximation, LDA, in which the conduction electrons in the lattice are represented by an inhomogeneous distribution function, n(r), while the exchange-correlation energy is taken to be that of a homogeneous electron gas [69c]. However, for the semi-infinite lattice problem, even the LDA approach is difficult. Thus, the much simpler jellium model comes into play.

2.3.1.1. Jellium model It is well known that a metal lattice is transparent to conduction electrons. This occurs because the wave describing a conduction electron propagates freely in a periodic lattice structure and because the Pauli exclusion principle assures infrequent scattering among conduction electrons. The jellium model is an approximation to this situation. It

1.0 \\

u~ ~D

L

Q)

0.5

Ld

i

Positive background

\\\%=2

- 1.0

- 015

0

0,5

1.O

Distance(Fermi wavelengths) Fig. 4. Electron density profile at a jellium surface from two choices of the background density, rs. After Lang and

Kohn [71].

Photochemistryat adsorbate/ metalinterfaces

99

replaces the discrete positive ion cores in L D A with a uniform positive charge background that fills half of the space. Fig. 4 shows the calculated electron density as a function of distance (z) from the surface in a jellium model [71]. There are two important features in these profiles: first, the electrons spill out into the vacuum (z > 0), which leads to a surface dipole layer; second, the electron density function, n(z), oscillates as it approaches the value which compensates the positive background. In the jellium model, the semi-infinite ion core lattice is smeared out into a uniform positive charge background. This model nicely approximates metals with delocalized s or p conduction electrons (completely empty or completely filled d-bands), e.g., Na, K, Cs, AI, etc. It provides us with good insight into some physical properties, such as heat capacity, thermal and electrical conductivity, and optical (electrodynamic) properties. To understand other important physical properties, e.g., the distinction between metals, semimetals, semiconductors, and insulators, we need the less naive band theory, which takes into account the periodicity of the lattice and the relatively localized nature of some valence electrons, e.g., d-electrons in transition metals. Detailed presentations of band theory can be found in refs. [39,69]. Here, we will discuss band theory in a more intuitive way, following H o f f m a n n [69d,e].

2.3.1.2. Band theory. The solution of the Schr/Sdinger equation for a periodic potential is given by the Bloch theorem [69], which says that the eigenfunctions are the products of a plane wave exp(ik • r) times a function with the periodicity of the crystal lattice, where k is a reciprocal lattice vector and r is a position vector. In chemical language, this simply means that the wavefunction for a periodic lattice potential is a symmetry-adapted linear combination of atom-localized wavefunctions. Solids contain about 1022 a t o m s / c m 3, and the accompanying huge number of atomic wavefunctions combine to give system wavefunctions where energies essentially form a continuum, i.e., an energy band. Fig. 5 shows the calculated band structure of a four-layer Ni slab which serves as a model for the Ni(100) surface [69d]. Here, the energy eigenvalue is plotted as a function of k varying along certain specific directions within a two-dimensional Brillouin zone. The number of lines in this figure is equal to the number (n) of orbitals in the unit cell. The total number of states or energy levels is nN, where N is the number of unit cells. It is intuitively difficult to discuss the individual energy levels from among an Avogadro's total number of them. An alternative presentation of the band structure is in terms of density of states (p), defined as p ( E ) d E = number of levels between E and E + d E .

(41)

Fig. 6 shows p ( E ) for the Ni slab in fig. 5. It is proportional to the inverse of the slope of E ( k ) versus k and counts the number of levels in each energy interval. Electrons occupy these energy levels, from bottom to top, according to the Pauli exclusion principle. At 0 K, the highest occupied molecular orbital is defined as the Fermi level, so that all levels above the Fermi level are empty. At other temperatures, thermal excitation assures that some levels above the Fermi level are populated, while some below the Fermi level are empty. For the Ni slab in fig. 6, the Fermi level lies at the top edge of the d-band, which overlaps the s- and p-bands. For m a n y solids, an energy gap, E~, exists between the so-called valence band, the highest energy band that is completely filled by electrons, and the conduction band, the lowest-lying energy band, which at most is only partially filled. The magnitude of Eg serves as a criterion for the distinction between metals, semimetals, semiconductors, and insulators (fig. 7). In metals (fig. 7a), the conduction band is partially filled by electrons at all temperatures. In semimetals, such as bismuth, the valence and conduction bands overlap in a narrow range of energies; the valence band is almost filled and the conduction band is nearly empty at absolute zero (fig. 7b).

X.-L Zhou, X.-Y. Zhu andJ.M. White

100

-2

Ni (I00) slab

W -8 -10

-qO -12 -12

-14

-14

r x M ; Fig. 5. Band structure of a four-layer Ni slab that serves as a model for a Ni(100) surface. The flat bands are derived from Ni3d; the more highly dispersed ones above these are 4s and 4p. After Hoffmann [69d].

DOS Fig. 6. Density of states of a four-layer Ni slab as in fig. 5. The fiat bands are derived from 4s and 4p, and the intense band ( - 8 - - - , - 1 2 eV) is the 3d band. The Fermi level is located near the upper edge of the d-band. After Hoffmann [69d].

I n p u r e s e m i c o n d u c t o r s , t h e c o n d u c t i o n b a n d b e c o m e s e m p t y as T a p p r o a c h e s 0 K , b u t t h e o c c u p a t i o n i n c r e a s e s w i t h t h e t e m p e r a t u r e (fig. 7c). I n i n s u l a t o r s (fig. 7d), t h e c o n d u c t i o n b a n d is n e a r l y e m p t y at all r e l e v a n t t e m p e r a t u r e s . T h e d i f f e r e n c e b e t w e e n s e m i c o n d u c t o r s a n d i n s u l a t o r s is t h a t Eg is l a r g e r in the latter, so t h a t t h e t h e r m a l p o p u l a t i o n o f the c o n d u c t i o n b a n d in i n s u l a t o r s is n e g l i g i b l e for n o r m a l l y a c c e s s i b l e e x p e r i m e n t a l t e m p e r a t u r e s . T h i s c a n b e

i (a) metal

(b) semi-metal

m (c) semiconductor

m (d) insulator

Fig. 7. Schematic diagram illustrating electron occupancy in energy bands of four materials: (a) metal, (b) semimetal, (c) semiconductor and (d) insulator.

Photochemistry at adsorbate / metal interfaces

101

explicitly illustrated by the Boltzmann population equation, Nc/N v = e x p ( - Eg/RT), where Nc and N v are populations of electrons at the bottom edge of the conduction band and the top edge of the valence band, respectively. The electron population in the conduction band, N~, increases with temperature but decreases with increasing Eg. The optical properties of metals, semiconductors and insulators reflect their different electronic band structures. For metals, the free electrons absorb incident photons by making transitions to higher energy bands. However, most polished metals are highly reflective in the IR; their reflectivities decrease as the incident wavelength decreases from the I R to UV. For semiconductors, the optical absorption coefficients are very small in the I R (incident light is reflected a n d / o r transmitted). With decreasing wavelength, strong absorption occurs abruptly when the photon energy matches the energy gap of a direct semiconductor. This strong absorption is due to band gap excitation from the valence to the conduction band. Semiconductors are essentially opaque to radiation of shorter wavelengths. Insulators are essentially transparent in the I R and visible regions, but strong absorption occurs in the UV when the band gap energy requirement is met. The absorption edges are usually slightly lower than the band gap energies, due to excitons, which are excited state levels of an electron-hole pair formed by absorption of energy. There is one other electronic state concept which must be mentioned, i.e., surface states [69]. In the above description of substrate band structure, we did not distinguish between bulk and surface band structures. Theoretical calculations show that electronic states on the surface differ from those occurring in the bulk. These surface states arise because the surface and bulk have different boundary conditions: semi=infinite versus infinite. Therefore, caution is needed in applying bulk band structures to surface problems. H o w these surface states are involved in surface photochemistry remains an open question. But we will incorporate this concept into the so-called adsorbate-substrate complex, which will be discussed later.

2.3.2. Elementary excitations As discussed previously, excitations in the substrate operate throughout the surface photochemical event, from initial excitation to final relaxation. Since they are involved in almost every field in surface science, ranging from adsorption and reaction to'all surface spectroscopies, we are certainly in no position here to discuss these in any detail. Instead, we will maintain our posture of introducing some of the basics related to our discussion on light-adsorbate-substrate interactions. These related elementary excitations include single-particle-like electronic excitation, collective electronic excitation (plasmons), and vibrational excitation (phonons). For a more comprehensive overview of the fundamental physics in the elementary excitation processes, interested readers are referred to several excellent books [39,69]. We view excitations of a solid substrate (surface and bulk) as a perturbation to the equilibrium structures, be they electronic or crystal structures; we consider these excitations to include the corresponding response in the solid. When external perturbations, e.g., electromagnetic fields (photon) or electron beams, are applied to a metal surface, the excitation is not localized. Precisely speaking, the initial and final states involve Avogadro's number of electrons and ions. However, a few simple assumptions can h e l p u s to get out of this complex m a n y - b o d y problem. Let us first look at electronic excitations. 2.3.2.1. Single-particle electronic excitation. When a photon is absorbed by a metal surface, an electron is excited from the occupied state to a previously unoccupied state above the Fermi level. The screening effect of the metal ensures that the excited electron and hole have little mutual interaction, and we can therefore avoid treating the two-body problem called excitons.

X.-L Zho~ X.-K Zhu andJ.M. White

102

Band structure

Photoelectron Ekinetic

--1064 -- 532 ...... 3 5 5

03

nm nm nm

h\ C

E Ferm,

7

I

intensity (D

:~

LJ

v=O i v=l

hv I •

,

I

i

,

i

0 P

DOS

Fig. 8. Schematics showing the relationship between nascent photoelectron distribution and the band structure.

.

,

/

~

ll

L

I

Energy

t"FI

t

J

2

""

,

,J

,

~

J

3

,

,:~

,

4

(eV)

Fig. 9. Calculated distribution of hot electrons generated by absorption of 1064, 532 and 355 nm radiation of Pt. The zero of energy is the Fermi level. Also indicated are the threshold energies for desorption of rotationally and translationally cold NO in the u = 0 and v = 1 vibrational states. After Buntin et al. [51].

It is well k n o w n in surface photoemission studies that, if the initial single electron state is spatially extended, as in the case of metals, the removal of an electron does not significantly alter the effective potential seen by the remaining electrons. Thus, we can treat the excitation process using the single electron picture described by eq. (9) and b a n d structures like those presented in fig. 6. W h e n the p h o t o n energy exceeds the surface work function, the final state lies in an energy continuum. As a first approximation, the transition rate is proportional to the n u m b e r of available states at a given energy. Therefore, the energy distribution of photoelectrons is simply an image of the initial filled states. This is shown schematically in fig. 8 [39]. Here we have ignored possible complexities such as: the strong spatial variations of the initial- and final-state wavefunctions, particularly near the surface; selection rules which can modify the actual strength of the photoexcitation matrix elements; inelastic scattering of the excited electrons in the escape process; and many-particle effects associated with the screening of the hole left behind. Interestingly, experimental photoemission studies show that, despite these complexities, in most cases the single particle picture remains an adequate description of the photoemission spectra. Experimental evidence showing the involvement of photoelectrons in surface photochemistry will be discussed in section 3 of this review. W h e n the p h o t o n energy is lower than the surface work function, the distribution of n a s c e n t (as formed) hot electrons can also be calculated using eq. (9). Here, the final states are those above the Fermi level in the b a n d structure (see fig. 6). Fig. 9 shows the nascent photogenerated hot electrons in the bulk at three p h o t o n energies [42b,51]. This calculation employs M o n t e Carlo sampling of the Brillouin zone and assumes a constant transition-matrix element. It is important to keep in mind that these calculated distributions are for n a s c e n t hot electrons in the bulk; for those hot electrons arriving at the surface, we need to consider other complexities,

Photochemistry at adsorbate / metal interfaces

103

such as surface band structures (surface states), surface fields (e.g., the dipole layer in fig. 4), and inelastic scattering processes. The last effect is well known in surface photoemission, where featureless secondary electron backgrounds are always present in photoemission spectra. Similarly, we can expect a background of secondary hot electrons arriving at the metal surface. Recent studies of photochemistry on metal surfaces have shown that hot electrons or holes play important roles in many systems; some results are summarized in section 3. In addition to the single-particle-like electronic excitation described above, electrons oscillating in the metal lattice or on the metal surface can be excited collectively and in quantized energy increments, called plasmons. While theoretical descriptions of plasmons are available in refs. [69], we will discuss these in an intuitive way [72]. To illustrate the origin of plasmons, we use the free-electron jellium model presented before. If, on a microscopic scale, we apply a disruption of the electrical balance of the free-electron gas in the solid, classical physics predicts oscillation of the electron gas. These oscillations propagate as longitudinal waves with a frequency %, and have energy quanta of hvp, characteristic of the bulk free-electron density. Within a free-electron gas picture, the frequency of a b u l l plasmon is given by [69]

2.3.2.2. Plasmons.

[

2

,

~1/2

6op = ~ne ~ t o m )

,

(42)

where o~ = 2~r%; n is the conduction electron density; e and m are the charge and mass of the electron; co = 107/4~rc 2, the permittivity of free space (c is the speed of light in vacuum). For most metals, the conduction electron density is in the range of 1022-1023 electrons/cm 3. Therefore, the plasmon energy of most metals is on the order of 1-10 eV. As in band-structure calculations, the presence of a semi-infinite boundary condition also results in differences between surface plasmons and b u l l plasmons. Surface plasmons are electromagnetic excitations of surface charges which propagate along the boundary of a solid whose electrons behave as a free-electron gas in two dimensions. The surface plasmon oscillation decays away rapidly and exponentially into the bulk free-electron gas. Both experimental and theoretical studies show that the surface plasmon frequency is usually 1/v~-vp [69]. The above description approximates jellium-like metal surfaces very well, but deviations can be expected for transition-metal surfaces [72]. The source of disruption, or more precisely, resonant excitation of the plasmons, can be energetic electrons or photons. One must be cautious when discussing the role of plasmons in surface photochemistry because a photon does not couple directly to a surface plasmon on a plane surface. The frequency v and the wavenumber k of an incident photon cannot simultaneously be matched to those of a surface plasmon. Therefore, direct photoexcitation of a surface plasmon on a plane surface is forbidden. However, conservation of both energy and momentum can be satisfied if the surface is rough or in the form of small spheres, because roughness or small spheres take up the excess momentum of surface plasmons. Another important fact is that bull plasmons can be excited only by p-polarized light incident on a plane surface. The electric field of s-polarized light is perpendicular to the plasmon electric field and is, thus, not effective. The energy absorbed by the surface in the creation of a plasmon must eventually be dissipated. There are several available dissipation pathways of potential significance in surface photochemistry: First, a plasmon can decay, through so-called Landau damping [39], into a single particle excitation, thereby becoming like the single energetic electrons described above. The resulting hot electrons or photoelectrons can then induce adsorbate excitation. Second, a

104

X.-L. Zhou, X.-Y. Zhu and J.M. White

plasmon can decay into heat, as phonons which couple to the adsorbate. Third, a surface plasmon can re-radiate light at the surface plasmon frequency [72]. The emitted electromagnetic field interferes with that of incident light to give a microscopic shape or size-dependent optical response, which is important for surface-enhanced R a m a n spectroscopy (SERS) on rough metal surfaces [73) and could be important in surface photochemistry. It is not known whether a surface plasmon can directly couple into a localized adsorbate electronic state.

2.3.2.3. Phonons. Similar to the plasmons discussed above, quantized energy increments called phonons can result from the collective motion, not of the electrons, but of the crystal lattice. In solid-state physics, lattice vibrations are usually represented by the elastic displacement of a crystal plane from its equilibrium position. This displacement vector can be decomposed into the sum of longitudinal and transverse components. In classical dynamics, the elastic energy is often represented as a quadratic function of the relative displacements. Cubic or higher-order terms are usually not important at low temperatures but can be at high temperatures. Solving the classical wave equations for the bulk infinite lattice gives three orthogonal wavevectors (q), one longitudinal and two transverse. The energy of a phonon is related to the frequency (vp) of its wave as hpp. Recall that the presence of a large number of electronic states in a solid results in band structures. Likewise, the presence of Avogadro's number of lattice units results in a distribution of phonons. Termed the phonon dispersion relation, it expresses the energy E of a phonon as a function of the wavevector magnitude q. Just as gaps m a y exist in an electronic band structure, E(k), the phonon dispersion function E(q) may be discontinuous. A phonon gap occurs when q lies on a Brillouin zone boundary. This is because lattice vibrations with wavelengths shorter than the interatomic spacing are not possible. As in band structures and plasmons, the presence of the semi-infinite surface boundary condition introduces surface vibrational modes (surface phonons) that are not allowed in the bulk sohd. These vibrational waves propagate along the surface with their wavevectors parallel to the surface; their amplitudes decay away exponentially into the bulk. Phonons can be excited by I R photons, low-energy electrons, and other energetic particles, such as neutrons and atoms. On the one hand, excited surface phonons can couple to the adsorbate and induce desorption. Experimental evidence showing the involvement of phonons in surface photochemistry has been found on non-metal surfaces. As presented earlier (and discussed in section 3), the IR-induced desorption of pyridine from KC1 has been attributed to the involvement of surface phonons [61]. On the other hand, excitation of phonons serves as the last step in relaxation processes. Excited electrons or holes can eventually lose their energy to phonons, within 10 -12 s [57]. Quenching of the excited adsorbate or adsorbate-substrate complex can also lead to surface phonon excitation. Details of the quenching process will be discussed later. 2.4. The adsorbate Since in surface photochemistry, we are interested in chemical changes in the adsorbate, particularly intra-adsorbate bond breaking, the adsorbate itself is obviously another key to work in this area. Since gas-phase and condensed-phase photochemistry and spectroscopy are relatively mature research fields and have been discussed in detail in many excellent books [20,74,75], we will only review some of the basics here. The large body of data and photochemical rules that have been developed serve as very important resources for choosing adsorbates and interpreting observations. In addition, the strong interaction between the adsorbate and the

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metal substrate may operate throughout a surface photochemical event. Therefore, in many cases, surface photochemistry essentially occurs in the so-called adsorbate-substrate complex. A review of the analogous organometallic photochemistry is then helpful in this regard. In the following, we will discuss briefly the gas/condensed phase photochemistry and organometallic photochemistry, respectively.

2.4.1. Gas and condensed phase photochemistry A photochemical reaction comprises a series of events from photon absorption to final product formation. This complicated process can be divided into two separable processes primary and secondary. As defined by Noyes and Leighton [74a], the primary process includes the initial act of photon absorption and the processes that follow before collisions intervene. The secondary process consists of the collision-related reactions of the primary products. Here, we will focus on the primary process. Chemistry that occurs as a result of single photon absorption within adsorbate molecules generally requires electronic excitation in order to meet the energy requirement for bond breaking. Of course, under extremely intense laser irradiation, multi-photon (several tens) vibrational excitation can also induce chemical changes, but this is beyond the scope of our discussion. The excitation of electronic states in simple molecules usually requires UV (200-400 nm) or vacuum UV ( < 200 nm) light; exceptions include the very interesting cases of 03 and NO 2, where visible light is effective. Clearly, one guide to candidate molecules for surface photochemistry studies is their gas-phase photochemistry which, in many cases, is known. Another important result of gas and condensed phase photochemistry is the common appearance of numerous competing relaxation pathways that follow on the heels of photon absorption; bond dissociation does not always occur, even when photons with sufficiently high energy are used; fluorescence, phosphorescence, non-radiative quenching, and combinations of these with bond breaking or isomerization are all well-documented. Furthermore, as implied by the properties of eq. (9), there are important symmetry requirements on the initial and final states involved in the transition, the so-called "selection rules" [20,74,75]. Depending on the nature of the interactions with the surface, many of these symmetry restrictions will change when the molecule is adsorbed. Another important general result is that electronic excitation is usually accompanied by vibrational and rotational excitations and a change in the equilibrium molecular geometry. Electronic excitations are governed by the F r a n c k - C o n d o n principle, which says that, since the frequency of light promoting an electronic transition (1015-1016 Hz) is much higher than that of molecular vibrations (10~2-1013 Hz), there is practically no change in either the molecular geometry or nuclear motions during photon absorption. This is illustrated in fig. 10 for diatomic molecules. According to the F r a n c k - C o n d o n principle, only vertical transitions contribute. In terms of spectroscopy, if the excited state is repulsive (fig. 10a), the absorption spectrum is continuous. If the excited state is bound (figs. 10b and 10c), vibrational, as well as rotational, excitation should accompany electronic excitation, as long as the excited state equilibrium internuclear distance differs from that of the ground state. In the particular case shown in fig. 10b, the u °-2 transition has the highest probability. These vibrational, as well as rotational (not shown), excitations are responsible for the fine structure on the optical absorption curve. The fates of electronically excited species are summarized in fig. 11. Only two of these, dissociation and photon emission, are discussed in this section. We will only examine simple gas-phase molecules at the low-pressure limit so that collisional energy transfer after excitation need not be considered. The excited states can be divided into three categories: (i) repulsive

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#/

V .

_

r (A-B) (a)

(b)

(c)

Fig. 10. Schematics showing potential energy curves for a diatomic molecule (AB): (a) excited repulsive state; (b) excited bound state with the excitation energy lower than the bond dissociation energy; and (c) excited bond state with the excitation energy higher than the bond dissociation energy. This excited state may cross over to a repulsive state (dashed curve).

state; (ii) b o u n d state with the excitation energy less t h a n the b o n d d i s s o c i a t i o n energy; a n d (iii) b o u n d state with the excitation energy exceeding b o n d dissociation energy. T h e first case is dissociative (fig. 10a), a n d molecules in this class c o n t i n u e to be of great interest in surface p h o t o c h e m i s t r y . In an o r b i t a l picture, these excitations generally involve p o p u l a t i n g , with an electron from a b o n d i n g or n o n - b 0 n d i n g orbital, an o r b i t a l that is a n t i b o n d i n g with respect to some b o n d . T h e excited state lifetime, or b o n d r u p t u r e time, is < 10 -13 s [17,76,77]. Since this p o t e n t i a l energy curve has no m i n i m u m , the a b s o r p t i o n s p e c t r u m is c o n t i n u o u s a n d dissociation always follows a b s o r p t i o n . F o r example, the p h o t o d i s sociation of the p s e u d o - d i a t o m i c molecule, CH3Br, occurs at p h o t o n energies higher t h a n 4 eV, a n d the cross section increases m o n o t o n i c a l l y with p h o t o n energy before reaching a m a x i m u m [20]. Since the lifetime of the excited state is c o m p a r a b l e to that of q u e n c h i n g b y the substrate,

A + B (dissociation) /AB //..../~ AB* ~..._ ~'~~AB " " ~

++ e (ionization) BA (isomerization) + hv (luminescence)

~ Intermolecularenergytransferor reaction Intramolecularenergytransfer

Fig. 11. Schematics illustrating the fates of an excited gas-phase molecule.

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photodissociation of CH3Br on metal surfaces is competitive with quenching and is indeed observed [5,78,79]. The second case is non-dissociative (fig. 10b). The fate of the excited state often involves transitions to lower levels with emission of light (fluorescence). Although the lifetime of this kind of excited state is long ( > 10 -8 s), when such an excited state is near a metal surface, non-radiative quenching usually dominates [80]. If quenching occurs on a 10-13 s time scale, direct dissociation of such states, because of coupling to the substrate, is not expected. The only way for chemistry to occur would be through vibrationally or rotationally excited ground states formed by quenching. The third case (fig. 10c) is more complicated than (i) and (ii). The excited state can either decay radiatively (fluorescence) or cross over to a repulsive curve (dashed curve in fig. 10c) and dissociate, in a process termed predissociation. Other processes are possible as well, particularly conversion, in which the electronically excited state converts into a vibrationally excited ground state, and intersystem crossing, in which an excited singlet state crosses over to a long-lived triplet state. Bound to a metal surface, the properties of these molecule-based states will be perturbed to a greater or lesser extent, depending upon the strength and nature of the bonding. Roughly speaking, the metal orbitals in the valence and conduction bands (and there are typically a very large number per unit energy interval) will mix with the molecule-based orbitals. Thus, in very narrow energy intervals there are very many possible changes from one state to another. The questions are - What is the probability of this happening? How will it affect the chemical outcome? Whenever there are strong interactions, perturbations of the adsorbate electronic structure must be considered. In the ground state, the adsorbate-substrate complex may be so weakly coupled that adsorbate orbitals differ very little from those of the isolated molecule. But for electronically excited states, we must be more careful. As discussed below, this brings the adsorbate-substrate complex into the picture and, by analogy, the photochemistry of organometallic complexes. In the above description of gas-phase molecules, the pressure is low, and we assume that there is no perturbation by neighboring molecules in either light absorption or the subsequent decay of excited states. At higher pressures, or in condensed phases, inter-molecular energy transfer for the excited states becomes important. In addition, the fine features in a molecular absorption spectrum are usually broadened at high pressures. The latter is a result of perturbation by the fields of neighboring molecules at the instant of light absorption. This effect is more pronounced in the liquid phase, where every optically active molecule is always embedded in the potential field of its neighbors. If the inter-molecular or solute/solvent interactions in a liquid phase are altered through electronic excitation, the gas-phase wavelength response of the molecule will be shifted. Depending on the nature of the electronic transition (e.g., n - o * , o - o *, n-~r*, ~r-~r* ) and that of inter-molecular interactions, both blueand red-shifts are possible. For example, in n - o * type transitions in polar media, some blue-shift usually occurs; it arises from the alteration in the strength of dipole-dipole interactions during electronic transition. For a molecule in the solid phase, perturbation of the gas-phase wavelength response by inter-molecular potentials could be stronger than in the liquid phase. Strong crystal field effects can be expected for a molecule with polarizability and permanent dipole moment. These perturbations usually lead to a blue-shift in the absorption continua and a suppression of various selection rules. For example, solid OCS shows two absorption continua at 4.3 and 4.7 eV, not observed in the gas phase, and assigned to the 3X + and 3A excited states, respectively. There is also a 0.29 eV blue-shift from the gas phase of the 1A state [81]. Clearly, these condensed phase concepts must carry over to the photochemistry at the

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adsorbate/substrate interface (the surface is part of a condensed phase). Throughout the crucial part of the temporal evolution of a dissociated bond, there are continuous interactions with the substrate, and, except for very low coverages, with neighboring adsorbates.

2.4. 2. A dsorbate-substrate complex / analogies with organometallics As noted above, strong interactions between the adsorbate and the substrate, excited or ground state, will always modify the properties of the two separate entities, particularly the optical properties. Intuitively, we expect this to be especially important for adsorbates in the first monolayer, where adsorbate-substrate chemical bonds are formed in most cases. For molecules adsorbed weakly without rehybridization of the molecular orbitals, the alteration of the ground-state properties will be minimal. This is the case for alkyl halides, which adsorb weakly on metal surfaces through the lone-pair electrons of halogen atoms with little distortion of their molecular structures from the gas phase [82]. For strongly adsorbed molecules a n d / o r those involving rehybridization of the molecular orbitals, the properties may be significantly different than in the gas phase. For example, molecularly adsorbed 02 at - 100 K on some metal surfaces is essentially in the form of peroxo and superoxo species [83a,84b,85a] whose optical responses will be quite different from 02 . Another example is carbon monoxide. When CO chemisorbs on metal surfaces, its electronic structure is changed significantly. There are two important orbital interactions: (1) charge donation from the 50 orbital (HOMO) of CO to the metal and (2) charge backdonation from the metal to the antibonding 2•* orbital (LUMO) of CO. The electronic structure of chemisorbed CO has been studied by EELS, photoemission, inverse photoemission and theoretical calculations [46]. In the gas phase, the energies of the 40, l~r and 50 orbitals are spaced apart by about 3 eV. However, for chemisorbed CO on metals, the 5o is lowered in energy by - 3 eV and becomes nearly degenerate with l~r [46]. The 2"~* interacts with metal d- and sp-orbitals to form a partially filled metal-2-~ * bonding orbital and an unoccupied metal-2~r * antibonding orbital [46]. The bonding interaction between CO and metal surfaces is very similar to that in metal carbonyl complexes [86,87]. From a molecular point of view, organometallic species are the closest analogy one can make for the adsorbate-substrate complex. Significant effort in the surface science community has been devoted to exploring the similarities between molecules adsorbed on metal surfaces and organometallics [86,88]. In many cases, but certainly not all, the bonding modes of molecules to metal surfaces are readily related to those in organometallics [86,88]. Moreover, the thermal chemistry of organometallic complexes sometimes exhibits the adsorption-desorption and dissociation characteristics found in molecule-metal systems. For example, heating sometimes results in thermal decomposition to metal atoms and free ligand molecules, such as certain carbonyls [89]. Photon irradiation of organometallic complexes often dissociates metal-ligand and intra-ligand bonds; this is analogous to photodesorption and dissociation of molecules adsorbed on surfaces. The photochemistry of organometallics has been relatively well studied [87]. The types of one-electron excitations in organometallic complexes include: (1) ligand field (metal-centered), (2) intra-ligand (ligand-centered), (3) ligand-to-metal charge transfer, (4) metal-to-ligand charge transfer, (5) metal-to-solvent charge transfer and (6) metal-metal bond excitation (metalcentered where two or more metal atoms are bonded together in the organometallic). Excitations in (1), (4) and (6) are metal-centered and can be related to substrate excitations in surface photochemistry at adsorbate/metal interfaces. However, there are distinct differences: in organometallics, the metal-centered orbitals are discrete, while in adsorbate-substrate systems they are typically part of a continuum. Like intra-ligand excitation in organometallics, excitation of an adsorbate-substrate corn-

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plex can take place within the adsorbate (intra-adsorbate excitation). A priori, adsorption of a molecule on a metal will result in perturbation of its electronic structure, and comparison with the excited states of organometallics may be useful. If one can account for surface screening, which will cause further broadening and shifting, the correspondence in m a n y cases is quite good. To place this comparison on a sounder footing, electronic spectra of adsorbate-substrate systems are needed, but are available in only a few cases. For pyridine on Ni(100), no electronic excitation was detected when the adsorbate plane was parallel to the surface because of strong bonding interactions, between Ni d-orbitals and pyridine ,n or ~ * orbitals, which severely broaden the ~r ~ ~r* excitation. In a tilted adsorption geometry, however, three ~r ---,~r* bands (5.2, 6.3 and 7.3 eV) appear. In this geometry, the d-~r interaction is weak or absent, and the surface image screening effect alone does not significantly perturb the transitions [90]. Similar observations were made for benzene and pyridine on I r ( l l l ) [91]. Pyridine, with perpendicular or tilted bonding on I r ( l l l ) , showed a substantial ~r---,~r* transition; for benzene, bonding flat to I r ( l l l ) , no ~ r ~ r * transitions occur. For benzene, pyridine, and pyrazine adsorbed on A g ( l l l ) , only small energy shifts and broadening (0.1-0.2 eV) relative to the corresponding gas-phase ~---, ~r* transitions were seen, and there was no significant difference between flat and tilted pyridine [92]. This is ascribable to these molecules' weaker interactions with A g ( l l l ) than with Ni(100) and I r ( l l l ) . The 50 ~ 2~r* transitions for chemisorbed CO on metal surfaces, though broadened, are essentially unshifted from their free CO positions since the shifts of both 50 and 2~r * orbital energy levels are nearly the same, - 3 eV [46]. Clearly, further such investigations will yield a much deeper understanding of surface photochemistry. Analogous to organometallics, charge-transfer (CT) excitation can also take place in adsorbate-substrate systems. In CT excitations, the initial orbital is usually substrate-dominated and the final one is adsorbate-dominated. The final orbital usually lies between the Fermi and vacuum levels. CT excitation differs from attachment excitation by hot electrons, though both processes end up with a negatively charged transient adsorbate ion and subsequently lead to decomposition or quenching. Photon absorption in the former is a localized process, i.e., it occurs within the adsorbate-substrate complex and has its own transition dynamic dipole, while in the latter it is a delocalized process, independent of the adsorbates. Thus, the two should be distinguishable on the basis of polarization experiments. Metal-to-adsorbate CT has been reported in a few systems. For benzene and pyridine on I r ( l l l ) [91], a transition was observed at 4 - 5 eV which is absent for the free molecules and was assigned to d ~ ~r* CT excitations. For pyridine and pyrazine on A g ( l l l ) , Avouris and Demuth [92] found a CT excitation at 2-2.5 eV. They also found that CT transitions are present only for molecules in direct contact with the surface. This indicates that electronic interactions between metal and adsorbate are necessary for a CT excitation channel. For CO on N i ( l l l ) , a sp~ ~ r * CT excitation at 5 - 7 eV has been reported [46]. 2.5. Dynamics

In our previous discussions of the light, the substrate, and the adsorbate, our focus has been on the excitation processes. One immediate question following this is: what happens to the excited state? Here, the focus is not just on chemical changes, but also on the time- and energy-evolution of the system. The answer to this question lies at the heart of dynamics, the time evolution of changes. While gas-phase dynamics has been studied for m a n y years, molecule-surface dynamics is a much newer area, coming into its own only in the last decade. The goal of such studies is to understand, at the atomic and molecular level, just how and how

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fast the energy introduced (in our case by photon absorption) is partitioned into the various available modes of the products. However, due to a lack of experimental means of directly following the time evolution of changes on the surface, present surface dynamic studies still rely predominantly on information obtained in the gas phase, i.e., mode-selectively detecting the desorption product in the gas phase by time-of-flight mass spectroscopy (TOF-MS) for velocity and angular distributions, and either laser-induced fluorescence (LIF) or resonance-enhanced multi-photon ionization (REMPI) for internal energy distributions. In the following, we will discuss the last key factor in surface photochemistry: the dynamics associated with desorption products. We will start with the information one can obtain in a dynamics experiment, and then discuss the underlying potential energy surfaces (PES) and their consequences that enters interpretation, including (i) M e n z e l - G o m e r - R e d h e a d , and Antoniewicz models, (ii) theoretical calculations, and (iii) lifetime and quenching. 2.5.1. Experimental information - TOF, L I F and R E M P I

We now turn to a brief overview of experimental methods used for dynamical studies. To provide time resolution, photodynamic studies typically use a pulsed laser as the light source. Typical detection modes are time-of-flight mass spectrometry (TOF-MS) for velocity and angular distributions, and either LIF or REMPI for internal energy distributions. A brief discussion of these techniques has been given in section 2.1.2. TOF-MS will provide only information concerning translational energy or velocity distribution of desorbing molecules and fragments, averaged over the internal states that are populated. If LIF or REMPI is used in a TOF detection mode, the translational energy distribution of a single internal state can be obtained. Of the recent reviews on this subject [4,93-101], those by King and Cavanagh [4] and Zacharias [101] are especially interesting, since they pay particular attention to the dynamics of thermal and non-thermal photon-induced desorption of N O adsorbed on sohd surfaces. Zacharias [101] also reviewed dynamics of photofragmentation of NO 2 that was adsorbed on N O / P d ( l l l ) . Because its internal energy content is relatively easy to detect and analyze, N O is a favorite with experimentalists. Here, we focus our discussion particularly on photodynamics (dynamics of non-thermal photodesorption and photofragmentation processes) at a d s o r b a t e / substrate interfaces. In addition to information on the translational and internal (vibrational, rotational and spin-orbit) energy and angular distributions of the desorbing photofragments (or parent molecules), such studies provide insight into how the photofragments form, what the surface adsorbate-substrate energy surfaces are like, and what surface interactions and orientations are important. Molecules adsorbed on surfaces are aligned and, compared to the gas phase, their orbitals are usually perturbed, especially in the chemisorbed layer. Compared to the gas phase, surfaces also provide additional pathways for acquiring and dissipating excitation in the admolecules. Therefore, we expect the dynamics of photodissociation and photodesorption of admolecules to be very different from their gas-phase counterparts. For gas-phase molecules in dynamic equilibrium with each other and the surroundings, the translational and internal temperatures are uniform, and the molecules have a Maxwellian velocity and Boltzmann internal state population distribution. Departures are expected in non-thermal photochemical processes in either the gas or adsorbed phase. These departures are of great interest in dynamical studies because they provide insight into state-specific excitations and the temporal evolution thereof. When a photon (or other energy) is deposited non-randomly into an adsorbate-substrate system, a non-equilibrium situation is created, at least transiently, in which the substrate, the adsorbate or the adsorbate-substrate complex is excited. With time, the deposited energy may dissipate completely into the substrate by interaction of

Photochemistry at adsorbate / metal interfaces

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the excited state with electron-hole or phonon acceptor channels. The net result of this kind of process is substrate heating with no direct chemical effects. However, as discussed earlier, even on metals, chemical transformations can compete strongly with relaxation channels. In such cases, either an adsorbate-substrate bond or intra-adsorbate bond will dissociate under the non-equilibrium situation. In m a n y cases, only a fraction of these are retained at the surface; the remainder desorb, typically with more energy than the surface temperature would predict. In addition, the translational and internal state temperatures will be different [51,62,103d], since the flow of energy among different degrees of freedom occurs on different time-scales. Because energy exchange between the admolecules and the substrate occurs while the molecules are within range of the surface potential, non-equilibrium product distributions are expected and, in fact, are found. If the desorption rate is too large, then near the surface the local number density of molecules can be high enough to destroy the nascent energy distribution by gas-phase collisions. This should be avoided by lowering the incident photon fluence per pulse. Once the molecules or the fragments are far from the surface, only intra-molecular energy exchange will occur because, under the U H V conditions used in these a d m o l e c u l e - s u r f a c e photon experiments, essentially no gas-phase collisions take place. 2.5.2. Interpretation - potential energy surfaces (PES) While the aforementioned experiments readily provide information on the translational and internal state distribution of desorbing products in some light-adsorbate-substrate systems, the extraction of dynamic information from these experimental results is more difficult. Our present knowledge about potential energy surfaces involved in ground, and particularly, excited adsorbate-surface interactions, is very limited. Although detailed quantum-mechanical treatment is not possible at present, simplified models have been proposed to account for desorption induced by electronic transitions (DIET). In the following, we will present these models and discuss their consequences. Other theoretical treatments, such as those of Tully and co-workers [104] and Zare and co-workers [105], on the dynamics of molecule-surface scattering and laser-induced thermal desorption will not be discussed here. Interested readers are referred to refs. [104,105]. 2.5.2.1. M G R and Antoniewicz models. One widely accepted model of D I E T is that proposed in 1964 by Menzel, G o m e r [106] and Redhead [107] (fig. 12). This model is analogous to direct photodissociation of a gas-phase diatomic molecule (fig. 10a). In fig. 12, the initial excitation is a F r a n c k - C o n d o n type electronic transition to a repulsive state. Following excitation, the adsorbate undergoes nuclear motion on the excited state potential energy surface. The presence of the metal surface opens up efficient decay channels that are not available in the gas phase. Quenching of the excited state brings the adsorbate back to the ground-state PES. If the kinetic energy (E~) gained by the adsorbate on the excited state PES is sufficient to overcome the barrier for desorption, desorbing molecules with kinetic energy of E k = E k - D ' will be detected in the gas phase by TOF-MS; if E k < D ' , the adsorbate will eventually lose its kinetic as well as vibrational energy to the substrate phonon bath and be captured. Due to the fast quenching rate on a metal surface, the probability of direct dissociation on the excited state potential energy surface is usually assumed to be relatively low. The probability of desorption is then proportional to a product: the initial excitation probability times the probability of survival on the excited state PES to gain the required kinetic energy ( > D ' ) . It must be mentioned that the M G R model assumes, in the quenching process, that all the electronic energy is converted to substrate excitation, which has no effect on the adsorbate. It is a generalized, qualitative picture which provides us with no details on the origin of the initial

X.-L. Zhou, X.-Y. Zhu and J.M. White

112

M+A" LU i'Ek

20

(D tIll

...M+ A

z

Distance from surface Fig. 12. Schematics showing the M G R model for de-

sorption induced by electronic transition (DIET). The adsorbate is initially in the ground state with a binding energy of Do. Excitation brings the adsorbate to a repulsive state with a finite lifetime. De-excitation can result in both quenching and desorption, depending on the relative magnitude of E~ and D'.

Im

At- A 2 distance Fig. 13. Schematics (harmonic potential energy curves) showing the excitation of internal vibrational modes during the same time sequence of excitation and de-excitation as in the MGR model (fig. 12).

excitation, the nature of the ground or excited state potential energy surfaces, or the nature of quenching processes. Any theoretical calculation will require detailed information on these points (see below). If detailed information on the potential energy surfaces and the excitation and quenching processes were available, the M G R model in fig. 12 could be used to predict the kinetic energy distribution of photodesorption products. How about the internal energy of desorption products? While this is not immediately obvious in the M G R model, it can be qualitatively understood. Suppose the adsorbate in fig. 12 is a diatomic molecule (A1-A2) and the X-axis corresponds to the distance of the center-of-mass to the surface. The internal motion of A ( A 1 - A 2 stretch) can be described by harmonic potentials in fig. 13. The excitation and quenching processes corresponding to those in fig. 12 will result in a vibrationally excited ground state. If, in fig. 12, E k > D ' , A 1 - A 2 will desorb with the vibrational energy gained in fig. 13. In addition, the diatomic molecule may experience a hindered-to-free rotor type transition during the desorption process. Thus, a translationally, vibrationally, and rotationally excited desorbing molecule can be detected in the gas phase. We can also extend the M G R model to describe intra-adsorbate bond cleavage on metal surfaces. In fig. 14, an absorbed molecule AB is excited via a F r a n c k - C o n d o n transition to a repulsive ( A - B bond) PES. Quenching brings the excited adsorbate back to the ground PES. If the amount of kinetic energy gained on the excited PES is sufficient, cross-over to dissociative chemisorption well may occur. In addition, if the lifetime on the excited PES is long enough, a direct cross-over to dissociative chemisorption may also contribute. Another very successful model for photon- and electron-induced desorption was proposed in 1980 by Antoniewicz [108]. In contrast to the M G R model, the Antoniewicz model predicts desorption from bound excited states; the process begins with the desorbing adsorbate moving toward the substrate. This model (shown in fig. 15 for neutral desorption) is particularly useful in describing surface photochemical systems where desorption is a result of metal-to-adsorbate charge transfer (CT) excitation. It can also account for ion desorption, which will not be discussed here.

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,../M

+ A-

orM+A +

.....•._£..................

LLI

~~

A+B+M M+A

A--B/M

A - M and/or B-M

Reaction Coordinate Fig. 14. Schematic potential energy curves for photoinduced intra-adsorbate bond cleavage. This is similar to the M G R model in fig. 12. Here, the potential energy curves describing the adsorbate-surface interaction (left) are coupled to those of chernisorption for photodissociation products.

Distance fromsurface Fig. 15. Schematics showing the Antortiewicz model for DIET. Different from the MGR model in fig. 12, the excited state in this model is bound. Excitation and de-excitation in this model involve the formation and neutralization of an ionic adsorbate (positive or negative).

The mechanism depicted in fig. 15 consists of two steps: the initial excitation and formation of an adsorbate ion (e.g., a negative ion from electron attachment or a positive ion from ionization), and the subsequent quenching and desorption of the neutral particle. The initial F r a n c k - C o n d o n transition creates an ion on the excited PES. This ion experiences An attractive image potential .and starts to move toward the surface. After a characteristic period of time, the charge-residence time or lifetime, the excited ionic adsorbate is neutralized via electron tunneling to or from the metal surface. This is again a F r a n c k - C o n d o n transition, which preserves the kinetic energy gained on the upper PES, and returns the system to the ground-state PES. Once neutralized, the adsorbate immediately finds itself on the repulsive wall of the ground PES. As in the M G R model, if the preserved kinetic energy plus the potential energy on the ground PES at the position of neutralization is greater than the binding energy, the adsorbate is repelled with sufficient energy to desorb. Like the M G R model, the Antoniewicz model is qualitative, and any theoretical calculation will require further detailed information on the potential energy surfaces, as well as details on excitation, neutralization, and exit channels. This is the task of our discussion in the next section. We set aside the very interesting topic of chemistry driven by core level excitations [109]. 2.5.2.2. Theoretical calculations. Nuclear dynamics calculations of desorption induced by electronic transitions includes the popular classical trajectory method and the semiclassical wavepacket approach. These calculations usually employ empirical potential energy surfaces for the nuclear motion. More fundamental potential energy surfaces can be obtained from electronic structure calculations, such as the local-density-functional method. As far as the results on dynamics are concerned, our purpose here is to intuitively illustrate the interpretive aspects of these theories. Details can be found in refs. [104,105,110-113]. The classical trajectory calculation is popular because of its simplicity and applicability in m a n y large systems, such as the surface problem in our discussion [104,105], where a

X . - L Zhou, X.-Y. Zhu andJ.M. White

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quantum-mechanical treatment is not feasible. To illustrate these calculations in surface photochemistry, we consider the M G R model in fig. 12. Since ab initio potential energy surfaces of high quality are not presently available, various empirical potential energy surfaces are usually chosen. For example, the ground-state potential energy surface can be modeled as a sum of the weak van der Waals type interaction and the chemical interaction between the adsorbate and a chosen ensemble of surface atoms (in the form of a modified Morse potential with an empirical repulsive contribution). The repulsive potential is also usually empirically chosen (for example, that of Stechel et al. [19]). Once these empirical PES surfaces are chosen, the motion of the particle on them is given by Newton's classical equations. For example, in a simplified one-dimensional picture (fig. 12), the velocity of the particle at the time of quenching is given by

v(z)-- (2[V*(z)-

V * ( z o ) ] / M } 1/2,

(43)

where V* is the empirical excited state potential function and M is the reduced mass of the A - M system. If the adsorbate in fig. 12 is a diatomic molecule, the vibrational motion can also be described by the classical equations of motion using either the harmonic potential, as shown in fig. 13, or other Morse type potentials. In this kind of classical treatment, the rotational motion of the diatomic molecule is usually modeled by assuming a hindered rotor on the ground PES and a free rotor on the excited state PES. Solving classical equations of motion then gives the translational, rotational, and vibrational energy distribution for the desorbing product. A recent calculation of this kind, which ignores the vibrational motion for the sake of simplicity, agrees well with experiments, showing a strong correlation between translational and rotational energy distributions [113]. In the semiclassical wavepacket dynamics approach, the particles used in the classical trajectory calculation are replaced by Gaussian type wavefunctions, i.e., wavepackets, but the equations of motion remain the same, i.e., classical Hamiltonian. This approach is exemplified in Gadzuk's recent study modeling the laser-excited hot electron-induced desorption of N O on Pt(111) [111]. This is shown, within the framework of the Antoniewicz model, in fig. 16. In this picture, the ground-state NO-Pt(111) interaction potential was simply chosen to be the Morse potential, of the form

Va(z ) = D [1 - exp(fl, Z)] 2,

(44)

with ~o = (2fl~D/M) 1/2, the harmonic frequency at the equilibrium position; M is the reduced mass. The ground-state wavefunction is that of the Gaussian vibrational oscillator on V,(z):

COo( z ) = ( 2 M w / h )'/4 exp [ - ( ~rM~o/h ) z 2].

(45)

Excitation of this ground state involves trapping a photoexcited electron in the 2-~* orbital of NO. The transient formation of this negative ion state initiates an additional image-type attractive potential, so that the excited state potential can be simplified by

V_(z) = c* + Va(Z ) -- eZ/4(z + Zeq).

(46)

The last term is simply the attractive electrostatic potential between N O - and the image charge in the metal; Zeq is the distance between the effective image plane and the equilibrium position of N O - and z is the instantaneous location of N O - with respect to the equilibrium position. Since this transition is Franck-Condon-like, the ground-state wavepacket is undistorted. Once on the excited PES, the wavepacket immediately finds itself in a non-stationary state. It starts to spread and move toward the surface from time t = 0. The motion of the wavepacket is

Photochemistry at adsorbate/ metal interfaces

115

NO-

v..(z )

t~O

':'i{ i '--° °

i

(a)

/

Ve(z )

NO\ ~int(TR) ~ O~NO

~

(b)

Fig. 16. Potential energy curves of center-of-mass translational motion of chemisorbed and negative ion NO with respect to the surface, showing wavepacketpropagation throughout the time sequence involving the negative ion 2rr* resonance. The distribution of final NO states, between vibrationally excited bound and desorptive continuum states is shown as P(¢) versus c. (b) Intra-molecular potential energy curves for NO and NO- and wavepacket propagation throughout the same time sequence as in (a), illustrating the mechanism for internal vibrational excitation in desorbed NO. After Gadzuk et at. [111]. governed by the classical Hamiltonian with the potential function of eq. (46). After a time interval of ~'R, the trapped electron on N O - is transferred back to the metal via resonance tunneling. This results in a second F r a n c k - C o n d o n transition to Va(Z ) and the wavepacket moves away from the surface, again governed by a classical Hamiltonian. As a result, the molecule desorbs with the final-state distribution, P(c), shown in fig. 16a. Meanwhile, the internal vibration of NO is represented by the motion of the intra-molecular wavepacket (the ground-state vibrational wavefunction of NO) on the ground and excited harmonic potential energy surfaces shown in fig. 16b. In this particular calculation, the rotational motion is ignored. When needed, it can be modelled using the hindered-to-free rotor transition and wavepacket dynamics, such as that of Stechel et al. [19]. The above approaches employ empirical potential energy surfaces. More realistic potential energy surfaces can be found in calculations by Avouris et al. [10,19]. They employed local-density functional calculations for D I E T on jellium-type surfaces, such as the desorption of F ÷ from A1 surface via valence ionization. In this calculation, the repulsive nature of the excited state in the M G R model was demonstrated theoretically. We will not go into this kind of calculation in detail, but note that, although the local-density functional calculation provides more realistic potential surfaces, its extension to non-jellium surfaces is difficult and, because the method is inherently time-independent, it may not give good results for electron dynamics.

2.5,3. Lifetime and quenching In the above description of desorption dynamics, we did not discuss in any detail either the lifetime of the excited state (~'R) or the nature of the quenching process. These two factors are integral parts of the dynamics and are of central importance to the final result of photoexcitation. Below we go into these in some detail and see how they govern the photochemical event. The excited state lifetime is an important quantity since, to a large extent, it determines whether photochemistry is possible for a molecule adsorbed on a metal surface. For gas-phase photochemistry, this information is relatively easy to obtain and has been investigated for many

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years. For adsorbate-substrate systems, however, comparable information is very limited because surface photochemistry is much more complicated and has been less thoroughly studied. Available experimental methods include measurements of absorption and scattering (both optical and electronic) lineshapes, luminescence quantum yields, and direct determinations of excited state decay rates using luminescence, absorption or scattering techniques. Measured lineshapes can yield reliable lifetimes only if effects such as site inhomogeneity, coupling with electron-hole pairs and phonons, dipole-dipole interactions, and the dephasing time of the adsorbate-substrate system are negligible. Since ultrashort laser pulses are now available with durations as short as 6 fs, direct measurement of excitation lifetimes may be possible in the near future [141]. Theoretical calculations, ranging from classical dipole coupling [117] to the recent calculation combining an accurate density-functional potential with a complex scaling theory [43], sometimes yield information comparable with experiments. Certainly theory must play a central role in the development of this field and there are splendid opportunities. Regardless of the excitation pathway, an excited molecule on a metal surface will have a limited lifetime and will dispose of its initial excitation energy in various ways. The energy relaxation mechanisms include: (1) radiative decay (fluorescence or phosphorescence), (2) non-radiative decay through energy or charge transfer to the metal substrate, and (3) chemical transformation. Clearly, for photodissociation to be detectable on a metal surface, (3) must compete strongly with (1) and (2). The probability of chemical transformation depends on the excited state lifetime and the time required for the nuclear motion needed to break a bond (adsorbate-substrate or intra-adsorbate). In this section, we discuss some fundamental aspects of these decay channels, particularly those that quench the chemistry of interest. As discussed below, the lifetime of electronic excitation for adsorbates in the first layer on metal surfaces lies between 10 -15 and 1 0 - 1 3 S (1-100 fs). Since resonant energy and charge transfer occurs on a shorter time-scale than non-resonant transfers, the shorter lifetimes, toward 10 -~5 s, are expected for resonant processes. Fluorescence or phosphorescence of an excited (vibrationally or electronically) molecule is strongly quenched near a metal surface [114]. For a molecule separated from the surface by distances of the order of the molecular emission wavelength, the excitation lifetime oscillates as a function of distance from the surface [115,116]. This phenomenon is explained by a simple interference model, in which the radiating molecule interacts with its own radiation field, part of which is reflected by the metal surface [116]. The radiative rate (inversely related to the excitation lifetime) increases when the reflected field is in phase with the directly emitted field of the molecule and decreases when the reflected field is out of phase. For molecules separated from the surface by distances much smaller than the emission wavelength, both theoretical [117] and experimental [114,118] results indicate that the radiative quenching probability increases rapidly as the distance to the metal surface decreases. This is attributed to efficient non-radiative energy transfer, mediated by electromagnetic field coupling to the metal. In the classical theory of Chance, Prock and Silbey (CPS) [117], the molecular excited state, located at a distance, d, above the surface, is treated as an emitting point dipole oscillating at the frequency of its emission, while the solid is modeled as a continuous medium of frequency-dependent dielectric constant c(o~). All of the interfaces are assumed to be infinitely sharp and planar, and the oscillating dipole is assumed to be located in a lossless medium. The dipole field interacts through space with the metal and excites plasmons, electron-hole pairs and phonons, thus losing energy and quenching its excitation. If the energy transfer is via resonant excitation of surface plasmons, the CPS model predicts a radiative lifetime that is exponentially dependent on the distance from the surface. If, on the other hand, quenching occurs via

Photochemistry at adsorbate / metal interfaces

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non-resonant excitation of electron-hole pairs in the metal, the model predicts a d 3 o r d 4 distance dependence, depending on whether volume or surface contributions of the metal dominate. A model calculation of an emitting dipole near a surface, characterized by small and random roughness, showed that the non-radiative energy transfer to the solid was enhanced by surface roughness and was strongly influenced by surface plasmons [119]. Experimental results [120] showed that surface plasmon excitation is definitely significant but only when the distance is large and the emission frequency is in resonance with the surface plasmon. When the emission frequency is non-resonant or the distance is small, < 100 A, other energy transfer channels dominate. In most cases, electronic energy transfer via phonon excitation is ineffective, since the emission frequency is so much higher than the maximum phonon frequency of the solid and since multi-phonon processes have low probabilities [121]. By way of comparison, low-frequency vibrational excitation can decay via substrate phonon excitation [122,123]. In order to distinguish between volume and surface contributions to the energy transfer, Persson and co-workers [124-127], by considering m o m e n t u m conservation in the energy transfer process, used a formalism quite distinct from the CPS model and predicted that a d 4 dependence dominates for some cases: (1) for metals and excitation energies where there is a very long mean free path (1 = 100 A) for an electron on the Fermi surface (which is the case for almost all metals in the I R region where there is mainly vibrational quenching of adsorbed molecules); (2) for noble metals where molecular radiative emission is below the threshold energy for interband transitions; and (3) for free-electron metals. They also predicted a d 3 dependence for short l. The CPS model, though containing questionable assumptions, survived experimental tests for a molecule-surface separation down to 10 A [80,115,118,128-133]. However, deviations from the CPS model are known [134]: for example, it does not adequately describe the distance-dependent lifetime of biacetyl (3mr*), separated from A g ( l l l ) by spacer layers of N H 3. Although the radiative emission is below the interband transition, the results cannot be fit by the d 4 dependence predicted in Persson's model. But if a distance-dependent radiative rate is added to Persson's model, a d 4 dependence gives a good fit. The CPS model breaks down for shorter separation distances, since the treatment of the excited states as point dipoles is no longer appropriate [114,135]. In addition, for a molecule chemisorbed on a metal surface, energy relaxation processes more efficient than field coupling will operate. For example, I R reflection-absorption studies of the C - O stretch of C O / C u ( 1 0 0 ) find a vibrational excitation lifetime of 1.3 x 10 -12 s [136], while the dipole field coupling theory gives 1.7 x 10 -11 s [127]. As shown below, resonant charge transfer, which occurs on a much shorter time-scale than field coupling, will account for the short lifetime. Fig. 17 shows schematically the energy decay mechanisms for an electronically excited molecule chemisorbed on a metal surface. For simplicity's sake, the adsorbed molecule or atom is assumed to have two non-degenerate electronic levels - o, a fully occupied bonding orbital located below the Fermi level ( E F ) , and e, an empty antibonding orbital located above E F. Now, let one electron in level o be excited to level e, leaving a hole in level o. Due to the image force, the energy level e may drop below E F. First, we consider the case in which level e ends up above E v. Since the excited state is coupled to the metal via an orbital coupling potential, Vk, the excitation can be quenched via resonant electron tunneling from level e to an energy level, E k, in the unoccupied part of the metal conduction band (step 1 in fig. 17a). The resulting ion usually relaxes via an Auger neutralization process in which a metal electron fills level o with ejection of a metal Auger electron (step 2 in fig. 17a). In the limit of Vk ---, 0,

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X.-L. Zhou, X.- Y. Zhu and J.M. White

- .2-,-t----tEF

E

1

i i i

EF

'

Ek

i t

EF

-i--0

~i~i!i i:ii li :iii:ii::::!

\ iii~ili~!i~ili!ii!i!ili~ili!i!iiiii!iii!iii~iii~i~iii~ \

\\

a

\

b

c

Fig. 17. (a) Schematic illustration of the resonant electron transfer from the excited level, e, of the adsorbate to the empty level ( E k) of the metal (step 1) and Auger decay of the resulting ionic adsorbate (step 2). (b) Auger de-excitation process of the excited adsorbate. (c) De-excitation process via electric field coupling.

energy conservation demands that the inequality, E e - E o - U o + U o e > E F - E o - U o - v , be met for the resonant electron tunneling (charge transfer) [127]. E e and E o are energies of orbital e and o, respectively; Uo and U o e are Coulombic repulsion terms involving two electrons in level o and one electron in o and the other in e, respectively; and v is an image screening term. Avouris and co-workers [125,137] studied electronic excitation and relaxation of noble-gas atoms on metal surfaces. In these cases, the inequality is satisfied, and the linewidths of the np 6 ~ n p S ( n + 1)s transitions are - 0 . 5 eV, consistent with density functional calculations [138] (noble-gas atoms on jellium surfaces) which give linewidths of - 1 eV. These linewidths correspond to an excitation lifetime of - 1 x 10 -~5 s, indicating that resonant electron tunneling is a very rapid quenching process. Now, consider the case in which level e ends up below E F. In this case, resonant electron transfer from e to E k is not possible. But the excitation can decay via an Auger de-excitation process in which the hole in o is filled by an electron from the conduction band, with the electron in e excited (Penning ionization) (fig. 17b). The Auger process is about a factor of ten slower than the resonant electron tunneling [18]. In fact, for non-resonant excitation decay, much longer lifetimes ( - 100 fs) are predicted [19]. In addition to Auger decay, the excitation can also, as discussed above, decay via energy transfer by dipole field coupling between the excited adspecies and the metal (fig. 17c). Unlike resonant electron tunneling and Auger de-excitation, in which charge transfer is involved, the dipole field coupling process involves the transfer of energy but not charge. Indeed, it does not require orbital overlapping between an adsorbate and the metal. Unless the spacer layer has a band structure containing delocalized orbital wavefunctions, dipole field coupling is the only possible decay process for an adsorbate in a physisorbed multilayer or one separated from a metal by a spacer layer. As mentioned earlier, charge transfer quenching leads to much shorter vibrational excitation lifetimes than predicted by classical dipole coupling theory. Avouris and Persson [127], using CO as a particular example, explained charge transfer quenching in the following way. U p o n adsorption, the affinity level of CO (2,n*) broadens and shifts toward lower energy in the vicinity of the Fermi level and is in resonance with it. As a result, the 2,~ * orbital is partially filled. As the C - O vibrates, the affinity level energy rises and falls, causing electron flow back

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and forth between the metal and the affinity level. This, in turn, damps the vibrational motion. In this case, the vibrational lifetime is proportional to the inverse square of the average fluctuation of the number of the electrons during one vibration. Based on this description, they calculated a vibrational lifetime for CO on Cu(100) of 1.8 x 10 -a2 s, close to the experimental value of 1.3 x 10 -12 s [136]. Similarly, the lifetime for vibrational excitation of N 2 on Pt(111) was estimated to be - 4 x 10 -12 s, compared to the experimental value of 1.8 x 10 -12 s [127]. Clearly, the fifetime for vibrational excitation is 2 - 3 orders of magnitude longer than for electronic excitation. So far, we have discussed quenching of excitations localized in the adsorbate. For excitation involving electron attachment or metal-to-adsorbate charge transfer, the adsorbate does not have a positive hole in the valence band. Instead, an anionic adsorbate is formed by at least partial tilling of an excited state orbital of the neutral adsorbate. The energy of the excited orbital will be significantly higher than in fig. 17, since there are no adsorbate-localized holes to provide stabilization. Quenching can proceed via resonant electron transfer back to the conduction band of the metal (fig. 17). The resulting conduction band electron will decay via inelastic scattering. The lifetimes of these transient negative ion states are estimated to be on the order of 10 -15 s [111]. Theory [43,117,124] predicts much shorter lifetimes of electronic excitation for adsorbates in the first layer than in the second or higher layers. For pyrazine on A g ( l l l ) [92], the lifetime of the electronically excited 1B2, state, estimated from ELS spectra, was - 5 × 10 -15 s for the first monolayer and - 3 x 10-14 s for the second monolayer. The lifetime for adsorbates even further ( > 10 ,~) from the surface is even longer, since radiative emission can be detected on a n s ( 1 0 - 9 S) time-scale [80,128]. At this point, we have discussed energy transfer from excited adsorbates to metals. Other energy transfer channels, including inter-molecular and intra-molecular (radiationless) energy transfer, are commonly observed in gas, liquid and solid phase photochemistry. For intramolecular radiationless transitions, the total energy within the molecule does not change. The time-scale involved is typically 10 - 9 S or longer [75], much longer than the time-scale for substrate quenching. Therefore, intra-molecular energy transfer may not be an important quenching pathway in the adsorbate phase. Inter-molecular energy transfer, on the other hand, does change the energy of the excited molecule. Since it is a bimolecular process, the time-scale for inter-molecular energy transfer depends on molecular concentrations. For concentrations typical of adsorbate layers, lifetimes of 10 -a2 s can be predicted [75]. Thus, inter-molecular energy transfer [108] may be an important quenching process for adsorbates in physisorbed multilayers, but is expected to be less competitive than substrate quenching in monolayers. Inter-molecular quenching, though speculative, does successfully explain a measured decrease, with increasing coverage, of the initial photodissociation cross section of C12CO on A g ( l l l ) [139b,c]. Another interesting case is the photodissociation of C H 2 I 2 on A1203 at 308 nm, where the quantum yield ( < 0.1) is much smaller than in the gas phase [140a]. Since the band gap of AI203 is > 8 eV, higher than the excitation energy, excitation decay via electron-hole pairs and charge transfer is unlikely. Decay via intersystem crossing and internal conversion, enhanced by the heavy atom effect and facilitated by intra-molecular randomization [140a], was thus proposed. Other good evidence for inter-molecular quenching exists in the D I E T of O ÷ from CO on N i ( l l l ) [140b]; initially, the yield of O ÷ increases linearly with CO coverage but then it maximizes and decreases near saturation. This was interpreted in terms of quenching due to inter-adsorbate interactions at high coverage. Having discussed quenching and the lifetime of excited states, we turn to the chemical transformation of these states. There are no direct measurements of adsorbate dissociation

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X.-L. Zhou, X.-Y. Zhu and J.M. White

times, but femtosecond gas-phase experiments are yielding new information. Using a classical mechanical model and assuming that the typical speed of a fragment attained during dissociation is o -- 105 c m / s and that dissociation requires a typical bond extension, l, of 2 ,~, Bersohn and Zewail estimated that the time for bond dissociation is then l / v .~ 200 fs [17]. This calculation agrees very well with the experimental result ( - 2 0 0 fs) for photodissociation of I - C N [76]. The time-scale for photofragrnentation of C H 3 - I and ICF2CF2-I was determined to be < 500 fs [142]. A recent experiment involving 200 fs laser-induced desorption of N O on P d ( l l l ) indicates that photodesorption occurs on a time-scale of < 200 fs [62]. Cowin and co-workers [5d], using a simple classical calculation, estimated a time-scale of - 1 5 fs for C H 3 - B r dissociation. Beswick and Jortner [77] deduced a lifetime for dissociation from a dissociative continuum of 10-14-10 -~5 s, using the time evolution of the initially excited nuclear wavepacket inferred from either energy-resolved or time-resolved observables. It is clear that the time-scale for bond dissociation is much shorter than for fluorescence. That photochemistry occurs readily in many metal-adsorbate systems effectively shows that, unlike fluorescence, bond dissociation is very competitive with quenching. Compared to gas-phase properties, it is traditionally expected that the presence of strong quenching in adsorbate-metal systems will result in smaller, or even negligible, cross sections. Several papers have indeed reported evidence of reduced cross sections for photodissociation of molecules directly adsorbed on metal surfaces. For example, Chuang and co-workers reported a quantum yield ( ~ ) of < 0.01 and < 10 -4 ( ~ = 1 in the gas phase) for photodissociation of chemisorbed CH2I 2 on A1 films and Ag(ll0), respectively [165]. Partial quenching of photodissociation was also reported for CH3I chemisorbed on A g ( l l l ) [79c] and P t ( l l l ) [143], for CH3C1 on N i ( l l l ) [144a,b], and for CH3Br on B r / N i ( l l l ) [5]. Although substrate quenching is important, the additional excitation channels introduced by underlying substrates can offset the quenching, yielding a net enhancement in photolysis cross sections compared to the gas phase. Examples include H C I / A g ( l l l ) [145], and alkyl chloride, bromide and phosgene on A g ( l l l ) [79,139,146,147] and Pt(lll)[78,148-150].

3. Experimental evidence for various processes

Having discussed in considerable detail the important features that enter the description of surface photochemistry, particularly that photochemistry involving intra-adsorbate bond breaking, we now briefly present experimental evidence from the recent literature for specific processes and, where appropriate, incorporate results from relevant related literature. For organizational purposes, we divide this section into several subsections, according to the kind of excitation, as follows: (1) processes driven by direct adsorbate excitation, (2) processes driven by photoemitted electrons, (3) processes driven by photoexcited electrons that have insufficient energy to surmount the work function barrier (subvacuum level energies), (4) processes associated with plasmon excitation, (5) processes associated with phonon excitation, and (6) processes associated with other kinds of excitation. Among these processes, (1) belongs to direct excitation, i.e., the incident photons are initially absorbed by the adsorbates, and (2)-(6) belong to substrate excitation, i.e. the initial absorption step is within the substrate. The direct and substrate-mediated photochemical processes can be distinguished experimentally. As discussed in part 2 (Methods and important factors), the dependences of photolysis rate, yield or cross section as a function of wavelength [7,24,41,52,56,144,152,153], polarization, and angle of incidence [41,51,84,150,154-156] are

Photochemistry at adsorbate / metal interfaces

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distinctively different for the two excitation mechanisms. Dynamic studies also shed light on the excitation mechanisms [5,144,157-161]. 3.1. Processes driven by direct adsorbate excitation

In addition to the important substrate-mediated excitation channels to be described below, direct excitation of adsorbates or adsorbate-substrate complexes, depending on wavelength, can make important contributions for adsorbate-metal systems. In determining whether direct excitation is possible at a given wavelength, optical absorption spectra of the adsorbates in the gas and condensed phases and of the corresponding organometallics can serve as qualitative guides. The role played by direct transitions is most clearly established for photochemistry on insulator surfaces where, using UV light, single photon substrate excitation does not occur, except perhaps at defects. The work on the surface-aligned photochemistry of alkyl halide molecules, by Polanyi and his group, bears directly on many of the adsorbate-metal systems where the same, or closely related, adsorbates have been used [158,160,161]. Recent work by Cowin and co-workers involving methyl bromide on LiF [225] and by Stair involving methyl iodide on MgO [162] also falls into this category. On LiF, Polanyi and co-workers, using excimer radiation, examined the direct photolysis of CH3Br [158], OCS [161] and HES [160] by following the angle and wavelength dependences of desorbing photofragments. The peaks of the translational energy distribution of the photofragments are lower than in the gas phase, whereas the maximum translational energies are the same as those in the gas phase (e.g. CH 3 from CHaBr [158] and H from HES [160]) and, in some cases, a significant portion of the photofragments carry translational energies higher than the maximum in the gas phase (e.g. Br from CH3Br [158] and S and CO from OCS [161]). These observations indicate the importance of the substrate in the dissociation dynamics, even though these molecules are very weakly held in their ground electronic states. Studies involving insulator substrates are often troubled by the inability to produce clean well-characterized surfaces. Recent work by Cowin and co-workers [225] and Stair [162] addresses this issue. On metals, one can never escape from substrate excitation, at least for thin overlayers of adsorbates; such layers are optically thin and all metals readily absorb UV light. Nevertheless, there are numerous instances where direct absorption occurs; most of the evidence is for photochemistry occurring in the second and higher layers. As noted below there are only a few monolayer-on-metals cases where there is good evidence for direct excitation. In one of the earliest papers, Chuang and co-workers studied the photochemistry of CH2I 2 adsorbed at 90 K on A1 [163,164] and Ag films [165], on Ag(ll0) [165], and on A1203 [6,140a]. On all three surfaces, the fragmentation of adsorbed CH2I 2 at low laser fluences, 308 nm laser pulses, is consistent with a genuine photochemical effect. The authors attributed the excitation process to direct electronic excitation of CH212; gaseous CH2I 2 absorbs 308 nm light, a single photon band gap excitation of A1203 is impossible, and the photoprocess is molecularly selective. Somewhat later, Cowin and co-workers, using TOF and TPD, performed revealing dynamic studies of the photolysis of CH3C1 adsorbed on N i ( l l l ) , with [144c] and without spacer layers [144a,b]. Using a pulsed excimer laser, they found that, from submonolayer to multilayers, photolysis took place at 193 and 248 nm, but not at 351 nm. At 193 nm, there were two T O F peaks, one corresponding to ( E t.... ) = 1.3 eV and the other to ~E t.... ) = 0.6 eV. In the gas phase, photolysis at 193 nm is well-known and the CH 3 photofragment energy is about 1.7 eV, taking account of the C-C1 bond strength, the photon energy, and the vibrational energy of

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X.-L. Zhou. X.-Y. Zhu andJ.M. White

C H 3. Thus, the high-energy photofragment, observed at all coverages, with an energy onset of 1.7 + 0.06 eV, was attributed to direct excitation.

Metal carbonyl surface photochemistry has been widely studied, particularly on semiconductors. One of the earliest is that of Celii et al. [166,167] who studied photodissociation of Fe(CO)5 adsorbed on Ag(ll0) at 120 K using 337 nm laser radiation. During irradiation, they observed desorption of CO but not Fe-containing fragments. They also studied photodissociation of Fe(CO) 5 on Si(100) and AI203 finding similar results. For the same Fe(CO)5 coverage and the same laser irradiation condition, the CO desorption yield was independent of the substrate (metal, semiconductor and insulator). They concluded that the photodissociation resulted from direct absorption of the UV light by adsorbed Fe(CO)5 and that the dissociation rate of the resulting excited state was faster than any quenching mechanism. Detailed mechanistic studies of Mo(CO)6 photolysis were performed on Si(100), S i ( l l l ) , A g ( l l l ) , C u ( l l l ) and graphite [7,41]. In view of the lack of dependence on the substrate and rates that tracked the optical properties of the parent molecule (see fig. 57), direct excitation was concluded. Earlier, we identified polarization effects as potentially providing crucial evidence regarding the direct or indirect character of photochemistry observed at a d s o r b a t e / m e t a l interfaces. The only polarization evidence, at the monolayer level, for important contributions of direct excitation involves the photodissociation of ClzCO on P t ( l l l ) [150]. At 280 nm, the angular dependence using p-polarized light cannot be accounted for on the basis of substrate excitation alone. Consistent with known gas-phase absorption in this region, a contribution from direct absorption is needed to account for the results. At ?~ > 315 nm, unlike at 280 nm, the measured yield can be satisfactorily fit by metal absorbance. This is not surprising since phosgene does not absorb these wavelengths. In an interesting chemical transformation of a different kind, surface isomerization, Grassian and Pimentel [168] studied photochemical reactions of cis- and trans-l,2-dichloroethene (C1CzHzC1) adsorbed on P d ( l l l ) and P t ( l l l ) at 110 K using broad-band irradiation (~ > 200 nm). By considering reaction energetics, they concluded that the photodissociation resulted from directly excited singlet states; but 1,2-dichloroethane only absorbs ?~ < 220 nm photons, so this assumption requires red-shifted spectra associated with the adsorbate-substrate complex. Based on more recent work showing the importance of substrate excitations, this interpretation should be re-examined. 3.2. Processes driven by photoemitted electrons

As noted throughout this review, substrate-excited electrons play an important role in surface photochemistry at a d s o r b a t e / m e t a l interfaces. These electrons can be divided into two categories - those with and those without kinetic energies sufficient to surmount the measured work function barrier at the v a c u u m / s o l i d interface. This division is useful from an experimental point of view; those electrons that can surmount the energy barrier can travel relatively freely through overlayers, sometimes through multiple overlayers, and can be detected externally. Those with insufficient energy may easily attach to first layer admolecules but will be transmitted through multilayers only by tunneling or hopping. While many of the concepts are identical for these two groups, we will discuss them separately, considering first those that have energies sufficient to surmount the work function barrier. When the incident photon energy, hv, is higher than the work function, ~, excited electrons are generated with a kinetic energy, E k = hv - (4~ + Eb), where E b is the binding energy of the electron measured with respect to the Fermi level. A fraction of these primary electrons escape

Photochemistry at adsorbate / metal interfaces

123

(a) [~(RX)

?

-

R+X"

R+X

a(RX) R(R-X)

(b) ~.-~°

b'~

RX-+M

RX+M a v

R(RX-M) Fig. 18. Potential energy curves for intra-adsorbate bond dissociation (a) and molecular desorption (b) via DEA process.

the solid without suffering measurable energy losses, but most are scattered, exciting other electrons and producing a secondary electron distribution with lower average energies. Thus, both primary and secondary electrons arrive at the surface where they interact with adsorbatesubstrate complexes and bulk adsorbates. For low energies, typically produced by ultraviolet and vacuum ultraviolet radiation, ionization is not possible; rather, electron attachment processes are expected. Attachment, to form temporary anions, is analogous to electron-molecule interactions in the gas phase, and these have been studied extensively [169-172]. Depending on the nature of the electronic state formed, particularly the bonding or antibonding character of the orbital where the attaching electron resides, these intermediates can either dissociate or remain bound [169,171]. At surfaces, we are interested mainly in the dissociative electron attachment (DEA) channel. The cross section for dissociative electron attachment in the gas phase can be as high as 10 -14 cmz (e.g., 2.3 × 10 -14 c m 2 for HI [169]), but for most molecules, it lies between 10 -16 and 10 - 2 ° c m 2 [169]. Importantly, the attachment cross sections are strongly resonant. For example, the DEA cross section for ethyl chloride peaks at 0.17 eV with a F W H M of only 0.1 eV [170], and at 0.76 eV with a F W H M of 0.6 eV for chlorobenzene [171]. At solid surfaces, desorption induced by electronic transitions (DIET) of this, and other, kinds is well known [173]. Fig. 18 shows models for dissociation (cleavage of an intra-adsorbate bond) and desorption (cleavage of a substrate-adsorbate bond) of the negatively charged surface species. Figs. 18a and 18b resemble figs. 12 (MGR model) and 15 (Antoniewicz model), respectively. However, for convenience, we show them together again and discuss the competitive dissociation and molecular desorption processes of the negatively charged surface species in a comparative fashion. As an example, to illustrate the concepts and language, consider the alkyl halides, RX, where R is an organic group (or hydrogen) and X is one of the halogens. The

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X.-L Zhou, X.- Y. Zhu andJ.M. White

captured electron occupies the antibonding orbital of C - X and, for the lifetime of R X - , the C - X interaction is often repulsive. Consequently, after the electron attachment excites RX (point a to point b), R and X start to move apart on the R X - potential energy curve. In the absence of quenching, R X - will dissociate to R and X - . One or both may desorb, depending on the kinetic energy acquired during the dissociation. Indeed, this is one mechanism for ESD of negative ions [180,181]. Very recently, the first evidence for negative ion desorption during photolysis has been observed, C1- from C C I 4 o n Ag(111) [305]. As for other D I E T processes, there is an analogy here with negative ion ESD, particularly the angle-resolved work ( E S D I A D ) of Madey and co-workers [309]. As for ESDIAD, it would be very interesting to measure the angular distribution of C1- evolved from photon-driven CI desorption. Having considered direct dissociation and desorption, we now turn to other paths followed after excitation. Referring to fig. 18a, quenching of R X - , involving reneutralization by transfer of the captured electron back to the metal, might occur at c, where the R - X bond is lengthened but not yet dissociated. If quenching occurs to d, RX is in its ground electronic state, but it is vibrationally excited. Since vibrational excitation is a typical method for thermally activating a molecule, the vibrationally excited RX may still dissociate, particularly if it is interacting strongly with the substrate. In this case, the dissociation fragments will retain little kinetic energy, making their desorption very improbable, unless the fragments themselves are very unstable [139]. We now turn to the molecular desorption process involving the potential between the surface and RX (fig. 18b). Before electron attachment, the potential is represented by the (M + RX) curve, and, after the attachment, it is represented by the (M + R X - ) curve. As pointed out by Antoniewicz [108], there is an attraction of R X - to the surface by its image charge, and this will tend to shorten the equilibrium distance between M and R X - . Thus, after electron attachment brings the system from a' to b', R X - starts to move toward the surface. Quenching, via electron transfer back to the substrate, may occur at c'. In this configuration, the potential describing the admolecule-surface bond is repulsive, and if the potential energy at d ' plus AE ( E b, -- Ec, ) is higher than Ea,, desorption of RX can occur. Note that the processes described in figs. 18a and 18b are concurrent, making intra-adsorbate bond dissociation and molecular desorption competitive. In many cases, the character of the temporary anion is such that activation of only the admolecule-substrate bond is important. For example, photon-driven desorption of N O on P t ( l l l ) is due to substrate excitation involving an N O - transient intermediate, and dissociation of the very strong N O bond is not important [51]. For the alkyl halides, on the other hand, the C - X bond dissociation dominates [144,148,174]. The electron (restricted in this discussion to have an energy above the vacuum level) that attaches to the adsorbate can be supplied from a variety of sources: external electrons, secondary electrons, or photoemitted substrate electrons. In the gas phase, D E A usually occurs at low electron energies, i.e., < 10 eV [10]. Compared to the gas phase, the substrate will modify the nature of the DEA process, particularly in the first monolayer. Modifications, due to adsorbate-adsorbate coupling, are also expected in molecular multilayers. For example, the probability of A B - molecular ion formation can be considerably modified in adsorbate states, due to perturbations of the isolated electron-molecule potential and due to orientation of the adsorbed molecules [175]. Moreover, the lower symmetry, charge exchange, and any increase in the number of decay (quenching) channels all tend to reduce the lifetime of the transient anions, and, thus, the cross section for dissociation [174,176]. Because DEA processes, initiated by externally incident low-energy electrons, are strongly related to photon-driven processes involving substrate-excited electrons, we give a brief overview of ion desorption results for small molecules that are directly related to the photo-

Photochemistry at adsorbate / metal interfaces

125

chemical studies reviewed here [174,176-182]. There are studies of electron-stimulated desorption (ESD) of O - derived from multilayers of 02 condensed on polycrystalline Pt [176,180,181], condensed on noble-gas spacer layers on Pt [179-181], and chemisorbed on Mo and W at 300 K [183]. For energies below 10 eV, dissociation and desorption are dominated by dissociative electron attachment processes. For 02 on Pt, O - desorption, due to DEA, peaks at - 6.6 eV, nearly the same as in the gas phase ( - 6 . 5 eV). The desorption intensity of O - increases linearly with 02 coverage and saturates at - 6 ML [176]. The desorbing O - cross section is, however, - 102 lower than in the gas phase, a fact attributed to the quenching of anions in the condensed phase [184]. In addition, there is an O - peak at - 13 eV, which is attributed to the decay of O [ states, 2E~ a n d / o r 2E+, transitions to which are symmetry-forbidden in the gas phase. For a fixed coverage of 02, the desorbing O intensity increases with the thickness of Ar spacer layers, a result attributed to decreased influence of image charge effects [179]. For atomic O on W and Mo, ESD of O - starts at - 4 eV and shows a peak at - 13 eV, but there is no gas phase-like peak at 6.5 eV [183]. The energy dependences of O - and C - yields from D E A of CO condensed on Pt at 20 K are very different from those found in the gas phase, where DEA leads to a sharp onset at 9.65 eV, with a maximum at 9.8 eV for O - and two maxima around 10.5 and 11 eV for C - [176]. In the condensed phase, O - production sets in at 10.6 eV with two maxima at 12 and 16 eV, and C - sets in at - 12 eV with a maximum at 14 eV. The C - / O - ratio is - 102 larger than in the gas phase. For submonolayer CO on five layers of Kr on Pt, O - exhibits a single peak at 12 eV, which corresponds to the I I CO resonant state found in the gas phase. For multilayer CO, however, an additional peak, characteristic of the condensed phase, appears at 16 eV and is assigned to Y.- C O - states that are forbidden in the gas phase [178]. The delayed onsets are attributed to high reneutralization probabilities for low-energy O - and C - when scattering in condensed CO [176]. In another system with obvious connections to surface photochemistry, C1- from submonolayer C12 on Pt shows two ESD peaks at 2 and 5 eV. These correspond to gas-phase DEA processes involving the 2Hg and 2H u core-excited C12" resonant states, respectively. A third peak, absent in the gas phase, appears near 11.5 eV at high coverages, and is ascribed to C1ions formed via the 21-Iu resonance by electrons which have suffered energy losses through the excitation of low-lying electronic states of molecular chlorine. Unlike the gas-phase low-energy electron-induced processes, ESD (via DEA) of O - from submonolayer and multilayer N O and N20 on Pt shows significant perturbation [177]. O from N O shows a peak at - 9 eV in the gas phase, while there are four peaks (6.8, 9.1, 13.5 and 16.9 eV) on Pt. Dimerization, i.e., strong a d s o r b a t e - a d s o r b a t e interaction, is partly responsible. For condensed N20, O - ESD exhibits two peaks at 9.4 and 16.4 eV, while gas-phase DEA of N20 gives a strong O - peak at 2 eV and a weak one at 11 eV. In the examples given above, no surface analysis was performed after electron irradiation. This is particularly relevant for low coverages, where adsorbate and fragment interactions with the substrate are strongest. That the desorbing negative ion currents are typically more than 100-fold smaller than in the gas phase is attributed to reneutralization by interactions with the substrate, to electron detachment interactions with neighboring adsorbates and to the retention of ions by surfaces. In a recent study of low-energy electron-induced decomposition (EID) of monolayers of C I : C O on A g ( l l l ) [174], attention was focused on surface analysis after electron dosing. E I D of C12CO has a threshold near 0 eV; all the C1 is retained, whereas all the CO desorbs. The total effective E I D cross section increases with incident electron energy and has no distinguishable resonant structure, significantly different from that in the gas phase. This behavior was attributed to involvement of substrate-derived secondary electrons in the EID.

X.-L. Zhou, X.-Y. Zhu andJ.M. White

126

Many recent surface photodissociation results have been interpreted in terms of substratederived excited electrons [5,144,148,153,157[. Cowin and co-workers, in studying the photofragmentation of CH3CI adsorbed on N i ( l l l ) at both submonolayer and multilayer coverages, and with a spacer layer, observed two photodesorption channels. With time-of-flight (TOF) mass spectrometry and using 193 nm radiation, which is absorbed by CH3CI [20,185], they observed C H 3 translational energy distributions with two maxima, 0.6 and 1.3 eV [144a,b], respectively (see fig. 29). Since the 0.6 eV peak vanishes for CH3CI coverages higher than - 8 M L but the 1.3 eV peak does not, the authors attributed the latter to direct photon absorption and dissociation of CH3C1 adsorbates, and the 0.6 eV channel to substrate photon absorption followed by dissociative electron attachment to the adsorbate. Consistent with this interpretation, 248 nm radiation, which is not absorbed by CH3C1, gave only the 0.6 eV channel. Both photon energies exceed the work function, so electrons from the metal can escape the surface and travel through multilayers. Since these electrons have limited mean free paths in the adsorbate, electron-driven processes should, as observed, occur only within the first few layers. The same kinds of results were found for the photodissociation of CH3C1 on GaAs(100) [157] and CH3Br on B r / N i ( l l l ) [5]. In studying photodissociation of HC1 on A g ( l l l ) with 248 nm radiation [145], Polanyi and co-'workers found that the dissociation cross section decreased with increasing atomic C1 coverage. The rate of photon-driven dissociation qualitatively correlates with the photoelectron yield since, as the CI coverage increases, the work function also increases and the photoelectron yield (not measured) should decrease. In a more elegant study, the authors used Xe as a spacer layer and potassium to lower the work function [153]. They found that HC1 on X e / A g ( l l l ) did not dissociate for X > 248 nm. In this case, photoelectrons, though not directly measured, should be produced with a maximum kinetic energy, E k . . . . = 0.25 eV (hv = 5 eV, ~ = 4.75 eV).

MeCl/Pt(111) 100 ~

.

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I

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,

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40

~o

'i~,

i .0.8 -1.2

o

1

2

3

4

5

6

7

8

9

MeCI Coverage (ML)

Fig. 19. Relative photoelectron yield (filled squares) and work function change (open circles) as a function of CH3CI coverage on P t ( l l l ) at 50 K. The inset shows in semi-logarithmic form, the photoelectron yield, normalized to the result for two monolayers as a function of multilayer coverage. After Jo and White [148a].

Photochemistry at adsorbate / metal interfaces

127

These may not have sufficient energy to attach to and dissociate HCI, since, in the gas phase, it is dissociated only by > 0.5 eV electrons [186]. Without the spacer layer, i.e., HC1 adsorbed directly on A g ( l l l ) , dissociation occurred [145]. Two possible interpretations are suggested: First, since HC1 lowered the work function, 248 nm photons produced higher energy and, therefore, effective photoelectrons. Second, the HC1 is in direct contact with Ag, and this interaction could change the attachment cross section. When K was added, lowering the work function to - 2.3 eV, adsorbed HC1 on X e / K / A g ( l l l ) dissociated even with 350 nm (hu = 3.5 eV) radiation. For HI, dissociation also occurred at much longer wavelength radiations on X e / K / A g ( l l l ) than on X e / A g ( l l l ) . These results clearly point to the importance of substrate-excited electrons and their energies. In another very interesting system, for which photon-driven desorption dynamics were investigated using polarized and pulsed laser excitation, Ertl and co-workers found that the photodissociation and photodesorption of H 2 0 on P d ( l l l ) [155] can be readily ascribed to substrate excitation. At hu = 5.0 eV, photochemical effects are detectable, but cross sections are much smaller than at hp = 6.4 eV. Since the work function of H 2 0 / P d ( l l l ) lies near 5 eV, this suggests that electrons excited above the vacuum level are most effective. Adsorbate excitation is ascribed to electron attachment, forming H 2 0 - , which dissociates through subsequent interaction with the surface. More recently, the relation between photoelectron yields (which were not measured in any of the foregoing experiments) and bond dissociation rates has been directly established [148]. For CH3CI on clean P t ( l l l ) and C-covered P t ( l l l ) , both the photoelectron yield and the photondriven rate of C-C1 bond cleavage were measured as a function of CH3C1 coverage. Irradiation with photons below 4.3 eV, which is only slightly lower than the work function (4.45 eV), did not dissociate CH3C1 on either P t ( l l l ) or C / P t ( l l l ) . Thus, for this system, substrate electron excitations to energies below the vacuum level do not lead to dissociation. On the other hand, for an unfiltered Hg arc, ?~ > 230 nm photons, photon-driven C-C1 bond dissociation is found. Fig. 19 summarizes the observations on C-free P t ( l l l ) , and similar results were found on C / P t ( l l l ) . The measured photoelectron yield over the first monolayer, as expected, accurately reflects the work function change. However, between 2 and 9 ML, the yield drops in spite of a 0.1 eV drop in the work function. Assuming that all of the measured photoelectrons originate in the metal, we ascribe this drop to their attenuation by scattering, elastic and inelastic, within the adsorbate. Turning to C - C I bond breaking, since CH3C1 is transparent at X > 230 nm in the gas phase [20,185], we expect no direct optical absorption, at least for multilayers. If this is the case, the observed photon-driven dissociation should correlate with the photoelectron yield. Fig. 20 shows the pseudo-first-order dissociation rate coefficient, k ' (first order in the coverage of CHaCI; - d [ C H a C l ] / d t = k'[CH3C1]), for various initial coverages on both Pt(111) and C/Pt(111). The value of k ' decreases rapidly with increasing coverage in the first two monolayers on Pt(111), and then slows; it decreases slowly and monotonically on C/Pt(111) over the entire coverage range. Since the relative number of photoelectrons is known as a function of coverage, the data of fig. 20 can be converted, using the data in fig. 19, into a rate coefficient that takes account of their attenuation. Assuming the C - C I bond dissociation process is also first order in the number of photoelectrons, i.e., k = k'[e], and adopting a simple layer-by-layer analysis (see detailed procedure in ref. [148]), we show in fig. 21 the variation of k with thickness on both Pt(111) and C/Pt(111). Above 2 ML on Pt(111), and over the entire CH3C1 coverage range on C/Pt(111), the same value of k is calculated, within experimental uncertainty. The quantitative (and very satisfying) correlation is consistent with a photoelectron attachment process in which monolayers of CH3C1 on C/Pt(111) and multilayers on any substrate are indistinguishable.

128

X.-L. Zhou, X.-Y. Zhu and J.M. White

6 ~

pt(111) C/Pt(111)

e---

5

I

3

"4 ,4

3 2

1 "-~Y 1 1

0---'0 1

2

3

4

5

6

7

8

9

MeCI Coverage(ML)

Fig. 20. Pseudo-first-order rate coefficients for CH3CI photodissociation on Pt(111) (squares) and on C-covered Pt(111) (circles). After Jo and White [148a].

0

1

2

3

4

5

6

7

8

9

MeCI Coverage (ML)

Fig. 21. Rate coefficients for CH3C1 photodissociation on Pt(lll), first-order in CH3CI and first-order in electrons, calculated from the data in fig. 20 and the electron yield of fig. 19. After Jo and White [148a].

The higher k for CH3C1 on P t ( l l l ) below 2 ML is very interesting, particularly in view of our expectations that quenching should dominate in the first layer. Clearly, if quenching is stronger in the first layer, it is more than compensated by other effects. These might include: (1) a higher photoelectron attachment cross section due to the highly oriented geometry of CHaCI (CI end-down [187]) in the first monolayer, (2) dissociation of vibrationally excited CH3C1 after quenching, and (3) the presence of other dissociation channels, such as direct excitation of a CH3C1-Pt complex. Of these, we speculate that (3) is not important. In these experiments, and others, e.g. the pioneering work of Cowin and co-workers [5,144,157], the mean free path of electrons in condensed multilayers is an important parameter because it is a dominant factor determining the variation of the rate with layer thickness. Using the data in fig. 19, Jo and White plotted the logarithm of the photoelectron yield versus CH3C1 coverage above 2 ML (since the work function is constant above 2 ML of CH3C1) and obtained a linear plot, the slope of which yields a mean free path of 1.8 ML [148]. Using the same method, they determined mean free paths of 2.3 ML in H20, 1.4 ML in N20 and > 50 ML in Xe [188]. A recent theoretical study indicates that the mean free path of low-energy electrons in condensed H 2 0 is 4.9 A ( - 1 . 3 ML) for 1 eV electrons and 18.9 A ( - 5 ML) for 4.7 eV electrons [189]. 3.3. Processes driven by subvaeuum level excitation

Having discussed those substrate-excited electrons that have sufficient energy to surmount the work function barrier and their relation to other electron-stimulated processes, we turn now to a discussion of the second class of substrate-excited electrons - those that have insufficient energy to pass into the multilayer regime except by tunneling or hopping. The key distinction between these two groups is related to the classical potential energy barrier an electron must cross at the adsorbate/substrate interface. For an adsorbate near the surface, but not interacting at all with it, electrons from the metal must pass the work function barrier, i.e. the surface dipole potential, in order to reach the adsorbate. Bringing the adsorbate closer, i.e., into the region of the long-range attractive physisorption potential, will tend to lower the potential

Photochemistry at adsorbate / metal interfaces

129

barrier between the adsorbate and the metal, making it easier for an electron from the metal to move classically into the region of the adsorbed molecule. For a chemisorbed monolayer, orbital interactions between the metal and the adsorbate will lower the classical barrier even more. Quantum-mechanical electron tunneling of an electron, with energy below the vacuum level, into the lowest-lying unoccupied molecular orbital of the adsorbate or adsorbate-substrate complex must also be considered for the physisorbed and chemisorbed species. Generally speaking, because the chemisorbed species lies closer to the metal, the potential barrier is thinner and tunneling is more probable than for the physisorbed adsorbate. Electron attachment to the adsorbate is strongly resonant; the position of the lowest-lying unoccupied electron orbital, LUMO, of the adsorbate is a key factor and we are assuming here that this orbital lies below the vacuum level. The position of this orbital can, in fact, be above or below the vacuum level and is related to a property that is experimentally available for m a n y isolated molecules - the electron affinity, defined as the potential energy change involved in bringing an electron from rest at infinity into the L U M O of the molecule (with the algebraic sign reversed so that a positive electron affinity implies the system is more stable with the electron attached to the molecule). The position of the L U M O is an important quantity because it determines the amount of excitation above the Fermi level one needs for resonant attachment. In chemisorbed monolayers, all the orbitals, including the L U M O , will be referenced to the Fermi level, while in physisorbed multilayers, they will be referenced to the vacuum level. According to this description, work function increases would increase the electron affinity of a chemisorbed species because the L U M O , moving with the Fermi level, falls further below the vacuum level. However, the position of the L U M O with respect to the Fermi level, a quantity closely related to the minimum excitation energy required for electron attachment, will not change when the work function changes. In this picture of electron attachment, higher energy electrons are required for physisorbed than for chemisorbed species because the L U M O lies further above the Fermi level. This difference will be reflected in the photon energy thresholds of photon-driven processes that involve electron attachment, regardless of whether the threshold lies above or below the vacuum level. There is some experimental evidence that supports these notions. For phosgene [139] and alkyl halides [79,147] on A g ( l l l ) , the photodissociation extends to much longer wavelengths than in the gas phase. This red-shifted photodissociation is mainly attributed to substrate excitation. The photon energy thresholds for the first monolayer are lower than for the second or higher layers; those for the monolayer are much lower than the work function, while those for the second or higher layers are either above or only slightly lower than the work function [79,139,147]. Several papers report that observed surface photochemistry is driven by photon excitation of electrons a n d / o r holes in the substrate metals. These, discussed below, include photodesorption of N O on P t ( l l l ) [51], photodesorption and dissociation of N O 2 on N O / P d ( l l l ) [154], O z on P d ( l l l ) [156] and A g ( l l 0 ) [84] and Mo(CO), on K / C u ( l l l ) [41,190]. The nascent (before any scattering) energy distribution of excited electrons depends on both the band structure, particularly the density of states (DOS), of a metal surface and the incident photon energy. As discussed earlier, the distribution that reaches the surface differs from the nascent distribution because of elastic and inelastic scattering events. For each photon-generated excited electron with energy E e above the Fermi level, there will be a hole located at E h = hv-E e below the Fermi level. Either electrons or holes or both may participate in the photochemistry of adsorbates, depending on the nature of adsorbate-substrate systems. At this stage in the development of surface photochemistry, there is very little direct evidence regarding the role played by holes [56]. It is noteworthy that this subject is much more fully developed for

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X.-L. Zhou, X.- Y. Zhu and J.M. White

photoelectrochemistry at semiconductors [191]. Like excited electrons, to be chemically active, holes must migrate to the surface region where they interact with localized molecular states of the adsorbates. In semiconductors, excited state lifetimes may be longer than in metals because the band properties, particularly band bending, will enhance spatial separation of hot electrons and holes thereby reducing the probability of their recombination and allowing more time for chemical events. As discussed above in the context of higher energy electrons, scattering and mean free paths within the substrate are also important for those electrons with insufficient energy to surmount the surface barrier. The mean free paths, A, of electrons and holes have been measured by internal photoemission and lie between 102 and 10 3 ,~, depending on the metal substrates [50b,63]. Photon absorption and hot carrier attenuation have been discussed earlier. The ratio, ~, between the number of the effective carriers, those able to migrate to the surface, and the total number of photogenerated carriers is ~/= a A / ( a A + 1), where a is the optical absorption coefficient. It is clear that, if aA >> 1 then 71 = 1, and, if aA << 1 then ~/--- aA. For most metals, aA lies between 1 and 10 and, thus, 0.5 < ~ < 0.9. For semiconductors, a lies between 10 3 and 10 6 c m - ] . Thus, ~ will be close to unity. Even when a large fraction of the hot carriers reach the surface, only a small portion ( - 1 % [52,154,155]) of them typically induce chemical transformation. Turning to the examples mentioned above, pulsed-laser irradiation of N O on P t ( l l l ) with 355, 532 and 1064 nm photons induces N O desorption, which is characterized as non-thermal [51]. In this important experiment, the dynamics and, using polarized light, the dependence on incidence angle, indicate that the desorption is due to substrate excitation. The photon energies used are all lower than the work function and the lowest direct electronic transition associated with the adsorbed NO. Thus, hot electrons a n d / o r holes drive the desorption. The best explanation supposes that a single hot electron attaches to the initially unoccupied 2~ * levels of NO, which are located in a broad band centered approximately 1.5 eV above the Fermi level [192]. A transitory negative-ion resonance ( N O - ) results and it desorbs via the process described in fig. 18b. Another study, the non-thermal photodissociation of N O 2 on N O / P d ( l l l ) , also indicates that the underlying mechanism is substrate excitation [154a]. In the interpretation, photogenerated hot electrons attach to N204, the well-known dimer of N O 2, to form a transient anion. The anion dissociates to N O 3 and NO, with the latter ejected into the gas phase. In another interesting study, UV irradiation of anisotropically oriented 02 on A g ( l l 0 ) results in non-thermal photodissociation and desorption [84]. In this case the a d s o r b a t e - s u b strate bonding is strong, and intuitively, we anticipate that a successful description will not be possible in terms of slightly perturbed molecular oxygen; rather, an adsorbate-substrate complex model will probably be required. The O - O bond axis of the adsorbed 02 is parallel to the A g ( l l 0 ) azimuth, permitting azimuthal-angle-dependent measurements using polarized light at normal incidence. The photodissociation and desorption, both of which are observed for photons with energies that lie below, as well as above, the work function threshold, were also studied as a function of the incidence angle, using p-polarized light. All the results confirm that the photochemical effects are the result of Ag excitation (details summarized in section 4.2). The interpretation is given in terms of excited electrons that drive the dissociation by attaching to the ~r* level, located between the Fermi and vacuum levels, of a surface peroxo-type species ( 0 2 ) . The same excitation mechanism is involved in photochemistry of 02 on P d ( l l l ) [156]. The desorption is another matter; perhaps substrate-excited holes accept electrons from the peroxo species, thereby effectively neutralizing the initial charge and weakening the bond to the Ag.

Photochemistry at adsorbate / metal interfaces

131

As another interesting example, the wavelength-dependent photodissociation of Mo(CO)6 on clean C u ( l l l ) and S i ( l l l ) [41,190] trace the optical absorption spectrum of Mo(CO) 6 and the dissociation is, thus, attributed to direct electronic excitation. But in contrast, by lowering the work function - 2 eV with potassium, a new photodissociation channel opens on both K / C u ( l l l ) and K / S i ( l l l ) . This new channel is thought to involve Mo(CO)6 formed by substrate electron attachment. 3.4. Processes associated with plasmon excitation As discussed in section 2.3.2.2, surface plasmons are collective surface electronic excitations that propagate along the boundary of a solid whose electrons behave as a nearly free-electron gas in two dimensions. Bulk plasmons are analogous, except for their three-dimensional character. Both are resonant processes. The strong surface electromagnetic fields associated with surface plasmon excitations on rough surfaces is a major factor in the widely-studied surface-enhanced R a m a n spectroscopy (SERS) [73]. The wavefunction of a surface plasmon may interact directly with the localized molecular states of the adsorbate and cause a chemical transformation. However, if bulk plasmon excitation is involved, the plasmon wave must propagate to the surface to effectively induce chemistry of the adsorbates. Alternatively, the bulk plasmon wave can undergo scattering, producing secondary electron excitations which propagate to the surface and induce surface chemistry via a DEA mechanism described above. Recently, Hoheisel et al. [193-195] reported evidence of surface-plasmon-induced desorption of N a atoms from N a particles condensed on LiF(100) at - 77 K. For radiation between 410 and 647 nm, they observed a N a desorption maximum at 490 nm with a F W H M of 90 nm when the N a mean cluster radius was 50 nm. This result corresponds to surface plasmon excitation spectra for N a particles embedded in noble-gas matrices. They also observed that the desorption maximum red-shifted with increasing Na particle size [194]. They attributed the desorption of neutral Na atoms to the population of an antibonding and localized electronically excited state of N a into which the energy of the plasmon transferred. Lee et al. [196] recently reported that plasmon excitation accounted for the photon-induced desorption of AI atoms from smooth A1 films. Using 257 nm radiation, Chen and Osgood [197] reported evidence for enhanced surface photochemical deposition of Cd from CdMe z adsorbed on 10-300 nm Cd particles. The enhancement was attributed to a plasmon resonance, since for Cd spheres of this size, it occurs near 5 eV. Supporting this notion, on Au particles where there is no resonance at this energy, no enhancement of Cd photodeposition was observed. In the photodesorption of SO 2 on A g ( l l l ) [152] and the photodissociation of Mo(CO)6 on A g ( l l l ) [7,198] there is, in the wavelength dependence, structure indicating that bulk plasmon excitation plays a role. The resonant behavior of the SO2 photodesorption cross section appears between 280 and 360 nm for 1 ML coverage, and this corresponds to the absorptivity of bulk Ag (see fig. 54). Since the large variation in the latter is, for these wavelengths, attributed to bulk plasmon excitations, it is reasonable to correlate this excitation with the enhanced desorption cross section. For 2 ML coverage, the desorption cross section is very small and does not show any resonant behavior. This can be understood in terms of a model in which the bulk plasmons with energies less than the work function scatter to form low-energy electrons and these penetrate the surface barrier to form temporary anions, but only in the first layer. Since SO 2 is very weakly bound, the potential energy increment required to desorb it is very small. Thus, assuming that quenching by charge transfer back to Ag is extremely rapid, and that the Antoniewicz mechanism is operative, only low kinetic energy desorbing fragments will

132

X.-L. Zhou, X.-Y. Zhu andJ.M. White

form. If so, then the presence of the second layer will absorb a significant fraction of the accumulated momentum, so that desorption from both layers will be less likely, as observed. Interestingly, in the photodissociation cross section of Mo(CO)6 on A g ( l l l ) , there is a resonance in the cross section at - 330 nm [7,41]. This resonance, ascribed to Ag, is absent on C u ( l l l ) , S i ( l l l ) and graphite (see fig. 57) [7,41]. It is thought that this resonance is due to interband transition (d ---, sp) [7]. However, according to the band structure of Ag, the d ~ sp interband transition would have a threshold and would not be sharply resonant. Rather, the bulk plasmon excitation may be a good alternative explanation, just as for SO 2 on A g ( l l l ) . Interestingly, there is another resonance at - 2 9 0 nm which is attributed to metal-to-ligand charge transfer within the transition-metal carbonyl, i.e., a direct optical excitation. 3.5. Processes associated with phonon excitation As noted earlier, this review is concerned primarily with electronic excitation mediated by UV and vacuum UV photons. Nevertheless, it is of interest to briefly overview the results, extensively reviewed [3,61,199-203], of IR excitation of adsorbates, because of the central role played, regardless of the excitation, by relaxation paths which randomize the excitation and lead to thermally driven events, particularly molecular desorption. In adsorbates or adsorbatesubstrate complexes, only vibrational levels can be excited with IR photons (unless the intensity is high enough to make non-linear multi-photon processes competitive). These photons will excite phonons in any substrate, electron-hole pairs in metals, and electronic states in narrow band gap semiconductors. IR-induced desorption is an extensively studied subject [204] and has been reviewed recently [3,61,199]. Relevant to the present review, two IR-induced desorption mechanisms have been established. One is due to direct heating resulting from deposition and conversion of the photon energy into substrate heat. This pathway is potentially important for all substrates - IR transparent materials, such as KCI, IR absorbing materials, and IR reflecting materials, such as polished Ag [3]. It is non-resonant and the results are analogous to conventional thermal desorption. The second pathway is due to resonant heating, where the photon energy is deposited in specific vibrational modes of adsorbates, but does not lead to direct desorption. Rather, the deposited photon energy is quickly shared with its neighboring molecules a n d / o r the substrate. The randomization, via dipole-dipole coupling and very fast electronic a n d / o r phonon-mediated processes, results in local heating of the molecular layers as well as the underlying substrate and, eventually, leads to so-called indirect desorption by the thermal activation of some weak bond, usually the admolecule-substrate bond. Because vibrational excitation is resonant [3], this pathway differs from direct heating; the resonance harvests the electromagnetic energy and rapidly transfers it to other modes. Because heating, either direct or indirect, drives it, IR-induced desorption does not exhibit characteristics of molecular or isotopic selectivity in desorption [3,61,205]. In an exceptional case, Chang and Ewing recently reported evidence for a relatively inefficient direct vibrationally induced desorption of CO from NaCI(100) [204]. Of course, substrate phonons are also excited by IR absorption and, through energy transfer processes, may induce admolecule desorption. There are several recent reports of phonon-driven desorption [206-208]. Some of these were mentioned in section 2.3.2.3. Of particular interest, Ferm et al. [209] reported single phonon desorption of H D from LiF(100) at 1.5-4.2 K. Low-intensity IR radiation excited phonons in the substrate and the desorbed H D had a velocity distribution corresponding to approximately 21 K. They attributed the desorption to a

Photochemistry at adsorbate/ metal interfaces

133

process driven by high-energy phonons formed near the surface in the wake of optical absorption. 3. 6. Processes associated with other kinds o f excitation

Briefly, we now consider some other excitation channels, particularly those that are important on semiconductors and insulators. For these materials, the band gaps introduce important wavelength dependences; substrate-mediated processes must, if they are driven by single photons, involve photon energies in excess of the band gap. Thus, the wavelength dependence has a predictable onset. Band-gap transitions will, in some cases, induce adsorbate chemistry, a well-known property of numerous semiconductor-admolecule systems. For exampie, irradiation of Z n O [210] and TiO 2 [211] surfaces resulted in desorption, with threshold photon energies similar to their bandgap transition energies, of CO 2 formed from oxidation of carbon impurities. A mechanism of a band-gap excitation inducing CO 2 desorption was thus proposed. Such excitation produces holes in the valence band and electrons in the conduction band. The photogenerated holes migrate with a certain probabifity to the surface and recombine with CO 2 to form neutral CO 2 which desorbs immediately. The results from laser-induced oxidation of GaAs(100) are also consistent with a band-gap excitation mechanism [212a]. Recent studies of photoreactions of N O adsorbed on Si(111) [52] and GaAs(110) [56] at 90 K show that the photodesorption intensities of N O follow the absorbances of Si and GaAs, i.e., there is enhancement of the photodesorption rate when the photon energy exceeds the band gaps of Si and GaAs. The results were interpreted as the photodesorption caused by photogenerated carriers, i.e., electron-hole pairs, which migrated to the surface. Ying and H o [521 estimated that, at most, 1.3% of the total number of photocreated carriers actually induce desorption of N O on Si(111). Pointing to a strong role for surface electronic structure, the photodesorption of CO chemisorbed on oxidized Ni(111) has been attributed to a photon-induced interband transition, 0 2- 2p --* Ni 2+ 3d, since the wavelength dependence followed the absorbance of NiO [24]. The separation of the two bands in question is 2.9 eV, but they are broad and overlap each other [212b]. The CO bonds to Ni 2+ of NiO via a strong a donation from CO to Ni 2÷ and a weak back-donation via the d(Ni)-~r*(CO) interaction. In the proposed photo-induced process, 0 2- 2 p - * Ni 2+ 3d, there is a sudden increase of electron population in the Ni 2+ 3d band, which will drive charge out of the o bond and may lead to CO desorption.

4. Literature review

Having discussed the major factors that form the fabric of descriptions of photon-driven processes at a d s o r b a t e / m e t a l interfaces and having presented examples of the various excitation processes that play a role, we now turn to more detail and present a review of the literature. This section has been arbitrarily divided according to the adsorbate. Table 1 provides a handy overview. 4.1. Halogen-containing molecules

As indicated in table 1, there have been more investigations of the surface photochemistry of halogenated molecules, R - X , than any other class. R includes H, C H 2, CH3, C2H2, C2H4, C2H5, C6H 5 and CO; X includes C1, Br and I; and the metal surfaces include A g ( l l l ) ,

CW, > 230 nm

CW, > 300 nm Pulsed, 248 nm

Modified Ni(lll)

Si(100)

Ag(111)

LiF(100)

Xe/Ag(111) Xe/K/Ag(lll)

Ag(111)

CO

HCI

HX (X = C1, Br)

HX (X = C1, I)

NO

[145]

[239]

[24]

[214]

Ref.

CW, > 230 nm

Pulsed, 248350 nm

[230]

[153]

Pulsed, 193,248 nm [215]

Pulsed, 193352 nm

Si wafer

C12

Light source

Substrate

Adsorbate(s)

(a) Systems showing photon-driven chemistry

Wavelength-dependent photodesorption with a threshold of - 3.4 eV of N O adsorbed in atop sites and N20 formed during N O adsorption

No photodissociation of HC1 on Xe/Ag(111) at X > 248 rim; HCI photodissociates on X e / K / A g ( l l l ) at 2~ > 248 nm; photodissociation of H I on X e / A g ( l l l ) at 308 nm but not at 350 n m and on X e / K / A g ( l l l ) at all wavelengths; E t. . . . ~ 0.6 eV for desorbing H atoms for all photodissociation cases; charge transfer from the surface to HX(a) responsible for the dissociation

Photon-induced reaction (PIR) of 2HX(a)---, H2(g ) and X2(g ) at 248 and 193 nm for X = Br and only at 193 n m for X--- CI; P I R more efficient than photodissociation of H X ~ H + X; photodesorption of (HBr)~ (n < 4) clusters at very high HBr coverages

H - C 1 bond cleavage; photodissociation cross section is - 2 0 0 times the gas-phase value and decreases with increasing work function

Photodesorption with a threshold of - 2.5 eV

Photodesorption of CO occurs on oxidized N i ( l l l ) but not on clean and O-covered N i ( l l l ) ; desorption is first-order in photon flux and CO coverage and is attributed to O 2 2 p ~ N i 1+ 3d interband transition in N i O

Photodissociation of CI 2 and subsequent reaction of resulting C1 atoms with substrate; desorption of C1, SiCI and SiCI 2 in T O F

Major observations

Table 1 Photochemistry at adsorbate/solid interfaces: summary of systems studied

[56] [2331 [2311

[1031

[621

[2361

CW, > 457 nm Pulsed, 193 nm CW, > 230 nm

Pulsed, 193 nm

Pulsed, 620 nm

Pulsed, 193 nm Pulsed, 3551907 nm

GaAs(ll0)

Ni(lO0)

Modified Ni(lll)

NiO/Ni(lO0)

Pd(lll)

Pt(lO0)

Pt(lll)

[51,235]

[2891

CW, > 230 nm

Cu(lll)

Thermal and non-thermal desorption of NO at 355, 532 and 1064 nm and not at 1907 nm; thermal channel corresponds to weakly bound NO ( - 2 0 0 K in TPD) and non-thermal one to strongly bound NO ( - 340 K in TPD); non-thermal N O is highly translationally and rotationally excited and has a vibrational population in u = l of - 4 % at 355 and 532 nm and - 0 % at 1064nm, and inverted spin-orbital population; photodesorption due to substrate excitation

Et ....

Desorption of N O + ions with a third power of laser flux; peak - 0.25 eV, independent of laser flux

Non-equilibrium photodesorption of NO with 200 fs laser pulses with a third power of laser flux; < E t . . . . )/2k --~ 600 K; Tvib = 2200 K ; Tro t = 400 and 2600 K for low and high J, respectively

Molecular photodesorption from weakly bound NO; two N O desorption channels, one thermal and another one non-thermal; highly rotational excitation for non-thermal NO molecules and 1% of them in v = 1; ( E t. . . . )/2k ranges from 1000 K ( v = 0, J = 5/2, 2IIa/2) to 3000 K (v = 0, J = 53/2, 2FI3/2); underpopulation of 2H3/2 level for J < 5.5

Photodesorption of N O on clean, O-, S-covered and oxidized N i ( l l l ) with a threshold of - 1 . 5 eV; first-order desorption in photon flux and N O coverage; desorption attributed to transition of occupied N O 2 ~ r * - d to unoccupied N O 2 , t r * - d state or hot electrons

Photodesorption with N O highly vibrationally and rotationally excited

Photodesorption and dissociation of NO; wavelength correlation of photoresponse with substrate band-gap excitation

Wavelength-dependent photodesorption with a threshold of - 3.4 eV of NO adsorbed in atop sites and N20 formed during N O adsorption

k/i

~ ~,

~~"

~

~-

,~

O2

Pulsed, 220270 nm

Pulsed, 193 nm CW, > 230 nm

CW, > 230 nm

MgF 2

Ag(ll0)

Pd(111)

CW, > 300 nm

K/Si(lll)

A g ( l l 1)

CW, > 230 nm

Si(lll)

N O films

Pulsed, 532, 1064 nm

Pt foil

NO

Light source

Substrate

Adsorbate(s)

Table 1 (continued)

[83]

[84]

[238]

[237]

[232]

[52-55]

[234]

Ref.

Photoinduced dissociation, desorption and conversion between different O 2 adsorption states; threshold of 3.4 + 0.3 eV for desorption and conversion and 3.7 + 0.4 eV for dissociation; p-polarized light more efficient for dissociation and desorption than s-polarized

Desorption of 02 and dissociation to O(a); substrate excitation responsible for photochemical processes, desorption activated by dissociation

Explosive photoejection of molecular N O

Two molecular desorption peaks, a slow one with ( E t . . . . ) / 2 k = 160-280 K occurring only for thick films and high laser fluences and a fast one with (/~t . . . . ) = 0.22 eV occurring at all thicknesses of NO; the fast N O is highly vibrationally and rotationaUy excited

Photoinduced non-thermal desorption of NO, dissociation of N O and reaction to form N20; no photodesorption of N O at high K coverages

Photoinduced non-thermal desorption of NO, dissociation of N O and reaction to form N20; evidence of photogenerated hot carriers responsible for observed photoprocesses

Thermal and non-thermal molecular desorption at both X's; at 532 nm: non-thermal N O is highly rotationally excited and has a vibrational population of - 3 % , ( E t . . . . ) / 2 k for non-thermal N O increases from 1200 K at J = 3.5 to 2800 K at J = 19.5, inverted spin-orbital population; at 1064 nm, ( E t.... ) / 2 k and rotational temperature are - 30% lower

Major observations

f-

[1031 [1541

Pulsed, 193, 248 nm

Pulsed, 193 nm Pulsed, 351193 nm

LiF(100)

NiO/Ni(100)

NO/Pd(lll)

n2s

NO 2

[159,1601

[241]

Pulsed, 193, 248 nm

Quartz

H20 films

a ( E t. . . .

)/2k

of 770 K

Dissociation to form NO which desorbs immediately; two NO desorption channels with ( E t. . . . )/2k of 900 (fast) and 135 K (slow); fast channel highly rotationally excited but slow one rotationally cold, angular dependence of polarized incident light indicates dissociation due to substrate excitation; angular distribution of fast NO peaked toward surface normal with a cos48 dependence and that of slow one with a cos 0 dependence

Photodissociation and concurrent desorption of NO

Photodissociation to HS and H with HS*(v _> 1)/HS(v = 0) ratio much higher than gas-phase photolysis; coverage-dependent and bimodal angular distribution of desorption H fragment; beyond 0.1 ML, photoreaction to produce H2(g); photoejection of' H2S molecules with translational energies up to several eV and photodesorption of H2S with lower translation energy ( < 0.5 eV)

Two-photon photoejection of H20 with

Photodissociation and desorption with the former more effective occur preferentially in first layer; substrate excitation responsible for both processes; (Etrans)/2k of - 6 0 0 K and cos40 angular distribution for H20 desorption; strong isotope effects (H20 versus D20 ) of 1 : 1.4 for desorption and 1 : 2.2 for dissociation

[155,240]

Pulsed, 193, 248 nm

Pd(lll)

H20

Dissociation, desorption and conversion between different adsorption states; different wavelength dependence for dissociation and desorption

[84b,85]

CW, > 230 nm

Pt(lll)

Photoinduced (1) desorption of O2(a), (2) conversion from al-O 2 to a2,a3-O 2 and (3) dissociation to O(a); two channels for photodesorption with (Etrans)/2k of 800 and 120 K; substrate excitations are dominant primary processes, conversion and slow 02 desorption initialized by photodissociation

[156]

Pulsed, 193 nm

Pd(111)

2'

N.

CW, > 230 nm

Pulsed, 193 nm

Pulsed, 222 nm

Ag(lll)

LiF(IO0)

LiF(O01)

CH3Br

CW, > 230 nm

Ag(lll)

Pulsed, 222 nm

LiF(IO0)

SO 2

CW, > 230 nm

Ag(lll)

OCS

Light source

Substrate

~sorbate(s)

Table 1 (continued)

Photodissociation and desorption of submonolayer CH3Br; translational energy distribution of C H 3 (with a peak at 1.7 + 0.1 eV and a F W H M of 0.54 eV) and Br (with a peak of 0.22 eV and a F W H M of 0.39 eV) fragment is broader and shifts to lower energy than gas phase but narrower than multilayer C H 3 B r / L i F at 193 nm; angular distribution for desorption of cosS0 for C H 3 and cos 0 for Br; desorption due to UV absorption by LiF

Photofragmentation of multilayer CH3Br, much broader nonBoltzmann velocity distribution of C H 3 and Br fragments than gas-phase photolysis; desorption of molecular CH3Br due to collision with C H 3 and Br fragments

[2251

[158,1591

C - B r bond cleavage, retention of all Br and a fraction of C H 3 ; red-shift and enhancement more significant for monolayer than for multilayer; threshold of - 3.0 eV for monolayer and 3.5-4.0 for multilayer

Only molecular photodesorption confined in the first monolayer; resonance of photodesorption rate at - 330 nm correlating with metal absorption

[1521

[79b,1471

Photodissociation to CO and S; cross section is on average 10 3 times greater than in gas phase, decreases with increasing OCS coverage and is reduced significantly by H 2 0 spacer layers; photodissociation dynamics strongly perturbed from gas phase; photoejection and photodesorption of molecular OCS and photoreaction to form $2 also occur

Dissociation to S(a) and CO(g); red-shift and enhancement compared to gas phase

Major observations

[161]

[229]

Ref.

ta.

CH3C1

Pulsed, 193, 248 nm

CW, > 230 nm CW, > 230 nm CW, > 230 nm CW, > 230 nm Pulsed, 193, 248 nm

Pulsed, 193351 nm

CW, > 230 nm CW, > 230 nm

CW, > 230 nm

Br/Ni(111)

Pt(111)

C/Pt(lll)

Ru(001)

Cu/Ru(001)

GaAs(100)

Ni(111)

Pd(100), K/Pd(lll)

Pt(lll)

C/Pt(111)

Wavelength-dependent cleavage of C-C1 bond; correlation between rate coefficient and photoelectron yield; larger cross section for monolayer than for multilayer Constant dissociation rate coefficient from submonolayer to multilayer; correlation between rate coefficient and photoelectron yield; lower rate coefficient for submonolayer than on P t ( l l l )

[148,2171

[148]

Photodissociation of C1-CH 3 bond; dissociation rate enhanced on K/Pd(100) as compared to Pd(100)

Red-shifted C-C1 bond cleavage, ejection of C H 3 into gas phase; two photofragmentation channels (fast and slow) at 193 nm, only slow one at 248 nm and none at 351 nm; fast channel due to direct photolysis and slow one due to attachment of excited substrate electrons; cross section peaks at 4 ML; perturbation of photodynamics from gas phase, photodissociation strongly inhibited by H 2 0 but not Xe spacer layers

[144]

[218]

Two channels of photofragmentation to C H 3 and CI (direct photolysis and dissociative attachment of photoexcited electrons); coverage dependence of cross section for both channels; photodesorption of molecular CH3C1 due, partly, to collision with fast CH 3 and CI fragments

C - B r bond cleavage, retention of C H 3 and Br

C - B r bond cleavage, retention of CH 3 and Br

C - B r bond cleavage, smaller dissociation cross section and higher photon energy threshold than on P t ( l l l )

C - B r bond cleavage at longer wavelengths than in gas phase; retention of C H 3 and Br and also formation of CHx(a ) and HBr(a)

C - B r bond cleavage, desorption of C H 3 and Br at multilayers; smaller cross section than gas-phase photolysis; strong perturbation of photodynamics from gas phase; two photodissociation channels (direct and surface-mediated indirect excitations)

[157]

[224]

[224]

[223]

[78,149, 2171

[5]

5

CW, > 230 nm

Pulsed, 266 nm

CW, > 230 nm

Ag(lll)

LiF

Pt(111)

CH3I

CW, > 230 nm

Pulsed, 308 nm

A1203

Ag(111)

Pulsed, 308 nm

Ag films Ag(ll0)

C12CO

Pulsed, 308 nm

A1 films

CH2I 2

CW, > 230 nm

Pt(lll)

CH2CO

CW, 55 eV

Si(lll)

CH3F

Light source

Substrate

Adsorbate(s)

Table 1 (continued)

Post-irradiation TPD and SIMS evidence of C - C bond cleavage

Slower dissociation rate of C-I bond in monolayer than in multilayer, and than C-Br bond in CH3Br

Direct dissociative photodesorption at low fluences and explosive desorption at high fluences; two dissociation channels yielding I(2P3/2) and I*(2P1/2) are identified in TOF of desorbing CH 3 fragments; lower velocities of the desorbing fragments than in the gas phase

C - I bond cleavage, retention of all I and a fraction of CH3; red-shift for monolayer only; slight quenching for monolayer photodissociation

C - F bond cleavage and formation of Si-F and Si-C bonds; quantum yield of 0.04 + 0.02

Major observations

[139]

[6,140a]

[165]

Dissociation to Cl(a) and CO(g); enhancement and red-shift of photodissociation for monolayer compared to gas phase

Photodissociation of C - I bond and concurrent desorption of CH2I, I and CH 2 in TOF; explosive desorption of molecular CHzI 2 at multilayer with high laser fluences; lower quantum yield (< 0.1) than gas-phase photolysis

Little photodissociation or desorption for submonolayer CH212; photodissociation and concurrent desorption of CH2I, I and CH 2 for multilayer CH212, retention of small fraction of I

[163,164] C-I bond cleavage, retention of I and desorption of CH 2 and C2H 4 in TOF at 0(CH212) < 1 ML, also desorption of CHzI and I at 0 > 1 ML

[213]

[143]

[226]

[79c]

[216]

Ref.

3:

(,

CW, > 230 nm

Pulsed, 193, 248 nm

CW, > 230 nm

CW, > 230 nm CW, > 200 nm

CW, > 230 n m

CW, > 230 nm

CW, > 230 nm CW, > 230 nm

Pt(lll)

LiF(O01)

Ag(lll)

Pt(111)

Pt(111)

Ag(lll)

Pt(lll)

Pd(lll)

Ag(lll)

C2H3C1

C2H5C1

CIC2H2C1

CICzH4Br

(CH3)2N2

C6H5C1

[146,220]

[2281

[2211

[2221

[1681

[219]

[79a,147]

[304]

[1501

C-C1 bond cleavage, accumulation of C1 and phenyl; formation of C12H9C1 in multilayer

No photochemistry in monolayer; C - N bond cleavage to N 2 and C H 3 occurs in multilayer with a threshold of - 3.1 eV

C-C1 bond cleavage requires higher photon energy than C - B r bond; wavelength dependences are the same as for photodissociation of mono-halide analogs

C - C I bond cleavage requires higher photon energy than C - B r bond; wavelength dependences are the same as for photodissociation of mono-halide analogs

cis- and trans-isomerization in multilayers and C - C I bond cleavage in monolayer to form adsorbed C2H 2 and C1 at h > 237 nm, and adsorbed C2H 2 with loss of C1 into gas phase at 200 < X < 237 nm; red-shift for monolayer photolysis

C-C1 bond cleavage, accumulation of C2Hs(a ) and Cl(a)

C-C1 bond cleavage, retention of all C1 and a fraction of C2H5; red-shift and enhancement more significant for monolayer than for multilayer

Single photon process at 193 nm; two photodissociation channels (CI + C2H3) , photoelimination (HCI + C2H2) and photodesorption (C2H3C1) at < 1 M L and, in addition, photoreaction (CI + C2H3C1 ---, HC1 + C2H2C1 ) at higher coverages; peak translational energy: 0.66 and 0.15 eV for C1, 0.81 and 0.25 eV for C2H3, 0.18 eV for HC1, 0.27 eV for C z H 2 and < 0.1 eV for C2H3C1 at 0.2 ML, and 0.71 eV for HC1 and - 0.1 eV for C2H3C1 at 1.5 ML; the same cosn0 distribution for both C1 and HC1 with n = 1 and 3 for low and high coverage, respectively; no photolysis at 248 nm

Dissociation to Cl(a), CO(a) and CO(g); direct excitation dominates below 290 nm and substrate excitation dominates above 290

"~

~~

-~

\

~ ~ '~

q~

CW, 351,407 nm CW, 514.5 nm

Roughed Ag

Roughed Ag

Roughed Ag

Roughed Ag

Roughed Ag

C6HsCHO

C6HsNH z

Pyridine

Pyrazine

CW, 514.5 nm Pulsed, 193 nm Pulsed, 193, 248 nm

CW, 257 nm

Roughed Ag

AI/SiO2/Si

Sapphire

Quartz

Triazine

AI(CH3) 3

CW, 351-514 nm

CW, 351,407 nm

CW, 351,407 nm

Pulsed, 222 nm

LiF(100)

C6H5I

Light source

Substrate

Adsorbate(s)

Table 1 (continued)

[245]

[244]

[243]

[651

[65]

[64,268]

[64,2681

[64]

[64,268]

[227]

Ref.

CH 3

and AI(CH3) x (x < 3) in

Photodissociation of A1-CH 3 bond and deposition of AI atoms

A I - C H 3 bond dissociation with a quantum yield of near unity, and desorption of C H 3 fragments and formation of high-quality A1 film at 193 nm; 248 nm light is ineffective at removing CH 3 fragments

A1-CH 3 bond cleavage; detection of TOF

Two-photon fragmentation with an enhancement at metal surface rather than a short distance above the surface

Two-photon fragmentation with an enhancement at metal surface rather than a short distance above the surface

Photodecomposition via resonant two-photon absorption; decomposition rate slower at 351 than at 407 nm

Photodecomposition via resonant two-photon absorption; decomposition rate peaks at - 400 nm with gradual decrease in rate at shorter wavelengths and a sharp fall-off at longer wavelengths ( < 458 nm), and maximizes at a separation of 15-20 A from surface

Photodecomposition via two-photon excitation at 407 nm; no photodecomposition at 351 nm

Photodecomposition via two-photon absorption at 407 nm and single photon absorption at 351 nm; higher decomposition rate at 351 than at 407 nm

Photofragmentation and desorption, C6H5(1200 K), I(1500 K), C12H10(1050 K) and C6H5I(900 K) found in T O F with ( E t. . . . )/2k - separated by 0.94 eV, have in parentheses; I(2P3/2) and i . r~2 p 1/2), the same Etrans, different mechanism from photolysis of C H 3 I / L i F

Major observations

6,

[247]

[248]

[253] [167]

Pulsed, 193351 nm CW, 257 nm Pulsed, 193, 248 nm Pulsed, 193, 248 nm Pulsed, 248532 nm Pulsed, 222 nm

Pulsed, 193, 248 nm Pulsed, 337 nm

CW, 256, 265 nm Pulsed, 337 nm CW, 257, 524 nm

A1

Quartz

OH-quartz

OH-oxidized Si

Quartz

Quartz

OH-quartz

Ag(llO)

Ag(111)

A1203

Si(111)

AI(i-C4H9) 3

Cd(CH3) 2

Ga(CH3) 3

In(CH3)3

Zn(CH3) 2

Fe(CO) 5

/2k

Removal of CH-containing species at 193 n m and no photoeffect at 248 nm

Photodesorption of C H 3 and formation of Cd films

Photodissociation of C d - C H 3 bond and deposition of Cd atoms

Photon-induced non-thermal desorption of molecular Al(i-C4Hg)3; rupture of isobutyl groups to create C H 3 radicals and fl-elimination to create isobutylene

N o photoeffects at 248 nm; at 193 nm monomerization dominates for low laser intensity (20 m J / c m 2) and dissociation of A 1 - C H 3 bonds occurs at higher laser intensity (200 m J / c m 2)

Dissociative photodesorption of C H 3 fragments with a E t. . . . = 150 K; the C H 3 desorption transient is subthermal

[254]

Multiple, but not all, F e - C O bond cleavage only at 257 nm; change in bonding in adsorbed Fe(CO)5 at 514 nm

F e - C O bond cleavage, desorption of CO and retention of Fe(CO) x

Cleavage of multiple, but not all, F e - C O bonds

[166,167] F e - C O bond cleavage, desorption of CO and retention of Fe(CO) x

Photodissociation of Z n - C H 3 at 193 nm and not at 248 nm

PJaotodissociation to produce a film of In, detection of neutral species CH3, In, I n C H 3 and In(CH3) 2, and ions In + and InCH,in TOF; Etrans'S of desorbing fragments depend on coverage

[214,248] Photodissociation of G a - C H 3 bond at 248 and 355 nm but not at 532 nm; detection of G a atom desorption by L I F

[250]

[249]

[245]

[251]

[242]

Pulsed, 193, 248 nm

Si(100), (111)

AI2(CH3) 6

[246]

Pulsed, 193 nm

Hydroxylated A1203, SiO 2

CW, > 250 nm

CW, > 250 nm CW, > 230 nm

Graphite

Rh(100)

Pulsed, 355 nm

Silica

Cu(111) K/Cu(lll)

CW, > 254 nm

Porous Vycor glass

CW, > 250 nm

Pulsed, 337 nm

Si(100)

Ag(lll)

CW, > 230 nm

Si(100)

Mo(CO)6

Pulsed, 193720 nm

Si(lll)

Fe(CO)5

Light source

Substrate

Adsorbate(s)

Table 1 (continued)

[259]

[7]

[41,190]

Cleavage of at least two, but not all, M o - C O bonds; quantum yield of - 0.05

Multiple, but not all, M o - C O bond cleavage with UV but not visible photons; dissociation due to direct excitation

Multiple, but not all, M o - C O bond cleavage on both surfaces; direct excitation on Cu(111); on K/Cu(111) photodissociation enhanced and extended to longer wavelengths and substrate excitation channels open up

Multiple, but not all, M o - C O bond cleavage with UV but not visible photons; dissociation due to direct excitation and Ag plasmon excitation

Formation of Fe3(CO)I 2 with Fe(CO)4(SiO2) as intermediate

[258] [7,198]

Photodissociation to initially Fe(CO)4 which reacts with surface to form H - F e ( C O ) 4 - O S i and H - F e ( C O ) 4 O H ; further irradiation leads to formation of atomic Fe for low loading of Fe(CO)5 and of Fe3(CO)12 for high loading of Fe(CO)5

Photodissociation of F e - C O bonds, desorption of CO and retention of Fe(CO)x

Photodissociation of F e - C O bonds, desorption of CO and retention of Fe(CO)x

Resonant decomposition via multi-photon electronic excitation of Fe(CO)5 to Fe atoms with no significant incorporation of C and O by UV but not visible photons

Major observations

[257]

[167]

[256]

[255]

Ref.

/.

CW, < 1.2 keV Pulsed, 248, 351 nm CW, > 230 nm CW, 257, 514 nm Pulsed, 193720 nm

CW, > 230 nm CW, > 230 nm

CW, 330-610 nm

Si(lll)

Si(lO0)

Si(100), Mo(poly)

si(111)

Si(lll)

Pt(lll)

Pt(111)

Pt(lll)

CO + 0 2

NO + 0 2

H2+

0 2

w(co) 6

CW, > 250 nm

Si(111), K/Si(111)

[270]

[269]

[36]

[267]

[254,262]

[266]

[265]

[264]

[7,41,54, 190,254, 260-263]

Wavelength-dependent formation of OH and

H20 at

85 K

Inhibition of photodissociation of 02 by NO; enhancement of photodesorption of NO and 02 compared to N O and 02 on Pt alone

Formation of CO2; CO 2 formation rate controlled by 02 photodissociation

Multiple, but not all, W - C O bond cleavage by UV but not visible photons; strong wavelength-dependent direct electronic excitation for multilayer photolysis and quenched (compared to multilayer) with little wavelength dependence for monolayer

Multiple, but not all, W - C O bond cleavage only at 257 nm; change in bonding in adsorbed W(CO)6 at 514 nm

M o - C O bond cleavage to form Mo(CO)x (x < 6)

Photodissociation of M o - C O bonds at 248 nm but not at 351 nm

Photo-induced metallization with broad-band (IR to 1.2 keV) synchrotron but not with < 8 eV irradiation

Photon-induced non-thermal dissociation of multiple, but not all, M o - C O bonds on both surfaces, photodissociation with UV but not visible photons and via direct electronic excitation on K-free S i ( l l l ) ; on K / S i ( l l l ) dissociation is enhanced and extends to longer wavelengths involving an additional substrate excitation process

z

Adsorbate

C6H 6 H20 (CH3)2CO CH3OH

Benzene Cyclohexane Cyclohexane Acetophenone

HzO NH 3

C2H5OH

Substrate

Ag(lll)

Roughed Ag

Pt(lll)

Si(/00)

230 230 230 230

nm nm nm nm

> 250 nm

> 230 nm > 230 nm

> 351 nm

> > > >

Light source

Table 1 (continued) (b) System showing no photon-driven chemistry

[275]

[271] [272]

[64,268]

[273] [274] [274] [274]

Ref.

photoeffects photodissociation of O - H bonds intra-molecular bond cleavage intra-molecular bond cleavage

No observable photoelectronic effects

No photodissociation of O - H bonds No photodissociation of N - H bonds

No photofragmentation

No No No No

Comments

g~

7~

Photochemistry at adsorbate/ metal interfaces

147

Ag(ll0), Ag and AI films, K / A g ( l l l ) , N i ( l l l ) , Pd(100), K/Pd(100), Pd(111) and Pt(111). One of the reasons these molecules are so popular is the ease with which they can be prepared, dosed and photolyzed. Their gas-phase photochemistry, which almost always involves the direct dissociation of C - X bonds, is well established [20,185]. 4.1.1. Clt126

In one of the earliest papers, Chuang and co-workers used TPD, XPS and T O F to study the photochemistry of C H 2 I 2 adsorbed at 90 K on A1 and Ag films, A g ( l l 0 ) and A1203, all irradiated with 308 nm laser pulses. Gaseous C H 2 I 2 exhibits a strong absorption at 308 nm and is photodissociated into C H 2 I and I with a quantum yield of - 1 [276]. In general, non-thermal photodissociation occurs readily on these surfaces, but with lower quantum yields than in the gas phase. Explosive thermal desorption occurs for high (many layers) C H 2 I 2 coverages and high laser fluxes. C H 2 I 2 adsorbs molecularly on A1 films at 90 K [164]. During pulsed illumination of low coverages (8 < 1 ML), only C H 2 and C 2 H 4 a r e detected in TOF, but the signals are weak and undetectable for laser fluences, F, below 100 m J / c m 2 [163,164]. Note that direct C H 2 generation is not observed from gas-phase photodissociation of C H 2 I 2 at 308 nm. For higher coverages (8 > 1 ML), CH2I, I and small quantities of C H 2 and C2H 4 are detected at relatively low F ( < 30 mJ/cm2); explosive desorption of C H 2 I 2 and C H 2 I occurs when 8 > 10 ML, even for low F [163]. In the submonolayer range, the C H 2 and C 2 H 4 T O F signal intensities increase with 8 at a fixed F, and increase linearly with F at a given 8; the translational energy d i s t r i b u t i o n s , ( E t. . . . ~/2k = 450 K for C2H 4 and 191 K for C H 2, are independent of F and 8. All these are characteristic of non-thermal, i.e. photochemical, processes. The quantum yield for photodissociation at 8 < 1 ML was less than 0.01 [163]. For multilayer coverages, photodissociation gives mainly CH2I and I; for example, at 8 > 10 ML, the C H 2 I / C H 2 T O F intensity ratio was 20. On a relative basis, photodissociation contributes most when 8 < 1 ML, indicating an important role for direct A I - C H 2 I 2 interactions and, quite likely, an increased local heating effect resulting from nearby multilayer dissociation events. The formation of C 2 H 4 w a s ascribed to a reaction between photogenerated C H 2 radicals and neighboring C H 2 I 2. At 514 nm there is no non-thermal photon-driven chemistry. On A1203, a wide band-gap insulator material, C H 2 I 2, also adsorbs molecularly at 90 K [140a]. Here, substrate excitation should be negligible and any resulting photochemistry can be attributed to direct excitation. Irradiation of submonolayer C H 2 I 2 results in desorption of dominantly neutral C H 2 I and I, with a small amount of C H 2 [6,140a]; this is very different from thermal desorption, which gives mainly C H 2 I 2. No C H 2 I 2, 12 or other species are detected even for F up to 500 m J / c m 2. The ( E t. . . . )/2k is 640 K for CH2I, 580 K for I and 2030 K for C H 2. The desorption of CH2I, I and C H 2 is non-thermal in nature because variations of the coverage and laser fluence do not influence either the mass or translational energy distributions and because the desorption intensities increase linearly with 8 and F, with no apparent threshold. The optical absorption cross section for condensed C H 2 I 2 on A1203 was 8.8 X 10 -17 c m 2 and the quantum yield for photodissociation at 8 < 1 M L was < 0.1. For multilayer coverages, the photodissociation and desorption are non-thermal, but only at low F. For example, at F = 2.2 m J / c m 2, the mass and translational energy distributions are the same as at 8 < 1 ML. The T O F intensities increase linearly from 0.3 to 1.2 M L and then slowly to saturation at 5 ML. The slow increase is attributed to the possibility that the surface is not covered uniformly by C H 2 I 2, and the saturation is attributed to photofragmentation, with desorption, occurring only in the topmost layer. The threshold for explosive, i.e., thermally

148

X . - L Zhou, X.- Y. Zhu and J.M. White

driven, desorption of C H 2 I 2 depends on both 0 and F - lower laser fluence threshold for high coverages and lower coverage threshold for high laser fluences. The molecular selectivity in photodissociation and desorption of CH2I 2 on A1203 was tested by coadsorbing C H 2 I 2 with N H 3 [140a]. N H 3 does not absorb 308 nm photons. For a submonolayer C H 2 I 2 : N H 3 = 1 : 1 mixture, irradiated at 308 nm, no significant N H 3 desorption was detected, although CH2I and I signals were easily seen. Desorption of N H 3 becomes detectable at 0 > 3 M L and high F. However, much less N H 3 than C H 2 I desorbs. Furthermore, ( E t.... ) is much lower for N H 3 than for CH2I. The N H 3 desorption can be explained by a CH212-mediated local heating effect; in the multilayer case, there is sufficient energy transfer from the excited C H 212 to cause local heating and subsequent N H 3 desorption. When explosive desorption occurs, there is a large N H 3 signal, and molecular selectivity is lost [140a]. Under these conditions, the photon flux is high enough to drive the local temperature above the desorption temperatures of both coadsorbed species. Returning to metals, C H z I 2 adsorbs molecularly at 90 K on Ag(110) and polycrystalline Ag films, and the photochemical behavior on these surfaces is similar [165]. In contrast to both AI and A1203, no desorption of any species is detected for submonolayer coverages and F up to 50 m J / c m 2. This interesting observation may be related to enhanced quenching, an effect discussed below for methyl iodide on P t ( l l l ) (see section 4.1.2). At higher F, both C H 2 I 2 and C H 2 I are found in TOF, but are of mainly thermal origin. Photochemical contributions are very small, and the quantum yield is < 10 -4. For 0 > 1 ML and even for F < 50 m J / c m 2, non-thermal photodesorption of C H z I , I, and a small amount of C H 2 occurs. The {Etrans) values for C H 2 I 2 and CH2I are similar, 0.115 eV, but are higher for C H 2. Again, explosive desorption of C H 2 I 2 o c c u r s at high coverages and high F. On all three surfaces, the fragmentation of adsorbed C H z I 2 at low laser fluences is consistent with a genuine photochemical effect. The authors attributed the excitation process to direct electronic excitation of CHzI2; gaseous CH2I 2 absorbs 308 nm light, a single photon band-gap excitation of A1203 is impossible, and the photoprocess is molecularly selective. However, contributions from substrate excitation on A1 and Ag cannot be ruled out based on these results. Given the absorbed photon energy and bond dissociation energies, the m a x i m u m translational energies of the desorbing fragments are lower than the excess energy. This was attributed to internal excitations within the fragments. However, modification of the relevant potential energy surfaces at the a d s o r b a t e / s u b s t r a t e interface, with attending modifications of the photodynamics, should also contribute [5]. For example, photodissociation of C H 2 I 2 in the gas-phase produces 25% of the electronically excited I, whereas on these surfaces, all the photodesorbed I is in the ground state, since there is only one I T O F peak. The quantum yield of photodissociation in the first layer is lower than in the gas phase, especially on Ag ( < 10-4). This conclusion was largely based on the T O F results, but was supported by post-irradiation XPS analysis on A1203. However, since quantitative post-irradiation surface analysis on Ag was not performed, it is possible that a large fraction of the photofragments produced in the first layer remains on the surface. Indeed, this is the case for photodissociation of a similar molecule, CH3I, on Ag(111) [79] and Pt(111) [143]. Nevertheless, the authors attributed the much lower quantum yield on Ag, compared to A1203, to either a higher electronic relaxation rate a n d / o r a lower absorption coefficient. Quenching on metals can be much faster than on insulators because the metal has a large continuum of excitations available to the conduction electrons and usually has degenerate filled or unfilled orbitals to resonantly quench any adjacent excited or transiently charged species. As noted above, explosive desorption is molecularly non-selective and is a thermal process, which results from electronic excitation of C H 2 I 2 and its rapid transformation into local heating.

Photochemistry at adsorbate / metal interfaces

149

4.1.2. Alkyl halides on P t ( l l l ) and C / P t ( l l l ) In this section, we discuss the photochemistry of CH3CI, CH3Br, CH3I, C2H5C1, and CIC2H4Br on P t ( l l l ) and carbon-covered P t ( l l l ) . The photochemistry of these molecules in the gas phase has been well-studied [20,185]. The optical absorption spectra in the UV region are broad and continuous, typical of bound-to-free transitions that lead to dissociation. Although the absorption spectra nearly parallel each other, they shift to longer wavelengths in the order iodide > bromide > chloride. Photolysis leads to carbon-halogen bond cleavage, with other minor dissociation pathways at short wavelengths. These molecules adsorb molecularly on P t ( l l l ) and C / P t ( l l l ) at or below 100 K. In subsequent TPD, only CH3I decomposes at 200-250 K; the others desorb intact [187]. All are photoactive on P t ( l l l ) and C / P t ( l l l ) . UV irradiation (filtered 100 W Hg arc lamp) of monolayer and submonolayer coverages results in the dissociation of carbon-halogen bonds and the retention of all halogen atoms and a coverage- and substrate-dependent fraction of the alkyl fragments. There is no photodesorption of the parent molecules. Among the retained products, adsorbed CH 3 (from methyl halides) and C2H 5 (from ethyl halides) were identified by HREELS [78b,223,219] and halogen atoms by XPS [143,148,221]. In post-irradiation TPD, significant amounts of CH 4 were observed at 270-290 K for methyl halides on P t ( l l l ) [78,148], but C2H 6 was the major product for CH3CI o n C / P t ( l l l ) [148]. For C 2 H s C 1 / P t ( l l l ), C2H4, C2H 6 and H z were seen in TPD, and surface carbon in AES after TPD; H R E E L S identified CCH 3 as the decomposition intermediate of CzH 5 [219]. For C 1 C a H 4 B r / P t ( l l l ), C2H 4 and H 2 were observed in TPD [221]. HX is also a post-irradiation TPD product. The photodissociation was strongly wavelength-dependent. For example, monolayer (ML) CH3C1 on P t ( l l l ) photodissociates with the full Hg arc (~ > 230 rim) but not when the incident light is cut off below 300 nm. In the gas phase there is negligible photodissociation of CH3C1 or C2H5C1 with the full arc, since the cross section is < 10 -24 cmz at 2~> 230 nm [20]. Thus, on the surface the response is red-shifted. This also occurs for CH3Br on P t ( l l l ) [78a,c]. When 0.5 ML CH3C1 and 0.5 ML CD3Br are coadsorbed on P t ( l l l ) [217], both molecules photodissociate with the full arc; but with a 300 or 360 nm cut-off filter, only CH3Br dissociates; and with a 420 nm filter, neither dissociates. This molecule specificity is very interesting and, in view of all the known data, probably reflects varying local potential barriers for electron attachment and different wavelength-dependent direct excitation probabilities. The most detailed wavelengthdependent study to date involves the photodissociation of CH3Br on P t ( l l l ) [149]. The results are summarized in fig. 22 along with the gas-phase absorption spectrum. The photodissociation on P t ( l l l ) extends to much longer wavelengths (threshold of - 400 nm) than in the gas phase. The wavelength-dependent cross section is broad and shows an interesting resonance at - 345 rim.

Comparing the work function and the photodissociation threshold, it was found that, for CH3C1 dissociation on P t ( l l l ) and C / P t ( l l l ) , only electrons above the vacuum level are effective. As discussed earlier, a nice correlation between the dissociation rate and photoelectron yield was found (figs. 19-21) by measuring both the photoelectron yield and the initial photodissociation rate as a function of CH3C1 coverage [148]. As shown in fig. 21, the dissociation rate coefficient on Pt(111) is higher for lower coverages, even though quenching is expected to be faster. Clearly, the photochemistry of the first monolayer is different from that in the multilayer regime, as it is in other metal-adsorbate systems [79a,b,139,152]. The higher rate coefficient for monolayer CH3C1 on P t ( l l l ) has been discussed earlier. Among the possible reasons, but not yet demonstrated, is an additional dissociation channel due to direct excitation of the CH3C1-Pt complex. Photolysis of 1 ML CH3CI on P t ( l l l ) separated by water spacer layers was studied, and the data were analyzed in the same way as figs. 19-21 by assuming a

150

X . - L Zhou. X.- Y. Zhu and J.M. White

102 Q

'''" Z

I • chemisorbed

°,,%

i • gas phase L

% 10

E oo 101 'o

OQ g

X

Z

~ o

100

l.U O3 O 9

0c o l 0-1 0

O

200

250 300 WAVELENGTH (nm)

350

400

Fig. 22. Photodissociation cross sections of CH3Br in the gas phase and chemisorbed on P t ( l l l ) at 148 K. After Radhakrishnan et al. [149].

DEA mechanism involving photoelectrons only [188]. The results, as a function of water spacer layer thickness, 0w, are shown in fig. 23. With increasing 0w, both the total photoelectron yield, [e], and the pseudo-first-order rate coefficient, k', decrease, whereas both the work function due to water and the photodissociation rate coefficient, k = k'/[e], increase first and then level off for 0w > 2 ML. In the gas phase, the electron attachment cross section for CH3C1 is sharply resonant (peak at 0.17 eV and F W H M of - 0 . 1 eV) [170]. Since there is probably some broadening on the surface, the data were analyzed assuming that only those photoelectrons with energies less than 0.4 eV were active for CH3CI dissociation. The results are shown in fig. 24. For this model, the decay of k ' and [el* with 0w track each other and the photodissociation rate coefficient, k* = k ' / [ e ] * is independent of 0w. This strong quantitative correlation between photoelectron yield and CH3-C1 dissociation rate firmly confirms that the underlying mechanism is dissociative photoelectron attachment. As discussed earlier, the red-shift is due to a second channel (dissociative electron attachment via photoexcited substrate electrons). At shorter wavelengths direct excitation of the adsorbate (only slightly perturbed from the gas phase) becomes important. The photodissociation threshold of chemisorbed CH3Br on P t ( l l l ) is red-shifted [149], compared with the gas phase, to well below the work function (4.5 eV) [277]. Thus, subvacuum level excited carriers (electrons) a n d / o r direct excitation of the adsorbate-substrate complex must be considered. When CH3Br is separated from P t ( l l l ) by a layer of carbon, the red-shift, compared to the gas phase, is eliminated and the average photolysis rate using the full arc decreases [223]. In this case, the threshold ( > 4.0 eV) is nearly the same as the work function (4.2 eV), again indicating that above-vacuum-level electrons may be most effective. The distinction between these various paths remains to be established, as do their relative contributions at different wavelengths; this system is a good candidate for polarization studies at temperatures at or below 77 K.

Photochemistry at adsorbate / metal interfaces

151

2.0

61

5

k(=k'/[el) A~

-0.55 1.5 ,../-.

-0.75

4

O

-0.95 ~

3

9 -1,15 ~o

-1.35

0

0.0 0

1

2

3

4

5

6

Water Spacer Layer Thickness (ML) Fig. 23. Total photoelectron yield ([e]), pseudo-firstorder rate coefficient (k'), rate coefficient (k) and work function change (za~) for 1 ML CH3CI on Pt(lll) as a function of 1920 spacer-layer thickness. After Jo and White [188].

1

2

3

4

5

6

Water Spacer Layer Thickness (ML) Fig. 24 Effective photoelectron yield ([e*]), pseudofirst-order rate coefficient (k') and effective rate coefficient (k*) for 1 ML CH3C1on Pt(lll) as a function of D20 spacer-layerthickness. After Jo and White [188].

Although photodissociation of the first monolayer CH3C1 and CH3Br is promoted on P t ( l l l ) , this is not the case for CH3I. Based on the gas-phase UV absorption cross section [20,185], one would predict that, with the full Hg arc, CH3I would photodissociate more readily than CH3CI and CH3Br. However, using XPS to distinguish halogen atoms in C H 3 X and in P t - X [143] (for example, I(3d) for molecular CH3I(a ) is 620.8 eV and for I(a) is 619.8 eV), the first monolayer CH3I photodissociated much more slowly than CH3Br. As shown in fig. 25 [143], after a 2 h irradiation of 1 M L C H 3 I / P t ( l l l ), only - 0 . 1 ML photodissociated. In contrast, irradiation of 1 M L C H 3 B r / P t ( l l l ) for only 1 h resulted in dissociation of - 0.5 M L (Br(3p) for molecular CH3Br(a ) is 182.5 eV and for Br(a) is 181.1 eV). For 1.85 ML C H 3 I irradiated for 2 h, - 90% of CH3I in the second layer photodissociates. The iodide case shows strong evidence of quenching by the metal for those molecules in the first, but not in the second monolayer. Photodissociation of multilayer CH3I, not in direct contact with Pt, is apparently quenched less strongly than for the monolayer. Molecular specificity is indicated in the wavelength-dependent photolysis of coadsorbed CH3C1 and CD3Br on P t ( l l l ) [217]. For another kind of molecule, the dihalides, bond specificity is also observed, e.g., the photolysis of CIC2H4Br on P t ( l l l ) [221]. Post-irradiation T P D shows the desorption of HC1, HBr and H 2 for _ 380 nm irradiations. However, XPS shows that, after a 30 min irradiation of 1.1 ML CICzH4Br at 105 K with > 295 nm photons, C - B r bond dissociation is significant (25%) while C-C1 bond dissociation is negligible ( < 2%). This strongly indicates that the photon-driven dissociation process is bond-specific and that most of the C - C 1 bond cleavage is thermally activated during TPD. On the other hand, when the full Hg arc is used, the C-C1 bond dissociates readily but at a slower rate than the C - B r

152

X.-L. Zhou, X.-Y. Zhu andJ.M. White

~ i ~ \ ,

624

I 300c/s

622 620 Binding Energy(ev}

4380

\

61B

1

I-'4

624

622 Binding

620 Energy(eV)

84

185

618

150c/s

183

181

179

Binding EnePgy(eV) Fig. 25. Core level spectra of halogen regions after photolysis with an unfiltered 100 W Hg-arc lamp. The noisy spectra are the raw XPS data. The smooth curves are synthesized fits using Gaussian and Lorentzian profiles. (A) l(3d) spectrum for monolayerCH3I on Pt(lll) and photolyzedfor 2 h. (B) I(3d) spectrum for 1.85 ML of CH3I photolyzed for 2 h. (C) Br(3p) spectrum for monolayer CH3Br photolyzed for 1 h. The numbers beneath the panel labels are integrated peak areas. After Liu et al. [143].

bond. For example, after a 30 rain irradiation of 1 ML C1C2H4Br at 105 K, 41% of the C - B r bonds dissociate but only 28% of the C-C1 bonds; and for 3 ML C1C2H4Br, 30% of the C - B r bonds and only 14% of the C-C1 bonds photodissociated. The T P D results, which show that both bonds are clearly broken regardless of the wavelength, are attributed to thermal decomposition of C - C I bonds in CIC2H 4 fragments formed by photolysis. Thus, the wavelength dependence of C1-C2HnBr and C1C2H4-Br photodissociation is similar to CH3C1 and CH3Br on P t ( l l l ) , and for C1C2H4Br on A g ( l l l ) [222]. 4.1.3. A l k y l halides on Ag(111)

Using XPS, UPS, TPD, and zae#, the photochemistry of CzHsC1 [79a,147], CH3Br [79b,147], CH3I [79c], C6H5C1 [146,220] and C1C2H4Br [222] have all been studied on A g ( l l l ) at 100 K.

Photochemistry at adsorbate / metal in terraces I

153 I

--

Optical Absorption Cross section (gas phase) Photodissociation Cross Section 0 m v

Bromide

~

/

/'

°0~

"5

0

o o -1

Bromide -2

1 4

I 5

I 6

Energy/eV Fig. 26. Comparison of gas-phase absorption cross sections and photodissociation cross sections for 1 ML C2H5C1 and CH3Br on A g ( l l l ) (logarithmic scale). The zero of the ordinate corresponds to a cross section of 10 -20 cm 2. After Zhou and White [147].

Compared to most other metals, the interaction of these molecules, and their hydrocarbon fragments, is quite weak. The parent molecules all adsorb weakly with little distortion, compared to the gas phase, and only CH3I decomposes during TPD (35%), forming adsorbed C H 3 and I [82]. With a Hg arc lamp, photon-driven C-X bond dissociation occurs for all coverages. In post-irradiation TPD only the alkyl fragment recombination product, R 2, and AgX (X = CI and Br) or X (X = I) desorb, except for C1C2H4Br, where C2H 4 and AgX desorb. The presence of these products is interpreted in terms of a surface photochemical process that breaks C-X bonds to form surface R and X. Depending on the adsorbate, some R groups desorb into the gas phase during UV irradiation, but quantitative XPS analysis indicates that all of the X and a large fraction of the R groups are retained. It is noteworthy that, unlike Pt(lll), Ag(lll) is very inactive with respect to C-C and C - H bond breaking; on the other hand, it is very active with respect to C - C bond synthesis and R 2 is the dominant product when alkyl fragments are involved. Photodissociation of these molecules on Ag(lll), as on Pt(lll), is wavelength-dependent. The physisorbed multilayers and chemisorbed monolayers have different wavelength dependences and different dissociation cross sections. Compared to the gas phase, the photon energy thresholds are red-shifted for monolayers of all the adsorbates; the red-shift largely disappears for multilayers. Typically, the wavelength dependence is much weaker on the surface than in the gas phase. Fig. 26 compares photodissociation cross sections for chemisorbed C2H5C1 and CH3Br on A g ( l l l ) with the gas-phase optical absorption cross sections [147]. The first continuous band for gas-phase C2H5C1 and CH3Br is assigned to a n ~ o* transition [20,185]. The absorption cross section, after passing the maximum (7.2 eV for C2H5C1 and 6.1 eV for CH3Br), decreases rapidly with increasing wavelength at a rate of about one order of magnitude per 0.25 eV. In

154

X . - L Zhou, X.- Y. Zhu and J.M. White

contrast, the photodissociation cross sections of the adsorbed halides change much less dramatically. For example, reducing the photon energy from 5 to 3.5 eV lowers the cross section by only one order of magnitude. Above the threshold ( - 3.0 eV for CHaBr and 3.3-3.5 eV for C2H5C1), the surface cross section increases monotonically with photon energy. We attribute a major part of the dissociation to substrate excitation since the excited metal electron yield increases with photon energy. For physisorbed multilayers of C2HsC1 and CH3Br, the photolysis rates are lower and the thresholds are higher ( - 4.1 eV for CEHsC1 and 3.5-4.0 eV for CH3Br ) than for the chemisorbed monolayer [79a,b,147]. Based on the gas-phase optical absorption spectra, photolysis is not expected above 254 nm for CEHsC1 nor above 280 nm for C H 3Br. Comparing the thresholds and the work functions ( ~ 4.0 eV for 2 ML C 2H 5Cl/Ag(111) [79a] and 2 ML CH3Br/Ag(111 ) [79b]), the red-shifted photodissociation for the physisorbed C2HsCI, as for CH3C1 on Pt(111) [148] can be readily attributed to above-vacuum-level electrons while, for CH3Br, subvacuum excited electrons below (but very close to) the vacuum level may also contribute. Electronic quenching is expected to be faster in the first layer than in the second layer, but the rates are reversed. Presumably, the excitation rate in the first layer is much higher than in the second layer and this more than compensates the enhanced quenching. For both C2HsC1 and CH3Br, the average photolysis yields decrease with increasing initial coverages, an observation attributed to exchange of energy between neighboring adsorbates (self-quenching) a n d / o r changes in the local potential which controls the rates of electron attachment and transfer back to the metal (see section 4.1.7). As on Pt(111), the case for CHaI is different. On Ag(111), monolayer CH3I is photolyzed at a slower rate than multilayer, indicating sufficient quenching to compensate any enhancement in the excitation probabilities [79c]. The photodissociation of 1 ML CH3I/Ag(111 ) extends to 2.6 eV, 1 eV lower than in the gas phase (3.6 eV). This red-shift is largely lost in the physisorbed multilayer. Assuming that gas-phase absorption trends hold for adsorbed halides, direct excitation will be more important with adsorbed CH3I than CzHsC1 and CH3Br. Unlike halides on P t ( l l l ) , the photodissociation rate on A g ( l l l ) with the full arc is, in fact, on the order of C2HsC1 < CH3Br < CH3I, just as in the gas phase. This indicates that, though quenching of CH3I on Ag(111) is significant, it is weaker than on P t ( l l l ) and even weaker than for CHzI 2 on Ag, where, as discussed above, no photodissociation occurs in the first layer [165]. For monolayer halides on A g ( l l l ) , the red-shift in the thresholds is chloride (2.0-2.2 eV) > bromide ( - 1.5 eV) > iodide ( - 1.0 eV). The thresholds themselves are: chloride (3.3-3.5 eV) > bromide ( - 3 . 3 e V ) > iodide ( - 2.6 eV). This spacing is much closer than in the gas phase. In early work, the red-shifted photodissociation was discussed in terms of direct adsorbate-substrate excitation involving a stabilized orbital constructed from the antibonding o* (C-X) orbital of the halide with Ag orbitals [139,146]. However, to date, there is no direct evidence that such processes contribute in the red-shifted regime. The substrate-excited electron mechanism, as evidenced by dynamical studies of alkyl halides on Ni(111) (see section 4.1.4), was proposed to explain the A g ( l l l ) data. This was confirmed by the correlation between substrate-excited electron yields and C-CI bond cleavage rates for C H 3 C 1 / P t ( l l l ) [148], and polarization evidence indicates that substrate excitation is responsible for red-shifted photolysis [150,155]. Within the framework of substrate-mediated electron-induced dissociation, the observed threshold trend and its molecular specificity can be described in terms of electron tunneling through the local potential barrier at the adsorbate/substrate interface. Assuming the local potential scales with the monolayer work functions (3.75 eV for C2HsC1, 3.65 eV for CH3Br and 3.45 eV for CH3I ), we predict a lower effective threshold for tunneling, and, therefore, a redder threshold, in the iodide than in the bromide or chloride. Moreover, the local potential barrier may also depend on the adsorbate-substrate interaction strength, i.e., stronger

Photochemistry at adsorbate / metal interfaces I

I

~ (m/e=112)

/~/

155

I

Ie=154)

AgCI

(xl)

(x45)

1

r"

a

c

(D

E o

15¢

I

t

300

450

z//

I

800

950

Temperature/K Fig. 27. T P D spectra of C6HsC1, C12H10, and AgC1 for 1 ML C 6 H s C I / A g ( l l l ) : (a) no UV irradiation; (b) after a full arc irradiation, with a 100 W arc lamp, of 40 min; and (c) 90 rain. After Zhou and White [146].

interactions lead to a lower interfacial potential barrier (on A g ( l l l ) , the bond strengths are ordered iodide > bromide > chloride). As a result, electrons may be available for dissociative attachment processes at lower photon energies for the iodide. The molecular electron affinity, which is CH3I > CH3Br > C2H5C1 [278], should also contribute; i.e., the electron attachment process may involve a rather narrow resonance. In this description, the thresholds are determined mainly by the local potential barriers. Based on the photon energy threshold and the work function at 1 ML, the minimum electron energy, which has activity for the dissociation of the monolayer halides, is - 0 . 8 5 , - 0 . 7 and 0.25-0.45 eV below the vacuum level for iodide, bromide and chloride, respectively. Although different for various halides, the difference is not as great as in the direct gas-phase excitations. There, in contrast, the threshold photon energies depend largely on the energy gap between n and o* orbital energies, and these shift by - 1 eV in passing from chloride to bromide and another eV from bromide to iodide. In another kind of comparison, the cross section for photon-driven C-C1 bond cleavage in C2HsC1 is - 10 times that for CrHsC1. This demonstrates that the structure of R influences the photochemistry of R - X . Part of this influence may be the result of different bonding configurations for CrHsC1 and CEHsC1; i.e., the former lies flat (,~-bonded) on the surface, while the latter bonds to Ag through C1 lone-pair electrons with CzH 5 pointing away from the surface. Turning to non-alkyl halides, fig. 27 shows the post-irradiation T P D spectra of 1 M L C6HsC1 on Ag(111). The only products are biphenyl and AgC1. The formation of biphenyl was interpreted as recombination in T P D of phenyl fragments produced by photolysis at - 100 K. This interpretation was supported by experiments in which the phenyl groups were of two kinds, perdeutero and perhydrido, which are mixed in the biphenyl products. The results provide evidence that biphenyl is formed during T P D and not during irradiation at low temperatures [273]. Song et al. [220] disagree, based on vibrational H R E E L S data, and

156

X.-L. Zhou, X.- Y. Zhu and J.M. White

conclude that biphenyl is formed during irradiation, probably by reaction of photoproduced phenyl fragments with neighboring chlorobenzene molecules. For multilayer coverages, they observed Ca2HgC1 in the post-irradiation TPD. As it is unlikely that large concentrations of phenyl groups can be made thermally, this system, and others like it, illustrate the synthetic usefulness of low-temperature photon-driven processes. The photodissociation of CrHsC1 in the gas phase occurs at wavelengths shorter than 280 nm and is attributed to a ~--* "~* transition followed by intersystem crossing [279]. This dissociation is probably inoperative for CrHsC1 adsorbed on Ag(111) because electronic quenching is expected to be much faster ( < 10 -13 s) than intersystem crossing (10 -9 s). For 1 ML coverage, the photodissociation cross section is - 3 × 10 -21 cm 2 at 254 nm; it decreases monotonically to a threshold of 325-350 nm, which is red-shifted compared to the gas-phase threshold. As for the alkyl halides, the photodissociation is best ascribed to capture of photoexcited hot electrons by adsorbed CrHsC1 to form C6HsCI . Although capture of thermal electrons by CrHsCI in the gas phase does not lead to dissociation, dissociation of C6HsC1may occur on A g ( l l l ) via interaction with Ag atoms of the substrate. Using even simpler halides, Polanyi and co-workers recently reported the photodissociation of HC1 and HI on Ag(111) using 248, 308 and 350 nm laser pulses [145,153]. For HC1 on Ag(111), no photodissociation occurred at either 308 or 350 rim, but the photodissociation cross section at 248 nm was 6 × 10 ~9 cm 2, _ 200 times greater than that for gas-phase photolysis of HCI, (3 _+ 2 ) × 10 -21 cm 2. With increasing C1 coverage (and an attending work function

t (b)308nrn~

o.o

Time

of Flight

(Its)

41.o

Fig. 28. TOF spectra of atomic H resulting from photodissociation of HCI on X e / K / A g ( I l l ) (c) 350 nm. After Dixon-Warren et al. [153].

at (a) 248, (b) 308, and

Photochemistry at adsorbate / metal interfaces

157

increase), the cross section decreased by - 2 orders of magnitude. As for the halides described above, these results were attributed to dissociative electron attachment. The work function of H C 1 / A g ( l l l ) , though not measured, is likely lower than for Ag(111) ( - 4.7 eV); for a similar system, C H 3 C I / A g ( l 1 1 ), the work function is - 3 . 9 eV [82]. If so, at 248 nm (5 eV) the pulsed-laser irradiation will create electrons with energies sufficient to surmount the work function barrier, and these can dissociate adsorbed HCI via DEA. In contrast, the 308 nm (4.0 eV) and longer wavelength photons can excite only subvacuum level electrons and these apparently do not attach to adsorbed HC1, either because a negligible fraction can tunnel into the region of space where HC1 is located or because the attachment resonance energy is higher (the DEA resonance of HCI in the gas phase is - 0.5 eV [169,171]). As C1 is added, the work function increases by as much as 1.8 eV [280]. Therefore, with increasing C1 coverage, both the yield and energy of photoelectrons will decrease, resulting in a lower dissociation rate. Consistent with this description, they found, using T O F measurements, that no H atoms were detected during the 248 nm laser irradiation of HC1 on Xe/Ag(111) and inferred that the cross section was < 10-19 c m 2. In this case, condensed HC1 will not affect the electron potential barrier at Ag(111) ( - 4.7 eV) because it is separated by Xe layers, and 248 nm photons will create only < 0.25 eV photoelectrons - below the DEA threshold of HC1 in the gas phase ( - 0.5 eV). In a key experiment, potassium was added to Ag(111), lowering the work function to - 2.3 eV. For HC1 on X e / K / A g ( 1 1 1 ) , photodesorbing H atoms, from the photodissociation of HC1, are readily detected at 248, 308 and 350 nm (fig. 28) [153]. The average kinetic energy of H at 248 and 308 nm is 0.6 eV, and slightly lower at 350 nm. The cross section at 248 nm is -- 4 × 10--19 c m 2. The low work function allows above-vacuum-level photoelectron production for all these wavelengths. For X = 350 nm, the m a x i m u m electron energy is 1.2 eV above vacuum, sufficient to dissociate HC1 provided the gas-phase electron attachment resonance is not altered strongly. For H I on Xe/Ag(111) and X e / K / A g ( 1 1 1 ) , the photodissociation also extends to longer wavelengths when the work function is lower [153].

4.1.4. Alkyl halides on N i ( l l l ) and B r / N i ( l l l ) Cowin and co-workers, using T O F and TPD, have performed revealing dynamic studies of the photolysis of CH3C1 adsorbed on Ni(111), with [144a,b] and without [144c] spacer layers, and of CH3Br on Br/Ni(111) [5]. The light source was a pulsed laser with wavelengths at 193, 248 and 351 nm. CH3CI adsorbs weakly on Ni(111) and desorbs molecularly without thermal decomposition. From submonolayer to multilayers, photolysis takes place at 193 and 248 nm, but not at 351 nm. T O F showed ejection of C H 3, CI and CH3C1 at 193 nm; only C H 3 and CH3C1 were ejected at 248 nm. Single photon non-thermal desorption of molecular CH3CI was observed above - 3 ML and attributed to desorption driven by collisions of fast fragments with surrounding parent molecules. Fig. 29 shows the T O F of C H 3 fragments at 193 and 248 nm for various coverages [144]. N o CH3 fragments were detected below 1.5 ML; although photofragmentation still took place (with reduced probability), the fragments were retained. At 193 nm, there are two T O F peaks, one corresponding to ( E t r a n s ) = 1.3 eV and the other to ( E t . . . . ) = 0.6 eV. The 0.6 eV peak disappears at - 20 ML, while the 1.3 eV peak persists even at higher coverages. At 248 nm, there is only one T O F peak at ( E t.... ) = 0.6 eV, and it disappears at - 8 ML. The common peak energy (0.6 eV) for these two excitation energies indicates a common wavelength-independent precursor to the low-energy photofragment. They measured the total cross section (molecular desorption plus dissociation) as a function of CH3C1 coverage using T P D areas of CH3C1 before and after photolysis. The cross section at 248 nm increases nearly linearly with the coverage, maximizes at 4 ML (Omax = 2 × 10 -20 cm2),

158

X.-L. Zhou, X.-Y. Zhu andJ.M. White

!:I TOF at 193 nm CH3 •~,.~ ; ~ 2 6 •:

TOF at248nm

a)

ii

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6 Monolayers

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8 Monolayers

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10 Monolayers i

i ~

6 Monolayers i~k,~j~ ~ n ! '

ayer s

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[

[

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l

100.0

Time (/L se¢)

[

L

150.0

0.0

I

I

I

Monolayers I

50.0 100.0 Time (F sec)

I

I

150.0

Fig. 29. TOF spectra of CH 3 from photodissociation of CH3CI on Ni(lll). The peak at time zero is due to scattered light: (a) 193 nm and (b) 248 nm pulsed-laser irradiation. After Marsh et al. [144a].

and then decreases. The same trend was found at 193 nm, but the cross section is - 10 times larger. Cowin et al. also studied the photolysis at 248 nm of CH3CI on Ni(111) as a function of thickness of H 2 0 (0w) and Xe (0xe) spacer layers (fig. 30). With increasingly thick H 2 0 spacer layers, the T O F C H 3 signal from 1 M L CH3C1 first increases, peaking at 0w = 1 ML, and then exponentially decays. The total cross section, determined from T P D , increases, maximizes at 0w = 2 ML, and then decreases. In striking contrast, when Xe is the spacer layer, the C H 3 T O F signal from 4 M L CH3C1 is present even at 0xe = 64 ML. Earlier, other work was discussed that involved direct measurement of the photoelectrons associated with these spacer layer experiments. In the gas phase, photolysis of methyl chloride at 193 n m is well-known [20]. The C H 3 p h o t o f r a g m e n t energy is about 1.7 eV, taking account of the C - C 1 b o n d strength, the p h o t o n energy and the vibrational energy of C H 3. Thus, the high-energy p h o t o f r a g m e n t observed b y Cowin and co-workers [144] at all coverages, with an energy onset of 1.7 _+ 0.06 eV, is attributed to direct excitation. Consistent with this model, fast methyl desorption is absent at 248 nm, where gas-phase-like direct absorption is not expected. The low-energy 0.6 eV peak at both 193 and 248 n m was attributed to substrate-mediated D E A . In the presence of water spacer layers, the C H 3 peak appeared at 0.74 eV [144c]. In the gas phase, the C H 3 kinetic energy is lower [144c]. T h e y hypothesized that the several tenths of an eV extra energy seen in their study is due to shifts of the molecular C H 3 C I - anion dissociation potential energy surface by interactions with neighboring polarizable molecules. T h e y made no distinction between

Photochemistry at adsorbate / metal interfaces

159

,-. 50 =

~

40

t

0

Cross section

3

O M e t h y l Signal

+o

30

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~-.

2

o =

0 0

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'

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i

i

4

i

6

•

(

8

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Water Spacer Depth (monolayers)

"~

100 80

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o

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-

i

2

-

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i

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Xenon Spacer Depth (monolayers) Fig. 30. Upper panel: A double Y plot of the cross section and CH 3 T O F fragment yield versus the H 2 0 spacer depth for photolysis of 1 ML CH3CI on N i ( l l l ) at a laser wavelength of 248 nm. Lower panel: A plot of CH 3 T O F fragment yield versus Xe spacer depth for photolysis of 4 ML CH3CI on N i ( l l l ) at a laser wavelength of 248 nm. After Gilton et al. [144c].

electrons with excitation energies above and below the vacuum level. Based on recent results for CHaC1 on P t ( l l l ) [188], we expect that energies above the vacuum level are required. In this regard, we note that the estimated 2.5 eV work function decrease by adsorption of CH3C1 on N i ( l l l ) [144a] is too high; a recent measurement gives - 1 . 2 eV [144d]. Thus, 351 nm photons will not be able to produce photoelectrons. Photoelectrons are certainly produced by 248 and 193 nm photons. On the basis of relatively narrow T O F peaks, the C H 3 T O F signal was attributed to photofragmentation in the topmost layer. That no CI photofragment appears corresponding to the 0.6 eV C H 3 peak was discussed in terms of CI-, produced from CH3CI----~ C H 3 + CI-, which, because of low kinetic energies and attractive image forces, does not eject at all [144]. The fact that only a limited adsorbate thickness gave the low-energy methyl peak was attributed to the attenuation of excited substrate electrons. Thicker CH3C1 layers were photolyzed at 193 nm than at 248 nm. This can be attributed to a combination of the higher energy and higher yield of substrate electrons produced at 193 nm. As directly and quantitatively established by Jo and White [188], the electrons above the vacuum level have very short

160

X.-L. Zhou, X.-Y. Zhu and J.M. White

mean free paths in water and in CH3C1 overlayers, but a long mean free path in Xe overlayers (see section 3.2). Moreover, the energy distribution varies with spacer layer thickness, particularly in the first few monolayers. Thus, photolysis attenuates rapidly as 0W or 0cH3c ~ increases. In contrast, since photoelectrons have long mean free paths in the Xe layer, photolysis does not disappear even at 0xe = 64 ML. The CH3C1 or H 2 0 coverage-dependent cross section and photolysis yield are very different from those found for chlorides on Ag(111) [79a] and Pt(111) [148], where the photolysis of the first layer is more effective than the higher layers and where, as discussed earlier for Pt(111), the pseudo-first-order rate coefficient, which is comparable to the cross section in Cowin's work, decreases monotonically with increasing CH3C1 coverage or H 2 0 spacer layer thickness. While the difference between these cases is, at present, not clearly understood, it may reflect a much stronger quenching probability on Ni(111) than on either Ag(111) or Pt(111). In another important study, CH3Br on Ni(111) [5], Cowin and co-workers passivated the surface with bromine using several cycles of exposure at 65 K and heating to 550 K. This was necessary to prevent thermal decomposition of the parent molecule. On the passivated surface, which contains - 0 . 6 ML Br and - 0 . 1 5 M L C, adsorbed CH3Br is thermally stable and desorbs molecularly at - 123 K for the first monolayer and at - 118 K for multilayers. They measured the total photolysis cross section (photofragmentation and desorption) using TPD, in the same way as for CH3C1/Ni(111 ). At 193 nm, the cross section (in units of 10 -20 cm 2) is 3.5 +_ 0.3, 8.4 _+ 0.6, 7.2 _+ 0.5, 11 +_ 1 and 3.9 +_ 0.4 for 0.5, 1.0, 2.0, 3.0 and 10 ML, respectively, compared to the gas-phase value of 4.4 × 10 -19 c m 2. At 248 nm, it is (2.6 + 0.5) × 10 -21 c m 2 at 4 M L (the gas-phase value is 1.27 x 10 -20 cm2). At 193 nm, photoejection of C H 3 fragments from CH3Br was detectable provided the coverage was above 0.5 ML. This underscores the importance of fragment retention. Ejection of molecular CH3Br also occurred and was attributed to collisions with fast C H 3 and Br fragments. A fast, narrow, gas-phase-like C H 3 peak (4.8 x 105 c m / s , 1.79 eV, peak A in fig. 31) was observed, consistent with photodissociation of a directly excited C H 3 B r , with the C H 3 end up, without suffering energy loss due to collision with other molecules. There is also a slow C I q 3 peak (3 × 105 c m / s , 0.70 eV, peak B in fig. 31) which maximizes in intensity at intermediate coverages and disappears at high coverages. The integrated C H 3 T O F increases rapidly with coverage to 5 ML and then becomes nearly constant. Br is also found in TOF, and its velocity distribution is interesting (fig. 32). Compared to expectations based on gas-phase photolysis, the center of the measured Br distribution has about the expected velocity, but the high velocity limit is nearly twice what is expected. This was discussed in terms of m o m e n t u m transfer between a rebounding methyl and Br, which requires some adsorbed CH3Br oriented with the Br end up. At 248 nm and above 1 ML, C H 3, but no Br, was detected in T O F [5]. The integrated C H 3 T O F signal increases rapidly with coverage up to 4 ML, then decreases to a roughly constant value by - 10 ML. The c n 3 velocity distribution, unlike that at 193 nm, shifts to higher velocities with coverage and exhibits two distinct peaks (3.0 x 105 c m / s , 0.7 eV and 4.0 x 105 c m / s , 1.24 eV) at 6 ML. As for methyl chloride, the slow peak disappears at high coverages, while the fast one remains. Based on the C - B r bond energy in an isolated molecule, the photon energies used, and the vibrational energy of the C H 3 photofragment, Cowin and co-workers predicted C H 3 fragment velocities of 5.9 × 105 c m / s , 2.71 eV, and 4.0 × 105 c m / s , 1.24 eV, at 193 and 248 nm, respectively, and argued that these are consistent with their measurements, at least at the onset of the fast peak. They concluded that only direct photolysis of adsorbed CH3Br with C H 3 pointed away from the surface can yield such swift C H 3 fragments. The slow peak, 3.0 × 103 c m / s , 0.7 eV, at both 193 and 248 nm, is attributed to D E A involving

Photochemistry at adsorbate / metal interfaces 1

I

i

161

I

Methyl Velocity Distribution 193 nm

Ionolayers

molayers

noJayor~5

nolayers

nolayers

0

2

4

6

8

( V e l o c i t y 10 5 c m / s ) Fig. 31. Velocity distribution of CH 3 fragment from photodissociation at 193 nm of CH3Br on B r / N i ( l l l ) . The direct photolysis peak is labeled A, the electron attachment peak is labeled B and the CH3Br cracking peak is labeled C. Also a representation of the gas-phase photolysis peak (solid line) is overlaid for reference. Each velocity distribution plot is self-normalized. After Marsh et al. [5d].

photoelectrons. This resonant process, as in C H 3 C 1 / N i ( l l l ) , requires electrons with a narrow range of energies, independent of incident photon energy, and occurs only within a limited thickness of the CH3Br overlayer due to the short attenuation length of these photoelectrons. The absence of Br at 248 nm was attributed to the low cross section, which reduced the signal below detection limits. It is also important to account for attractive forces between B r - and the surface. Compared to higher coverages, a broader, lower intensity T O F and smaller cross section were indicated below 1.5 ML and attributed to enhanced electronic quenching. Using this as a model and making a simple classical calculation which defines the molecules as dissociated on the repulsive potential curve when 90% of the available potential energy has been converted into kinetic energy, they estimated a C H 3 - B r dissociation time of - 1 5 fs [5d]. They also estimated, by comparing T O F intensities and cross sections at submonolayer and multilayer coverages, that the charge transfer time is 6 fs or less. At submonolayer coverages, dissociation was attributed mainly to electronically quenched but highly vibrationally excited ground states.

162

X.-L. Zhou, X.-Y. Zhu andJ.M. White

4.1.5. A l k y l halides on other metal surfaces

R o o p et al. [224] reported that photodissociation of C H 3 B r on Ru(001) and C u / R u ( 0 0 1 ) to C H 3 and Br occurred readily when irradiated with > 230 n m photons; adsorbed C H 3 was identified using H R E E L S [224]. O n C u / R u ( 0 0 1 ) , post-irradiation T P D showed desorption of C H 4, and a small a m o u n t of C2H4, which were absent without U V irradiation. In an interesting process of a different kind, surface isomerization, Grassian and Pimentel [168] used b r o a d - b a n d irradiation ((A > 200 nm) to study photochemical reactions of cis- and trans-l,2-dichloroethene (C1CHCHC1) adsorbed on Pd(111) and P t ( l l l ) at 110 K. T h e y found that chemisorbed C1CHCHC1 is di-o-bonded at 110 K with the chemical structure analogous to dichloroethane. This species thermally decomposed to Cl(a) and C2H2(a ) on b o t h surfaces at 270 K. F o r multilayer coverages of either cis- or trans-C1CHCHCI on P t ( l l l ) and P d ( l l l ) , they found that U V irradiation (h > 200 rim) resulted in photolytic isomerization with a steady-state mole ratio, c i s / t r a n s = 0.67 + 0.20. There was some evidence for a small a m o u n t of C - C 1 and C - H b o n d cleavage. For chemisorbed monolayers on both surfaces, photolysis resulted in C - C I b o n d cleavage. For > 237 n m irradiation, b o t h C1 and C2H 2 were retained, but when the wavelengths were as low as 200 rim, only C2H 2 was retained. By considering reaction energetics, they concluded that the photodissociation resulted from directly excited singlet states; but 1,2-dichloroethane absorbs only X < 220 n m p h o t o n s [168], so this assumption requires red-shifted spectra associated with the a d s o r b a t e - s u b s t r a t e complex. Because more recent work shows the importance of substrate excitations, this interpretation should be re-examined. There were two favored reaction channels from the excited singlet: one is formation of b o u n d C1 and C2H 2 (RX1) and the other produces b o u n d C2H 2 and gaseous

|

Br Velocity

I

Distribution

193

nm

--Actual

jr.~ ¢..-. ; -,e-v---'N 11

/ ~ -... ~

Br

Distribution

- - - CH3Br Cracking

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/ :

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t'.ff"

Gas Phase

0

i

I

1

1

2

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Fig. 32. Velocity distributions of Br photofragmented from CH3Br on Br/Ni(lll) at 193 nm. The apparent Br distribution is shown as a dotted line, the CH3Br cracking contribution is dashed, and the actual Br distribution is shown as a solid line. Also a gas-phase photolysis peak (solid line) is overlaid for reference. All distributions are normalized to the same height. After Marsh et al. [5d].

Photochemistry at adsorbate / metal interfaces

163

excited CI~' (RX2) [168]. RX2 may be due to direct excitation, which dominates at short wavelengths, while RX1, which is red-shifted compared to the optical absorption spectrum of C1C2H4C1, might be attributed to substrate excitation involving electron attachment to form C1C2H2C1- or CIC2H2C1 z-, which then dissociates. Solymosi and co-workers [218], using TPD and UPS, studied photodissociation of CH3C1 adsorbed on Pd(100) and K/Pd(100) at 90 K. Broad-band continuous irradiation ( > 230 nm) indicated that, on clean Pd(100), photodissociation occurred readily, producing surface CH 3 and C1. On K/Pd(100), they found enhanced photon-driven C-CI bond cleavage, just as on clean Pd(100) under the same irradiation conditions. They attributed the photodissociation to substrate excitation. Since K lowers the work function, thus increasing the photoelectron yield, an enhanced photodissociation rate is expected. It would be interesting to study the electron energy distribution of this system, or one like it, in view of recent work indicating that electrons with kinetic energies in excess of 0.4 eV do not readily attach to adsorbed CH3C1 [188].

4.1.6. Alkyl hafides on semiconductor and insulator surfaces For comparison with photochemistry of alkyl halides on metal surfaces, we summarize some relevant results on semiconductor and insulator surfaces. Schneider et al. [157] studied CH3CI adsorbed on GaAs(100) at 40 K with pulsed lasers (193 and 248 nm). CH3C1 adsorbs weakly on GaAs(100) at 40 K with little thermal decomposition ( < 4% monolayer). At 193 nm, there were two distinct T O F peaks of CH 3 with energies of 0.56 and 1.23 eV. The 0.56 eV peak intensity increases with coverage and maximizes at 6 ML and then decreases to below the detection limit above 20 ML. The 1.23 eV peak intensity also increases with coverage and becomes constant above 6 ML. At 193 nm, CI and CH3C1 also appeared in TOF. There is a single T O F peak for C1 with an energy of 0.2 eV. CH3CI has a broad slow velocity distribution with a peak energy of 0.2 eV. The T O F intensities of both CI and CH3C1 increase from submonolayer to 6 ML and then decrease to 70% and 35%, respectively, of their maximum intensities. At 248 nm, there was a broad slow peak for CH3CI above 2 ML (attributed to molecular ejection due to collisions of fragments from neighboring molecules), the 0.56 eV T O F peak for CH3, and no signal for C1. The CH 3 T O F peak maximizes at - 4 ML and disappears above 10 ML. The cross sections are nearly one order of magnitude higher than for CH3CI on N i ( l l l ) [144], particularly at low coverages, but the same photolysis mechanisms are involved, i.e., direct excitation for the 1.23 eV peak of C H 3 and substrate excitation for the 0.56 eV peak of C H 3. Polanyi and co-workers [158,159] studied the dynamics (translational energy distribution and desorption angular distribution) of photodissociation and photodesorption at 222 nm of submonolayer CH3Br physisorbed, but aligned, on LiF(001) at 115 K. In the gas phase, photolysis of CH3Br at 222 nm produces equal amounts of ground-state Br(ZP3/2) and excited state Br* (2P1/2) separated by 0.456 eV. The corresponding CH 3 fragments have kinetic energy distributions peaking at 2.1 and 1.75 eV, respectively [281]. On LiF, both non-thermal photodissociation and molecular photodesorption of CH3Br were observed. The yield and dynamics of photodissociation and photodesorption depended on coverage, treatment of the crystal, laser fluence, and detection angle with respect to the surface normal. The photodesorption yield increases linearly with coverage, maximizes at 0.1 L dose and drops sharply for doses higher than 0.55 L; i.e., multilayer CH3Br starts to form. The yield also increases linearly with laser fluence. The translational energy distribution of CH3Br has a peak ranging from 0.01 to 0.1 eV with the corresponding F W H M from 0.04 to 0.27 eV (i.e., high-energy tailing), depending on the laser fluence, coverage and detection angle. The peak energy and F W H M increase with laser fluence and coverage but decrease with detection angle. The angular distribution of CH3Br depends strongly on coverage (expressed here as the dose in

164

X . - L Zhou, X.- Y. Zhu and J.M. White

langmuir, L): cos 0 (0.001 L), cos30 (0.004 L), cosS0 (0.007 L), cos80 (0.016 L), cos3°0 (0.12 L) and cosT0 (0.6 L). The coverage-dependent angular distribution is explained in terms of the increased hindrance with coverage of wide-angle desorption of CH3Br. Photodesorption is also observed at 308 n m where CH3Br is transparent. This result is attributed to photoacoustic excitation of the substrate induced by UV absorption of solid-state color centers. Due to acoustic mismatch of CH3Br multilayers with the substrate, the photoacoustic energy reaching the surface of the multilayers is expected to be less than that reaching the LiF surface, and the photodesorption yield from CH3Br ice is much lower. The photodesorption occurs after the laser pulse (15 ns), since propagation of the photoacoustic disturbance to the surface requires a mean transit time of 200 ns [158]. Unlike photodesorption, photodissociation occurs only at 222 nm, not at 308 nm [158]. On annealed LiF(001), the photodissociation yield (integrated C H 3 T O F signal) increases linearly with laser fluence and increases linearly with coverage at low coverages, but reaches a limiting value for doses > 0.1 L. The translational energy distribution of C H 3 depends strongly on surface treatment. On unannealed, rough LiF(001), there exists a wide range of possible adsorption sites for CH3Br. The translational energy distribution at monolayer coverage (ranging from - 0.1 and 2.28 eV) is very broad and displays low-energy tails, and the average translational energy is low. On annealed LiF(001), the distribution, with a peak energy of 1.7 + 0.1 eV and a F W H M of 0.54 eV, is narrower and the average translational energy is higher. The broader and lower translational energy on an unannealed rough surface is attributed to CH3Br adsorbed on defect sites where it is coupled more strongly to the surface. Even on annealed LiF(001), there are some defect sites on which CH3Br is preferentially adsorbed. Thus, for low coverages on annealed LiF(001), the T O F peak of C H 3 is broader than for high coverages. For thick CH3Br ice on both annealed and unannealed surfaces, the translational energy distribution of C H 3 is similar to submonolayer on annealed LiF(001), except that the translational energies are about 0.4 eV lower. This suggests that CH3Br ice overlayers are relatively well-ordered and homogeneous. The maximum translational energy of C H 3 for submonolayer CH3Br is close to the gas phase, but overall the distribution is heavily weighted toward the C H 3 + Br* branch. The authors speculate that one of the reasons for the shift of the maximum kinetic energy of C H 3 from submonolayer to multilayers (0.446 eV), close to the excitation energy from Br to Br* (0.456 eV), could conceivably be the exclusive production of Br* in multilayers. The overall shift (peak and maximum energies) could also result from the lateral binding in the CH3Br ice which increases the effective mass of the C H 3, with the result that the energy partitioning is more in favor of Br recoil [159]. The maximum C H 3 translational energy is independent of detection angle, indicating that there are no collisions of C H 3 fragments with photodesorbing species. On rough LiF(001), the angular distribution of photodesorbing C H 3 fragments is cos 0 for CH3Br coverages from submonolayer to monolayer. On annealed LiF(001), the distribution varies from - c o s 2 0 at 0.01 L to - c o s 5 0 at 0.18 L and there is no low-energy tail on the translational energy distribution of C H 3, suggesting that the detected C H 3 fragments did not rebound off the surface. The authors take this, along with the narrow surface normal-peaked angular distribution of C H 3, to indicate that CH3Br is oriented with the C H 3 - B r bond axis in the direction of the surface normal with Br bonded to the surface. The translational energy distribution of photodesorbing Br for 0.001 L dose of CH3Br on annealed LiF(001) is broad with a peak at 0.22 eV, a F W H M of 0.39 eV and a m a x i m u m energy of ~ 1 eV. Interestingly, as for CH3Br on B r / N i ( l l l ) [5d] (see fig. 32), about 1 / 3 of Br fragments have kinetic energies higher than the maximum (0.43 eV) in the gas phase. The explanation is the same; i.e. some CH3Br orients with C H 3 down and Br up and C H 3 rebounds

Photochemistry at adsorbate / metal interfaces

165

between Br and the surface, resulting in a total of 0.57 eV energy transfer from C H 3 to Br. The angular distribution of Br is cos 0 and the translational energy distribution is angle-independent. Tabares et al. [225] studied multilayer CH3Br on LiF(001) at 30 K. At 193 nm, the gas-phase optical absorption cross section of CH3Br is 4.5 x 10 -19 c m 2. Unlike on LiF(001) at higher temperature, 115 K [158], the C H 3 translational energy distribution, though it has nearly the same maximum energy as in the gas phase, is much broader, with a long low-energy tail. Significantly, the authors suggest that CH3Br does not wet (spread out into a uniform equilibrium coverage on) LiF(001) at 30 K [225], while it does at 115 K [158]. Thus, at 30 K multilayer CH3Br may be less dense and more disordered, leaving more possibilities for escape of C H 3 photofragments with collisionally degraded translational energies. The Br angular distribution is - cos 0 and the translational energy distribution also has a very slow component. However, about 1 / 3 of Br has kinetic energies higher than the maximum in the gas phase. Molecular CH3Br photodesorption was attributed to C H 3 B r - C H 3 and C H 3 B r - B r collisions, which also results in slow velocity components of C H 3 and Br fragments, rather than photoacoustic shock waves, since they observed no evidence for UV absorption by color centers. The total photolysis cross section at 2 ML, as measured by CH3Br TPD, was 1 × 10 -18 cm 2, - 2 times that in the gas phase. The enhanced cross section is attributed to CH3Br desorption induced by collision. Kutzner et al. [226], using T O F and REMPI, studied the dynamics of photodissociation of thick CH3I condensed on LiF(100) at 150 K with a 266 nm laser. In the gas phase and at 266 nm, there are two dissociation channels, C H 3 + I * ( 2 p 1 / 2 ) (95%) and C H 3 + 1(2P3/2) (5%) with an energy difference of 0.94 eV between I* and I [282]. As for thick layers of C H 2 I 2 on Ag [165] and A1203 [6,140], they found explosive desorption of CH3I for high laser fluences ( > 1.5 m J / c m 2, 4 - 5 ns pulse width). Direct dissociation dominates at low laser fluences. The C H 3 velocity distribution shows a slow peak and two fast peaks. The slow peak corresponds to a temperature of 130 K for the v = 0 state, nearly the same as the substrate temperature (150 K), and is attributed to the fragments formed beneath the surface. The two fast peaks exhibit peak velocities of - 3400 m / s (0.9 eV) and 4600 m / s (1.64 eV) with corresponding high velocity onsets of - 3 8 0 0 m / s (1.12 eV) and 5100 m / s (2.02 eV), respectively, for v = 0, and are attributed to direct photodissociation. The high velocity onsets are nearly the same as those found in gas-phase photodissociation to I* (3910 m / s , 1.19 eV) and I (5110 m / s , 2.03 eV), respectively. The (CI'I 3 + I) photodissociation channel has a higher population of C H 3 in vibrationally excited states v = 1 and v = 2 than the ( C H 3 + I * ) channel. C H 3 fragments from photodissociation of condensed CH3I have more rotational excitation than in the gas phase, a fact attributed to deformation of the dissociative potential surface(s) when CH3I is attached to a surface. As for photodissociation of CH3Br on B r / N i ( l l l ) [5d] and LiF(001) [158,225], the excited halogen, I*, velocity distribution extends to higher values than allowed in the gas phase. Villa et al. [227], using T O F and REMPI, studied iodobenzene ( - 1 ML) adsorbed on LiF at 150 K. Pulsed-UV-laser irradiation (222 nm) results in both photofragmentation and molecular photodesorption. T O F revealed phenyl (C6Hs), I, iodobenzene (C6H5I) and biphenyl (C12H10). All the T O F spectra can be fit by single Maxwellian-Boltzmann distributions. In summary, they observed that: (1) unlike photofragmentation of CH3I on LiF [226], both I and I* have essentially the same translational energy distribution; (2) I, though 1.7 times heavier than C6H5, has a higher translational temperature ( - 1500 K) than C6H 5 ( - 1200 K); (3) C12H10, formed from two C6H5, also has a higher translational temperature ( - 1500 K) than C6H5; and (4) C6I-I5I has a translational temperature of - 900 K. They speculate, based on the fact that

166

X.-L Zhou, X.- Y. Zhu and J.M. White

they observed C12H10 but not I2, that C6H5I is tilted with its aromatic ring downward at a certain angle and the I atom pointing outward. In this geometry, I is produced with little surface mediation, while C6H 5 has to rebound from the surface, suffering energy loss. The photodesorption of C6H5I is attributed to relaxation of the electronic energy from the C6H 5 ring to the surface. In addition, they found that irradiation of C 6 H s B r / L i F , in contrast to C 6 H s I / L i F , with a 222 nm laser results in preferential molecular desorption with little dissociation. Cho et al. [215] studied the photochemistry of HC1 and HBr adsorbed on LiF(001) at 85 K with 193 and 248 nm lasers. They found a very interesting surface aligned photoreaction of 2HX(a) ~ H2(g ) + X2(g), which is more efficient than the anticipated photodissociation, HX(a) ~ H + X. The photoreaction is observed from 0.1 to 10 monolayer coverages and for HBr(a) at both 193 and 248 nm. The translational energy of H 2 peaks at 0.018 eV, independent of wavelength, while that of Br 2 peaks at 0.058 eV at 193 nm and 0.075 eV at 248 nm. The cross section at 193 nm is 6 × 10-19 cm 2, twice the photodissociation cross section. Photoreaction of HCI(a) occurs only at 193 nm with a peak translational energy of 0.012 eV for H z and 0.042 eV for C12. The cross section is 1.5 × 10 -19 cm 2, an order of magnitude higher than the photodissociation cross section. The yields of H2(g ) and Br2(g ) are equal and are the same at 193 and 248 nm even though the yield of H atoms from photodissociated HBr(a) is - 102 times smaller at 248 nm. The results are interpreted in terms of HBr island formation down to low coverages; the photoreaction mechanism could involve the excitation of H X dimers, leading to direct formation of H2(g ) and X2(g ). Photoejection of clusters of (HBr), (n < 4) was found at 193 nm (not at 248 nm) for high coverages (80-12000 L dose) of HBr. The desorption yield of these clusters increases with coverage and laser fluence. The angular distribution of photoejected species is interesting. While the monomer peaks toward the surface normal, the clusters are peaked 40 ° off the surface normal, presumably because their molecular axes are tilted with respect to the surface normal before excitation. Sato and Kawasaki [214], using TOF, studied condensed C12 on Si at 100 K with 193, 248 and 352 nm lasers. For thin multilayers, the T O F spectrum for C1 is bimodal, while for thick layers, it has only the higher kinetic energy component. The m a x i m u m kinetic energy of C1 decreases with increasing wavelength: 2.9 eV (193 nm), 1.6 eV (248 nm) and 1.08 eV (352 nm). The high-energy component is attributed to direct photodissociation of C12 in the topmost layer and the slow component to C1 atoms photogenerated on the substrate surface but losing energy through strong interactions with the surface. T O F also shows the desorption of SiC1 and SiCI 2, which are attributed to reaction products of photogenerated CI atoms with the Si surface. Their kinetic energies are low and similar to the low-energy component of C1. Wen and Rosenberg [216], using photoemission spectroscopy (PES), studied photolysis of C H 3 F adsorbed on S i ( l l l ) at 30 K with 55 eV monochromatized synchrotron radiation. The C H 3 F coverage corresponds to a surface C H 3 F / S i ratio of - 0.5. The photolysis, with a cross section of (9 + 3) × 10 -~7 cm 2 and a quantum yield of 0.04 +_ 0.02, causes C - F bond scission and results in formation of surface S i - F and CH~ (x = 0 - 3 ) fragments. The photolysis mechanism is believed to involve production of secondary electrons which initiate dissociative attachment of adsorbed CH3F.

4.1.7. Phosgene on A g ( l l l ) and P t ( l l l ) The photochemistry of phosgene (C12CO) in the gas phase has been studied extensively [20]. In the near-UV region, absorption is continuous, starting at - 3 0 0 nm with a m a x i m u m

167

Photochemistry at adsorbate / metal interfaces

(o = 1 × 10-19 cm 2) at - 230 nm. Photodissociation produces C1 atoms and CO molecules. The dominant process in the near-UV is C12CO + h v --* COC1 + CI, COC1 ~ CO + CI, 2 CI "--' C12. Adsorption of C12CO on Ag(111) and P t ( l l l ) at - 1 0 0 K is molecular, and there is no detectable thermal decomposition in TPD [139b,150]. On both surfaces, photodissociation occurs readily in the monolayer regime, with higher cross section and lower photon energy thresholds than in the gas phase. The dissociation products are CI and CO. All C1, but only a small fraction of CO, is retained on Pt(111) at 100 K. On A g ( l l l ) , all C1, and no CO, is retained. The photochemistry of C12CO on A g ( l l l ) (continuously irradiated using a 100 W Hg arc lamp with a maximum energy flux to the sample of 75 m W / c m 2, A > 230 nm) was studied as a function of wavelength, coverage and irradiation time using XPS, T P D and MS. The XPS spectra before and after illumination with the full arc show that a monolayer of C12CO can be completely dissociated. The total XPS area of CI(2p), shifting from 201 eV for molecular C12CO to 197.3 eV for the dissociated surface C1 atom, is independent of irradiation time, while O(ls) and C(ls) peak areas decay monotonically with time to zero. The XPS data confirm the loss of all O and C, and the retention of all C1, i.e. no photodesorption of C12CO or C1. The only desorbing product during illumination is CO and the only products desorbed after illumination are parent CI 2CO and AgC1. The initial dissociation cross section (o) was measured as a function of C12CO coverage and wavelength. Fig. 33 shows o at 254 nm from 0 to 2 ML [139c]. The value of o is remarkably

~,

12

10

Phosgene/Ag(111)

~

3. = 254 nm

¸

E 0

'7

8

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0 L. 0

4

e-

O

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I

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0.5

1.0 Coverage/ML

!

1.5

2.0

Fig. 33. Initial cross section for photodissociation of C12CO versus initial coverageat 254 nm. The broken curve is the theoretical fit. After Zhou and White [139c].

X.-L Zhou, X.-Y. Zhu andJ.M. White

168

high (10 -is cm 2 for very low coverages), exceeding the gas-phase optical absorption cross section (7 × 10 -20 at 254 nm [20]). The cross section drops monotonically from 0.2 to 2 ML. Assuming a homogeneous adsorbate layer, independent of coverage, and one that adsorbs a very small fraction of the incident light (optically thin layer), we would predict a coverage-independent cross section, at least for submonolayers. In phenomenological terms, the coverage dependence can be explained by a self-quenching mechanism in which the excitation energy is rapidly shared among neighboring phosgene molecules, particularly as the surface adsorbate density approaches values appropriate for solids. Alternatively, if the dissociation is due to electron transfer from Ag to C12CO, fig. 33 can be readily understood in terms of local potential variations. In the first explanation with increasing coverage, the local potential around each C12CO molecule may increase due to dipole-dipole interactions, thereby reducing the electron transfer probability. Zhou and White [139c] proposed the following simple photolysis mechanism: CIzCO(a ) + hp ~ C12CO* (a)

k 1,

C12CO* (a) ---, 2 Cl(a) + CO(g)

k2,

C12C0" (a) ---, C12CO(a )

k 3,

Cl2CO(a ) + C12C0" (a) ~ 2 C12CO(a )

k 4,

(i) (ii) (iii) (iv)

where (a) denotes an adsorption bond to one or more Ag(111) atoms, (i) denotes excitation (including both direct and substrate-mediated), (ii) is the dissociation process of interest, (iii) denotes expected quenching by Ag, and (iv) denotes the inhibition of dissociation by neighboring ground-state C12CO molecules. This inhibition includes not only the kind of self-quenching that is well-known in the gas phase but surface site-blocking effects as well (e.g., if dissociation requires an empty site next to a parent molecule). The steady-state photodissociation rate derived from the mechanism is R = ( k l k 2 [ C ] ) / ( k 2 + k 3 + k4[C]), where [C] is the instantaneous concentration of the parent molecule. The measured coverage-dependent cross section is best fit by o = 7.2 × 10-1711/(55 + 65[C])] (broken curve in fig. 33). Notice that this simple model, which embodies a small number of physically intuitive ideas, gives an adequate description in terms of a single intrinsic cross section of 13 > 10-19 cm 2 at the zero coverage limit. The second explanation, a decreased electron transfer rate, would involve a coverage-dependent cross section for step (i) in the above mechanism; as the molecules crowd together, the local potential increases, and the number of electrons with energies that readily attach to phosgene decreases. The wavelength-dependent cross section is shown in fig. 34 for C12CO in both the first and the second layers [139@ Fig. 34 also shows the work function of Ag(111) with and without 1 ML CI:CO on Ag(111). Comparing, for 1 and 2 ML, the total photodissociation rates measured both during (by CO signal intensities) and after illumination (by the amount of residual parent) indicates an increment in the rate by adding the second ML for wavelengths below, but not above, 300 nm. The difference between the rates for 2 and I ML is taken as the photodissociation rate for the second-layer C 1 2 C O . For first-layer C12CO, there is an apparent threshold near 480 nm (2.6 eV), a steady rise up to 320 nm (3.9 eV), a sharp rise between 320 and 290 nm (4.3 eV), and finally a slow rise to 254 nm (4.9 eV). This is very different from the gas phase, where the optical extinction coefficient drops to negligible values for wavelengths longer than 300 nm [20]. Consistent with this, C12CO in the second layer has no measurable cross section above 300 nm. At 290 and 254 nm, the cross section for the first-ML CIzCO is 6 - 8 times higher than for the second-layer C12CO. Interestingly, all of the C1 is retained when the second-layer C12CO is photolyzed. There was no direct evidence regarding the excitation mechanism, but for the first m o n o -

169

Photochemistry at adsorbate / metal interfaces

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layer, above-vacuum-level electrons were excluded for wavelengths longer than 295 nm on the basis of the measured work function. Below 295 nm, direct excitation was speculatively favored on the basis of the gas-phase excitation. Related work, using externally generated electrons, shows that electron-induced decomposition of adsorbed C12CO on A g ( l l l ) occurs near zero eV with a cross section of 1 0 - ] 6 - 1 0 - 1 5 c m 2 [174]. Since even in the second layer, C12CO has a higher cross section than in the gas phase, electrons must be a important contributor. Although one would expect quenching to be faster for the first layer than the second, the photodissociation cross section is larger for the first layer. As for the methyl chloride-platinum (silver) system discussed earlier, a simple explanation is that the excitation probability for the first layer is much higher than for the second layer, compensating for the faster quenching. Experiments involving phosgene coadsorbed with other species have proven revealing. For 1 ML C12CO o n partially I-covered Ag(111) ( I / A g = 0.12), the work function is 0.25 eV higher than for 1 ML on clean Ag(111), and the photodissociation has the same photon energy threshold, but the rate is slower. The slower photodissociation rate is attributed to the faster quenching rate because C12CO bonds to I/Ag(111) more strongly than on clean Ag(111). However, when the photodissociation proceeds to the point at which the surface is completely covered by I and C1 atoms, the remaining C12CO photodissociates at a faster rate. The underlying mechanism is not clear and further experiments are needed.

170

X.-L. Zhou, X.-Y. Zhu andJ.M. White 151.

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,

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300

350

400

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In a very important set of experiments, the photodissociation of C12CO on Pt(111) was examined using polarized light [150]. Fig. 35 shows the wavelength dependence of the initial cross sections for 1 ML C12CO.They range from 1.2 × 10 -lg cm 2 at 254 nm to 2.4 × 10 -2o cm z at 365 nm. As on Ag(111), the data are interpreted in terms of a new process setting in at 290-300 nm and extending to shorter wavelengths. Variations in the photodissociation yield with polarization and angle of incidence reveal different dominant excitation mechanisms below and above the break. Fig. 36 shows the angular dependence of ClzCO photolysis yield (solid circles) using 280 nm p-polarized light. Comparison of the measured yields with the metal absorbance (A p, dashed curve) shows that substrate excitation, by itself, cannot describe these results; for example, there is a three-fold increase in the yield in passing from normal incidence to 60 °, but Ap increases by a factor of only 1.37. There are a number of possible fits that involve various contributions of substrate and direct excitation. The solid curve in fig. 36 shows the intensity of the total electric field, (E~ouj), i.e., the sum of (Ex2) and (Ez2). The dash-dot curve is a sum of Ap and ( E 2 ) , assuming that all the photolysis at normal incidence was due to substrate excitation. The curves were normalized to their values at normal incidence. It is clear that, within experimental error, both fits are satisfactory. Fig. 37 shows the angular dependence of the photolysis yield (solid circles) at X > 315 nm. Unlike at 280 nm, the measured yield can be satisfactorily fit by metal absorbance alone. It was found that, if direct excitation were important in this region, a combination of 17% (E. 2) and 83% ( E ~ ) would be required to fit the data. However, this assumes an average dynamic dipole oriented 79 ° off the surface normal, distinctively different from that at 280 nm. Such a dramatic change of dynamic dipole with wavelength seems unlikely. All the evidence indicates that direct excitation, in parallel with substrate excitation, is important below 290 nm, while substrate excitation dominates at longer wavelengths. Based on the results on P t ( l l l ) , we may reinterpret those on Ag(111). For wavelengths below the point at which the slope changes in fig. 34, direct excitation is important; above this

Photochemistry at adsorbate/ metal interfaces 4" A

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Angle of incidence (degree) Fig. 36. C12CO (1 ME) photodissociation yield versus incidence angle for p-polarized light with a 280 nm band-pass filter (light source: 100 W Hg-arc lamp). The data are averages of repeated experiments and the error bars reflect the scattering of individual measurement as well as uncertainties in power and angle measurement. Solid and dashed curves are the total electric field intensity (Et2otal = ~ E 2 ) + ~E2)) and metal absorbance (Ap), respectively. The dash-dot curve is a combination of ( E l ) and Ap. Each curve was normalized to its value at normal incidence. The photolysis yield at normal incidence was 10%. After Zhu and White [150].

-10

,

T

10

30

,

50

70

90

A n g l e of incidence (degree) Fig. 37. Normalized CI2CO (1 ME) photodissociation yield (solid curve) versus incidence angle for p-polarized light with a 315 nm cut-off filter (light source: 100 W Hg-arc lamp). The data are averages of repeated experiments and the error bars reflect the scattering of individual measurement as well as uncertainties in power and angle measurement. Solid and dashed curves are normalized substrate absorption and total electric field strength, respectively, for p-polarization at 365 nm. The dash-dot curve is the metal absorbance for p-polarization at 312 nm. The photolysis yield at normal incidence was 25%. After Zhu and White [150].

point, substrate excitation dominates. O n both A g ( l l l ) a n d P t ( l l l ) , the photolysis cross section at 254 n m is m u c h larger than in the gas phase. At least three factors might c o n t r i b u t e to the higher cross section: (1) the optical properties of the C l 2 C O - m e t a l complex are different from those of the gas-phase molecule (i.e. larger a b s o r p t i o n coefficient); (2) charge transfer (from metal to C12CO ) excitation can occur within this complex a n d m a y c o n t r i b u t e to or d o m i n a t e the photolysis process; a n d (3) substrate excitation also c o n t r i b u t e s to the observed cross section. Since second-layer C12CO has n o direct interaction with the metal (i.e. n o a d s o r b a t e - s u b s t r a t e complex), its optical properties m a y be analogous to those of the gas phase. Charge transfer excitation would be absent. Photolysis of the first-layer C12CO extends to m u c h longer wavelengths t h a n the second layer. This m a y be due to a lower electron potential barrier for the first layer, so that lower energy electrons well below the v a c u u m level are active. These effects will result in a m u c h higher excitation p r o b a b i l i t y for C12CO in the first layer, which, if it exceeds the faster quenching, could explain the observed larger photolysis cross section. 4.2. Oxygen Photochemistry of 0 2 adsorbed on P t ( l l l ) [84b,85], A g ( l l 0 ) [84] a n d P d ( l l l ) [83,156] has recently been studied. 0 2 chemisorbs molecularly on these surfaces at or below 100 K. Both p h o t o i n d u c e d desorption of molecular 0 2 a n d dissociation of O2(a ) to O(a) take place. I n addition, p h o t o - i n d u c e d i n t e r - a d s o r b a t e state conversion occurs o n P t ( l l l ) a n d P d ( l l l ) . This adsorbate is of particular interest because the strong chemisorption, a c c o m p a n i e d b y significant

172

X.-L Zhou, X.-•

Zhu andJ.M. White

8,5

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ENERGY LOSS (cm~ ) Fig. 38. Specular HREELS taken at 95 K: (a) 0.45 ML 02; (b) 0.25 ML O; (c) 0.12 ME 02 +0.05 ML O; (d) 0.45 ML 02 with full arc (0.05_+0.01 W/cm2); (e) 0.45 ML 02 followed by 6 rain irradiation with 295 nm cut-off filter (0.07 5:0.01 W/cm2). After Zhu and White 185a].

negative charge accumulation, makes the adsorbed molecular species very different from its gas-phase counterpart. Thus, the ground-state wavefunction of adsorbed di-oxygen involves significant contributions from the substrate and a description in terms of an adsorbate-substrate complex is required. Emphasizing the strong alteration of gas-phase properties, we note that on P t ( l l l ) at 100 K, 02 adsorbs predominantly in a charged peroxo-like ( 0 [ 2) form that gives an intense O - O stretching frequency at 870 cm-~ in HREELS, much lower than the gas-phase stretching frequency, and an increase in work function of P t ( l l l ) by 0.8 eV [283]. At 100 K, a maximum of 0.44 ML 02 can be adsorbed. Upon heating to 150 K, part of the peroxo species desorbs and the remainder simultaneously dissociates to form 0.25 ML of atomic oxygen, which desorbs as O 2 at higher temperatures (600-1000 K). Continuous UV irradiation of the peroxo species, with photons from a high-pressure arc lamp, drives both dissociation and desorption. This is clearly demonstrated in fig. 38 [85a], which shows the H R E E L S data of O z adsorbed at 95 K, with and without UV irradiation. Without irradiation, HREELS shows a strong loss at 865 cm-1 of peroxo-like 02, which shifts

Photochemistry at adsorbate / metal interfaces

173

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WAVELENGTH(rim) OF CUT-OFF FfLTERS (') Fig. 39. Wavelength dependence * of the average rate (first 6 min irradiation) of photodesorption (open squares) and photodissociation (solid squares) of 0.45 ML O z on P t ( l l l ) . * Each data point corresponds to the wavelength of the cut-off filter used. Therefore, the rate presented is actually an integrated rate at wavelengths _> the indicated numbers. The data points at 230 n m correspond to full arc. After Zhu and White [85a].

to 855 c m - I when it is prepared in the presence of atomic O. The loss at 475 c m - I is due to P t - O stretching from atomic O. After peroxo irradiation with the full arc (X > 230 nm) there are two new losses at 480 and 650 cm-~; the former indicates O - O bond dissociation to form atomic O, and the latter is ascribed to a new form of adsorbed 02 not well-characterized, but perhaps bridge-bonded peroxo. Further insight is gained by combining these results with T P D determinations of the total amount of oxygen remaining after photolysis [85]. The total area drops, indicating that oxygen is removed during illumination; analysis during irradiation confirms this and identifies molecular O z as the only photodesorbing species. These processes were studied as a function of wavelength distribution, determined by cut-off filters, for an initial 02 coverage of 0.45 ML with a constant energy flux of 0.07 __+0.01 W / c m 2 incident upon the surface at 57 ° off the surface normal. The results are shown in fig. 39 as average rates for photodesorption and dissociation versus cut-off wavelength for an irradiation of 6 rain. With no cut-off filter (?~ >_ 230 nm), the desorption rate exceeds the dissociation rate by a factor of about 2. As the photon energy distribution is shifted to lower energies by successively longer wavelength cut-off filters, both rates drop rapidly from 230 to 320 nm, with a threshold of - 3 0 0 nm for dissociation and 460 nm for desorption. The wavelength dependence for inter-state conversion of molecular 02 is qualitatively the same as for photodesorption. Thus, the desorption and the dissociation have very different wavelength dependences, implying that different pathways (transition states) are involved. Further support for two paths comes from measurements of photodissociation and desorption rates as a function of incidence angle. With the full arc, the dissociation rate decreases, while the desorption rate increases with increasing incidence angle. If the same transition state were involved, we would expect a constant ratio independent of the incidence angle. It appears that two mechanisms are involved in the desorption, one that dominates between 350 and 460 nm and the other dominating below 350 nm. For wavelengths between 230 and 315 nm, the average cross section is estimated to be 5.7 × 10 -2° cm 2 for dissociation and 1.2 x 10-~9cm2 for desorption. Interestingly, the thresholds

174

X.-L. Zhou, X,-Y, Zhu and J.M. White

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are close to those for direct gas-phase photolysis of hydrogen peroxide (dissociation) and [P(C6Hs) 3] 2PRO2 (desorption) [284], respectively. Although this could be taken as evidence that direct excitation is involved in both paths, work on other oxygen-metal systems implicates substrate excitation [84,156]. Since the photon energies ( < 5.4 eV) are lower than the work function (6.6 eV), the underlying mechanisms for the observed photochemistry of 02 on Pt(111) cannot be due to electrons excited above the vacuum level. Keeping these limitations in mind, the following speculative model has been proposed (fig. 40). Adsorption of 02 increases the work function of Pt(111) from 5.8 to 6.6 eV [283]. UPS indicates that the l~rg* level of 02 is located near the bottom of the Pt d-band [283]. Upon adsorption, the degeneracy of l~rg* is removed; the orbital parallel to the surface, ~rn, is largely non-bonding with respect to Pt, while the one perpendicular to the surface interacts strongly with the d-band (%: dxz and dyz). The coupling of the latter results in a bonding (nro) and an antibonding (~r*) orbital. The former is located mainly on 02 and the latter mainly on Pt. As in organometallic peroxo complexes, ~r, is either filled or partially filled. Direct excitation from ~rn to o*, as in H202 [20,185], would account for the dissociation. Excitation from ~r~ to ~r* would decrease the bonding between O and Pt and could lead to desorption. This model also accounts for different thresholds for dissociation and desorption. Quenching of "~ to ~r* excitation will lead to vibrationaUy excited Pt-O2, one decay channel of which may be conversion to another adsorption state, as observed. Assuming that the ~r~ to o* transition dipole vector lies in the O - O bond axis, while that for -n~ to o* lies along the P t - O 2 bond axis, the dependence of dissociation and desorption upon the incidence angle can be nicely explained.

Photochemistry at adsorbate / metal interfaces

175

Clearly, for both dissociation and desorption, there are alternatives to the direct excitation pathways outlined above. For example, subvacuum level excited electrons could be involved. These hot electrons can tunnel into o~* and induce dissociation. Hot holes can also operate; since v* in adsorbed 02 is filled [283], hot holes may induce desorption by capturing an electron, reducing the charge on 0 2 - and thereby suddenly changing the interaction energy between 02 and Pt. The energy of holes required for desorption may be much lower than the energy of electrons required for dissociation, in which case desorption would have a lower threshold, as observed. However, dissociation and desorption cannot both be due to substrate excitation. Otherwise, we would not be able to explain the dependence on incidence angle. Clearly, O 2 / P t ( l l l ) is an interesting photochemical system that deserves further mechanistic study; perhaps both direct and substrate excitation contribute. The three photochemical processes for 02 on P t ( l l l ) were also found on P d ( l l l ) , but it is not clear whether the same underlying mechanisms operate. The photochemistry of 02 on P d ( l l l ) has been studied both dynamically and kinetically using pulsed laser [156] and CW irradiations [83]. Adsorption of 02 on P d ( l l l ) below 100 K proceeds through population of three molecular states, Otl_3, corresponding to superoxo, peroxo-I and peroxo-II species. All three di-oxygen species bind side-on to the surface [285]. The saturation 02 coverage is 0.31 ML. Unlike P t ( l l l ) , the TPD spectra from P d ( l l l ) resolve the various molecular states of 02 on P d ( l l l ) ; this is of considerable help in establishing quantitative rates. In TPD, both molecular desorption and dissociation, to a maximum atomic O coverage of 0.25 ML, take place below 250 K [83b]. Yates and co-workers [83b] studied the O 2 / P d ( l l l ) system using a high-pressure Hg arc lamp and band-pass filters. They found a threshold energy of 3.4 + 0.3 eV (or lower) for photodesorptlon and conversion and 3.7 + 0.3 eV for photodissociation. The photodissociation threshold is slightly lower than that found in photolysis of gas-phase H202 (4.1 eV). The cross section for photodesorption (ades) is about 4 times that for photodissociation (Odis) ; for example, at 5.2 eV, Odes is (1.3 + 0.1) X 1 0 - 1 9 c m 2 and Odis (3.5 + 1.2) X 10 - 2 ° c m 2. The latter is one-sixth of the cross section observed for photodissociation of gas-phase H202. Photodissociation and desorption also have 02 coverage thresholds" The photodissociation starts at - 0.12 ML, increases to 0.2 ML and levels out at higher coverages, and the photodesorption starts at - 0 . 1 5 ML, increasing monotonically with coverage. The authors discussed the underlying mechanisms using a frontier orbital approach and suggested that, in the peroxo case, photodissociation could result from substrate excitation and photodesorption from intra-adsorbate excitation. In the superoxo case, they suggest that substrate excitation does not play a significant role and that intra-adsorbate excitation induces both desorption and dissociation. In a later experiment [83c] s- and p-polarized light ( < 5.4 eV), at an incidence angle of 60 o, was used to photolyze a saturated O2-Pd(111 ) surface. They found that p-polarized light is more efficient for both photodissociation and desorption and suggested that both photodissociation and desorption are due to substrate excitation, since Pd more readily absorbs p-polarized than s-polarized light. This evidence, while important and suggestive, is not sufficient to rule out significant contributions due to direct excitation. Ertl and co-workers studied the O2/Pd(111 ) system using pulsed-laser irradiation at 6.4 eV [156]. They observed the same three photochemical processes as Yates et al. [83] and provided mechanistic and dynamic information that gives considerable insight into them. For example, starting with a surface containing only al-O 2 (dosed to saturation at 153 K), the a I ~ a 2 ~ c~3 photoconversion is clearly evident (fig. 41). The a 2- and ot3-O 2 desorption peaks appear only after UV irradiation, and they develop at the expense of a~-O 2. This conversion is also indicated in HREELS where, for saturation coverage at 100 K, the a 3 O - O loss intensity

176

X.-L. Zhou. X.-Y. Zhu andJ.M. White

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initially increases and then decreases, while the a z_ 2 O - O loss intensities simply decrease with increasing irradiation time [156a]. Photodissociation is also evidenced in H R E E L S by the appearance of a P d - O stretching vibration at 480 nm (atomic O) after irradiation. The dynamics, as well as direct evidence for molecular desorption during illumination, are evident in fig. 42 [156a], which shows the time-of-flight (TOF) spectra of molecular 02. Fig. 42a is for saturation 02 coverage prepared at 90 K (containing eft_3 states) and irradiated at 90 K. Clearly, the velocity distribution of desorbing 02 is bimodal and can be fit by a superposition

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of two modified Maxwell-Boltzmann distributions, which yield average translational kinetic energies, /2k, of 800-1-50 and 120 + 20 K for the fast and slow component, respectively. The /2k value in the slow component is close to the surface temperature ( - 100 K during the laser pulse) and can be interpreted as being thermally accommodated to the surface (i.e., thermal desorption). The /2k of the fast channel greatly exceeds the surface temperature and, thus, indicates a non-thermal desorption process. By way of comparison, when only the a~-O2 state (prepared by saturating the surface at 153 K) is present initially, only the fast component appears (fig. 42b) but with a lower intensity than for fig. 42a. The T O F intensity increases initially, but then decreases coincident with the irradiation-time-dependent population of a 2- and ~t3-O 2 a s monitored in T P D (fig. 41). Thus, photodesorption seems to require the presence of a 2- and %-02. This is supported by higher temperature (153 K) T O F with only al-O 2 (fig. 42c); the signal is much weaker and tails off at longer times (lower energies). Thus, at 153 K, %- and a 3 - O 2 have very short mean residence times on the surface and, upon production from al-O 2 in one laser shot, will desorb thermally before the next shot.

X.-L. Zhou, X. - Y, Z h u a n d J . M . White

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A n g l e of I n c i d e n c e Fig. 43. Photodissociation yield at 193 nrn (6.4 eV) of 02 saturated at 130 K on Pd(ll) versus angle of incidence with respect to the surface normal for p- (filled circles) and s- (open circles) polarization, respectively. The solid lines show the calculated absorptivity of Pd for the two polarization cases (upper panel). The lower panel shows the ratios between the p- and s-polarization data in the upper panel. After Wolf et al. [156b].

The signal at short flight times indicates non-thermal desorption directly from a]-O 2 but the cross section must be much smaller than for a 2- and a3-O 2. The cross sections are (4-5.5) × 1 - 19 cm 2 for photodissociation, and 2 x 10-19 and 1.05 x 10-19 c m 2 for p h o t o d e s o r p t i o n of a 1- and a2_3-O 2, respectively. For a saturation coverage prepared at 90 K, the p h o t o d e s o r p t i o n angular distribution of molecular O2 follows cos20. The angle and polarization dependence of the rates of photodesorption and photodissociation, monitored by the integrated 02 T O F intensity and the post-irradiation O(a) H R E E L S signal, is shown in fig. 43. The data, obtained by saturating the surface with O 2 and irradiating at 130 K with pulsed s- and p-polarized light, can be fit nicely with the absorptivity of bulk Pd. Substrate excitation is adequate to explain both processes, and the data c a n n o t be successfully fit by direct a d s o r b a t e - s u b s t r a t e excitation alone; nor do symmetry considerations favor direct excitation. Turning to photoconversion, T P D results as a function of the polarization and incidence angle, with saturated cq-O 2 only, indicate the same mechanism for photoconversion. It appears that, in spite of certain near coincidences with gas-phase processes like those described above, most, if not all, of the observed photochemistry in this system can be accounted for by substrate-excited electrons and holes. It is suggested, based on the fact that pre-adsorbed atomic O dramatically reduces the saturation coverage of molecular 02 (especially al-O2), that the photoconversion and the desorption of 02 in the slow channel are driven by photodissociation of a]-O 2 [156a].

Photochemist~ at adsorbate / metal interfaces

179

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Another very interesting example involves the anisotropic chemisorption, in peroxo form, of 02 on Ag(ll0). The maximum coverage is 0.25 ML at 100 K and both desorption and dissociation occur thermally at 190 K. There is only one adsorbed molecular 02 state at 100 K, with the O - O bond axis parallel to the Ag(ll0) azimuth. If photodissociation is due to direct local excitation of adsorbed 02 molecules, the rate should be proportional to ](/~- E ) 12, where /t is the transition dipole vector of 02 and E the electric field vector of the incident light. Since, on Ag(ll0), the O - O bond axis aligns along the Ag(ll0) azimuth, the angle, a, between/~ and E can easily be controlled using polarized light at normal incidence. Variations in the photon-driven rates with the direction of E is expected if direct excitation dominates. However, as shown in fig. 44 [84a], both the photodissociation and photodesorption rates are independent of the azimuthal angle, suggesting that photolysis of 02 on Ag(ll0) is dominated by substrate excitation. This is further supported by the dependence of p-polarized photolysis on the angle of incidence; both photodissociation and photodesorption can be fit reasonably well by the substrate absorption of p-polarized light [84b]. Photodissociation is evident in HREELS, which shows A g - O stretching at 320 cm -t, and photodesorption is evident in TPD, which shows a loss in total surface oxygen coverage after irradiation [84b]. In these experiments, a high-pressure Hg arc lamp and bandpass filters were used. Since the photon energy ( < 5.3 eV) is lower than the work function (5.4 eV) only subvacuum level electrons are involved. Although some contribution from direct excitation of the A g - O 2 complex cannot be unequivocally ruled out, all the evidence indicates that dissociative attachment of hot electrons dominates. For A g - O 2, the wavelength dependence indicates that photodesorption and dissociation for 0.25 ML 02 share the same energy threshold of - 2.8 eV, and the ratio of the photolysis yields of the former to the latter is - 2, independent of wavelength. The common photon energy threshold and the constant yield ratio suggest a common transition state for the two pathways. Based on careful kinetic analysis, Hatch et al. [84b] concluded that photodissociation induces desorption of 02; dissociating O atoms, with a binding energy of 3.47 to 4.12 eV, transfer

180

X.-L. Zhou~ X.-Y. Zhu andJ.M. White

m o m e n t u m to adjacent molecules and induce desorption of molecular O 2, which has a much lower binding energy (0.1 to 0.4 eV). Similar mechanisms have been proposed for thermally initiated desorption of 02 on Ag(ll0) [286] and photoinduced desorption of 02 on P d ( l l l ) [156]. By examining the photodissoeiation of O 2 on P t ( l l l ) , P d ( l l l ) and Ag(ll0), we find a common feature; i.e., the difference between energy threshold and work function is nearly the same, 2.5 eV on P t ( l l l ) , 2.7 _+ 0.3 eV on Pd(111) and 2.6 eV on Ag(110). If the antibonding orbital of adsorbed O 2 governing the photodissociation is located at the same energy above the Fermi level, independent of substrate, this common feature is easily explained by substrate excitation. Although the work function differences between the metals would change the reaction probability, the thresholds would be the same because the key adsorbate orbital is located at the same energy above the Fermi level. Generally, more and more evidence points to the dominance of substrate excitation; the O2-Pt(111) system may be an exception, at least for some wavelengths, and further study of this case is needed.

4.3. Water The photochemistry of H 2 0 adsorbed on P d ( l l l ) at 90 K was studied by Ertl and co-workers [155] using pulsed excimer laser (6.4 and 5 eV) excitation, with T P D and T O F for detection. H 2 0 adsorbs on P d ( l l l ) molecularly in a bilayer structure and desorbs molecularly between 150 and 170 K without thermal decomposition. Laser irradiation at both 5 and 6.4 eV of the H 2 0 bilayer leads to both molecular desorption and to conversion to a new adsorbed state desorbing at 185 K. This state is attributed to surface hydroxyl, OH, from either direct photodissociation of H 2 0 ( a ) to OH(a) or reaction of HzO(a) with O(a) formed by photodissociation. Photodesorption of H 2 0 was detected in T O F and was fit nicely by a single modified Maxwell-Boltzmann distribution with a mean translational energy, ( E t.... ), of 600 K at both 5.0 and 6.4 eV. The angular distribution of photodesorbing H 2 0 shows a cos40 dependence. The cross sections for 6.4 and 5.0 eV excitation, respectively, are 4.6 × 10-20 and - 1 × 10-2~ cm 2 for photodesorption and 1.4 × 10-~8 and 5 × 10-20 cm 2 for conversion. The cross sections of both processes for D20 are lower than for H20; the ratios of the H 2 0 and D : O cross sections are 2.2 and 1.4 for conversion and desorption, respectively. The isotope effects were interpreted using the M G R mechanism; i.e., H 2 0 and D20 are initially excited onto an isotope-independent excited potential energy surface, followed by rapid de-excitation to the ground state; thus, of the total number of species excited, only a small, mass-dependent fraction actually fragments or desorbs. The excitation mechanism was determined by studying the photodesorption of H 2 0 as a function of angle of incidence using both s- and p-polarized light; substrate excitation was the dominant primary step. Because photodesorption and conversion have similar dependences, the same primary electronic excitation was assumed for both processes. The work function of H 2 0 / P d ( l l l ) was unknown but was expected to be < 5 eV. Excited electrons above and below the vacuum level may be involved in the photolysis. However, the distribution will be significantly broader and more intense, and will extend to higher energies at 6.4 eV than at 5.0 eV. The coverage dependence was interesting. As shown in fig. 45 [155a], with increasing H 2 0 coverage, the dissociation yield increases and saturates above - 0.6 layer while the desorption yield increases, peaks at 1 layer and then decreases rapidly. ( E t.... ) / 2 k decreases from - 600 to - 400 K as the H 2 0 coverage increases from 1 to 7 layers. Apparently, all the photon-driven chemistry is confined to the P d / H 2 0 interface; i.e., there is no photochemistry in the multilayer. Since ice is transparent for h~ < 7 eV, thick H 2 0 layers do not inhibit the electronic

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versus

excitation at the interface, but they do prevent the release of H 2 0 into the gas phase• By way of comparison, water molecules condensed on a quartz plate at - 100 K can be photoejected, but in a two-photon process, by 248 nm photons [241]. The photoejection is attributed to electronic energy exchange between an excited molecule and a ground-state molecule, which converts electronic energy into translational energy of the ejected molecule. Zhu et al. recently extended the H z O / P d ( l l l ) work using high-resolution electron energy loss spectroscopy (HREELS) [240] to give a more detailed description of the photo-reaction pathways and kinetics. U p o n UV laser (6.4 eV) irradiation, adsorbed water on P d ( l l l ) undergoes sequential dissociation to form surface hydroxyl groups and atomic oxygen• These dissociation products partially desorb as water in a reaction-limited peak in TPD, in agreement with earlier expectations• The second step of photodissociation ( O H ---, O + H * ) is accompanied by the desorption of molecular water, a result of photoinduced reaction between " h o t " hydrogen and surface hydroxyl groups• Kinetic modeling of the experimental data yields cross sections for each pathway: for photodissociation ( H 2 0 ---, O H + H), o = (9•0 + 0.3) × 10 -19 cm z, in agreement with the earlier study; for photoinduced reaction ( O H ---, O + H *; H * + O H --, HzO(g)), o = (1.3 + 0.3) × 10-19 cm 2. A polarization and angle-of-incidence study confirms the earlier conclusion that photodissociation is primarily a result of substrate excitation. The effect of coadsorbates on the photochemistry of H 2 0 on P d ( l l l ) has also been investigated• On both (v/3- × v/-3-)CO- and p(2 × 2)-covered surfaces, water adsorbs molecularly and photodesorbs with the same cross section of 2.3 × 10 -20 cm 2. N o significant photodissociation was observed on these surfaces• 4.4. Carbon monoxide

As described in section 1, there were a number of early photodesorption studies of CO from metals, mainly Ni and W. This work has been reviewed by Lichtman and Shapiro [1] and by Koel et al. [2]. In those studies, except that b y Kronauer and Menzel [26] on CO desorption from W, CO desorption was the result of photon heating of the substrates and the photodesorption yield for ~ > 300 nm was negligibly small ( < 10 - s desorbing CO's per incident photon). The Kronauer and Menzel [26] result was interesting; at ~, -- 250 nm, the yield of CO from W rose to 4 × 10 - 7 and the desorption was non-thermal.

182

X.-L, Zhou, X.- Y. Zhu and J, M. White

Recently, Yates and co-workers [24] studied the photon-induced desorption of CO chemisorbed on clean N i ( l l l ) , O/Ni(111) and oxidized Ni(111) using a Hg-arc lamp. They found that photon irradiation had no effect (desorption or dissociation) on either clean N i ( l l l ) or O / N i ( l l l ) . However, significant photodesorption was found for CO adsorbed on oxidized Ni(111). In their experiments, the temperature rise during UV irradiation was less than 1 K, so thermal effects should be negligible. The photodesorption is first-order in photon flux and first-order in CO coverage. The photon energy threshold is 2.7 + 0.5 eV and the cross section increases monotonically with photon energy. At 4.2 eV, the highest photon energy used, the cross section is 5.0 x 10 -~8 cm 2 for an O / N i AES ratio of 0.21, and 3.7 × 10 -18 cm 2 for an O / N i AES ratio of 1.1. The excitation pathway is attributed to 0 2- 2p ~ Ni 2+ 3d interband transitions which have a threshold of - 3.1 eV. This transition increases the electron density on Ni 2+, to which CO is bonded; in turn, o electron donation from CO to Ni is reduced, the N i - C O bond is weakened, and CO desorbs. In a study of the electron-beam-assisted adsorption of CO and CO 2 on Si(100), Ekwelundu and Ignatiev [239] reported non-thermal admolecule photodesorption. As a function of photon energy, the process starts at 2.5-2.6 eV, increases slowly to 3.6 eV, and then increases rapidly. The quantum yield varies from 10 -~ at 450 nm to 10 -s m o l e c u l e s / p h o t o n at 300 nm. A mechanism involving photogeneration of holes in both valence and surface state bands was used to explain the observed photodesorption. 4.5. Nitric oxide

The photochemistry of N O on metal and other surfaces is widely studied, particularly as a probe molecule for studying dynamics of molecule-surface interactions. One major reason is its readily excited and measured fluorescence spectra which reflect its internal energy. Although N O photochemistry does not involve intra-adsorbate bond cleavage, we review it here because the methods are applicable to the study of bond breaking within admolecules, and because of the central role which NO-based systems play in our present understanding of surface photochemistry. In two recent reviews, King and Cavanagh [4] and Zacharias [101] examined the literature published on this subject before 1988. They reviewed the dynamics of both laser-induced thermal and non-thermal desorption of N O on Pt foil [234], Pt(111) [51,235], Pt(100) [236], Pd(111) [287] and oxidized Ni(100) [233]. Here, we summarize pertinent results from these reviews as well as more recent work. UV laser-stimulated photodesorption of N O adsorbed on Ni(100), O(c2 × 2)/Ni(100) N i O ( l l l ) / N i ( 1 0 0 ) and NiO(100)/Ni(100) at < 170 K has been studied [103,233]. Fully state-resolved energy distributions (translational, vibrational, rotational and spin-orbit) have been measured by T O F and LIF. Photolysis has been mainly at 193 nm but, in some cases, also at 248 nm. On Ni(100), N O adsorbs molecularly with its molecular axis perpendicular to the surface and it desorbs at 345 K with a shoulder at 380 K. On NiO/Ni(100), it is weakly adsorbed with its molecular axis tilted by - 45° and desorbs at - 220 K [103b-d]. In early work, Weide et al. [233b] studied the photodesorption of N O chemisorbed on Ni(100) at 170 K using 193 nm laser light. Their results suffered from lack of surface characterization and, therefore, whether photodesorption was actually from a clean or oxidized surface is unknown. Later experiments indicate that photodesorption of N O is inefficient on clean Ni(100) but is enhanced by orders of magnitude upon surface oxidation [103]. Briefly, the results were: (1) photodesorbed N O is vibrationally excited with (~ = 1 ) / ( u = 0) almost equal; (2) the velocity distribution of N O (2FI1/2, high J ) peaks at - 1100 m / s and is much narrower than Maxwellian; (3) the velocity distribution shifts to higher velocities as the rotational energy

Photochemistry at adsorbate / metal interfaces

183

increases, a fact which they interpreted as suggesting more energy was transferred to the surface for low than for high rotational quanta; (4) the rotational distribution deviates from a Boltzmann plot and exhibits selective features (T~ot = 2 0 0 0 K for Ein t < 1000 cm -1 and Trot = 600 K for Ein t > 1000 c m - 1 ) . They interpreted these observations as the result of a resonant a b s o r p t i o n - d e s o r p t i o n mechanism. Later, Mull et al. and Kuhlenbeck et al. [103a-c], with careful surface characterization, f o u n d measurable p h o t o d e s o r p t i o n of N O at 193 nm only on oxidized Ni(100) (i.e., NiO), but no photoeffect on clean or oxygen-covered Ni(100). F e r m et al. [103d] found that the weakly b o u n d N O (desorbing at - 220 K in T P D ) is photoactive. The electronic excitation energy of NO, b o u n d strongly (345 and 380 K in T P D ) and directly to metallic Ni, is believed to dissipate rapidly into Ni b y electron-hole pair creation, thereby quenching photodesorption. In contrast, N i O is non-metallic and the b a n d gap makes electronic quenching slower and photodesorption significant [103]. At 193 nm, the p h o t o d e s o r p t i o n cross section is 7 × 10 -17 cm 2 on N i O ( 1 0 0 ) / N i ( 1 0 0 ) and 1.4 × 10 -17 cm 2 on N i O ( l l l ) / N i ( 1 0 0 ) , more than two orders of magnitude higher than the gas-phase optical absorption cross section of N O (1.1 × 10-19 cm2). The photodesorption q u a n t u m yield on oxidized Ni(100) is - 10 -z [103a-c]. W h e n Ni(100) was irradiated with 193 n m laser pulses and a steady N O gas flux, F e r m et al. found that the N O desorption signal intensity increases continuously with the n u m b e r of laser shots until it reaches a steady-state value after about 3000-5000 pulses [103d]. After T P D of the irradiated surface, AES showed significant O with a trace a m o u n t of N, which they later attributed to the photodissociation of an impurity in NO, i.e. N O 2 dissociating to N O + O. The increase in the N O desorption signal with the n u m b e r of laser pulses was attributed to oxidation of Ni(100), producing a very active N i O surface. Photodesorbing N O has two different s p i n - o r b i t states, 2111/2 and 2II3/2. Fig. 46 shows a series of T O F spectra for photodesorbing N O molecules (v = 0, 2IIt/2 state) with various rotational levels, J, on N i O / N i ( 1 0 0 ) at 193 n m [103d]. The spectra at low J are bimodal. The slow channel decreases rapidly with increasing J and disappears for J > 3 3 / 2 while the fast channel is observable even at J = 5 3 / 2 . The angular distribution of the fast channel follows

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184

X.-L. Zhou, X.-Y. Zhu and J.M. White

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E~ [cm-q Fig. 47. "Bohzmann plots", ln[Nj,,/(2J" +1)] versus Eint, for the rotational populations of "fast" NO molecules desorbing in the v = 0 level by 193 nm (6.4 eV) photons from NiO/Ni(100): (a) 2H1/2 manifold and (b) 21-I3/2 manifold. After Ferm et al. [103d].

cos20 whereas the slow c h a n n e l follows cos 0. R o u g h l y 1% of the N O in the fast channel, a n d an u n d e t e r m i n e d a m o u n t in the slow, is v i b r a t i o n a l l y excited (~ = 1). T h e t r a n s l a t i o n a l energy, in terms of < E t. . . . ) / 2 k , for the fast channel increases linearly with E l , t ( E i . t is s u m o f b o t h r o t a t i o n a l a n d s p i n - o r b i t energies) for b o t h 21-I~/2 a n d 21-[3/2 states; it ranges f r o m 1000 K (v = 0, J = 5 / 2 , 2I~1/2) to 3000 K (v = 0, J = 5 3 / 2 , 2 1 ~ 3 / 2 ) • F o r the same Eint, ( E t. . . . ) / 2 k is • 2 2 a b o u t 400 K higher for I~3/2 t h a n for 1-I1/2. F o r the slow channel, ( E t.... ) / 2 k = 330 _4- 50 K, i n d e p e n d e n t of E m r T h e r o t a t i o n a l a n d s p i n - o r b i t d i s t r i b u t i o n for the fast c h a n n e l is interesting• Fig. 47 shows a B o l t z m a n n p l o t d e s c r i b i n g fast N O molecules d e s o r b e d b y 193 n m p h o t o n s in ~, = 0 state for b o t h 2111/2 a n d 2H3/z branches. F o r the 21-[1/2 b r a n c h , the

Photochemistry at adsorbate / metal interfaces

185

Boltzrnann plot is fit by a straight line, yielding Trot = 360 ± 25 K. In contrast, the 21-I3/2 branch shows a very pronounced underpopulation for Ein t < 250 cm -1 ( J < 5.5). The population ratio, Nj(21/1/2)/Nj(2H3/2), is, for J < 5.5, much higher than unity, the value expected for molecules which are either scattered or thermally desorbed from a surface. For high J, the ratio approaches unity. For the slow channel, the 2II1/2 and 2II3/2 branches are equally populated for all J levels and the Boltzmann plot yields Tro~= 180 + 10 K. The T O F signal/noise ratio for p = 1 state was poor and a limited number of 21/1/2 levels could be analyzed. The ( E t.... )/2k v e r s u s Ein t plot shows no noticeable difference with respect to that in the i, = 0 state. The Boltzmann plot for the 21/1/2 branch is linear and yields Trot = 625 + 50 K. The z1/ 3/2, as in u = 0, is also underpopulated. At 248 nm, the
186

X.-L. Zhou, X.-Y. Zhu andJ.M. White

2I-[3/2 NO, lower than both the translational energy of desorbing NO and the m a x i m u m surface temperature (300-320 K). This result is ascribed to rapid laser heating (4 x 101° K / s , calculated) which causes N O to desorb during a 3.5 ns window within which the desorbing N O molecules are able to achieve nearly complete translational energy accommodation but are unable to achieve full rotational energy accommodation [252]. For the fast channel, there is an enhanced population in 2H~/2 for J < 6.5; for J > 6.5, the Boltzmann plot yields Trot = 425 + 25 K for b o t h 21-[1/2 a n d 21-[3/2 and the spin-orbit population is inverted - Nj(2~I3/2)/ Ns(2II~/2) = 1 . 6 + 0 . 3 , higher than the 0.65 expected for thermal equilibrium. The same inversion in the spin-orbit population was found in the u = 1 state, which has a Trot of 295 4- 40 K for 2F[1/2 and 2FI3/2 at Ein t > 75 cm -J. Using longer wavelength laser pulses (1064 nm) with power sufficient to induce a 100-120 K temperature jump, two desorption channels were observed [234]. The slow one was indistinguishable from that using the 532 nm laser. For the fast channel, however, { E t. . . . ) / / 2 k and Trot are both - 3 0 % lower than, while the internal state populations exhibit the same non-Boltzmann characteristics as, those observed using 532 nm laser. Burgess et al. [234], based on the observations at different wavelengths (532 and 1064 nm), suggest that the fast channel is mediated by metal excitation (hot substrate carriers). Based on these results, they attribute the slow channel to a thermal process and believe that the fast channel characteristics are due to both exit-channel effects and the non-thermal nature of the activation process. In related work, Buntin et al. [51,235] studied the dynamics and mechanism of laser (355, 532, 1064, 1907 nm) induced desorption of N O from P t ( l l l ) . Unlike the poorly characterized NO/Pt(foil), the interaction of N O with Pt(111) is well-characterized. For N O coverages below l / 8 ML, N O is bridge-bonded, while at 0.25 ML, it is atop bonded and desorbs at - 340 K. For higher exposures below 200 K, there is a weakly bound N O state with a desorption peak temperature of - 2 0 0 K. In their study, a saturation exposure of N O was made with the P t ( l l l ) at 117 K. A temperature jump of - 110 K was estimated during the laser pulse. As on NiO [103] and on Pt foil [234], T O F spectra of N O from P t ( l l l ) contain two desorption channels, fast and slow. The slow one is thermal in nature and is only present when the 200 K N O state is present; the fast channel is non-thermal and is associated with N O occupying the atop site. N o photodesorption was found from N O bound in bridge sites, an observation attributed to extremely rapid quenching of excited bridge-bound species. The slow channel decreases rapidly with increasing J; the fast channel dominates at high J. The angular distribution of fast N O in ~ = 0 is sharply peaked toward the surface normal and is fit by a cos"0 (n = 7 + 3) dependence, while that of slow N O is fit by cosnO (n = 2 ___1). For the slow channel, ~ E t.... ) / 2 k is 200 + 20 K, independent of J and laser wavelength (1064, 532 and 355 nm), and the rotational temperature is 92-114 K with the spin-orbit population in equilibrium. The rotational temperature is lower than the estimated surface temperature, again attributed to the short time-scale of the excitation and desorption [252]. The dynamics of the fast channel were studied in more detail at an initial surface temperature of 220 K [51]. The kinetic energy increases with increasing rotational energy and t h e 2I~3/2 level has - 10% higher kinetic energy than the 2II~/2 level for a given J. The average kinetic energy is nearly the same for 532 and 355 nm pulses while that for 1064 nm pulses is 35% lower. The lower kinetic energy at 1064 nm is not due to variations in the resorption laser intensity or in the surface temperature jump. There is no measurable N O photodesorption for 1907 nm laser pulses. As shown in fig. 48, the internal-state population versus internal energy in u = 0 shows non-Boltzmann behavior for Ein t < 300 cm-~ for all three wavelengths 1064, 532 and 355 nm. For Ei, t > 300 cm -~, the Boltzmann plot yields Trot for 2I-[3/2 (2[I1/2) levels of 203 + 15 K (268 +_ 19 K) at 355 nm, 226 + 15 K (251 + 20 K) at 532 nm and 213 +_ 20 -

187

Photochemistry at adsorbate / metal i n t e r f a c e s 2

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Fig. 48. Internal state p o p u l a t i o n as a function o f internal energy for laser p h o t o d e s o r p t i o n of N O f r o m P t ( l l l ) at the surface temperature of 220 K: (a) 1064 nm, (b) 532 rim, and (c) 355 nm. The surface was saturated with N O at 220 K. (2H1/2 states: R]I (X), Qn ([3); 21-[3/2 states: R22 (A), Q12 (O), /022 (©), Q22 ( + ) - After Buntin et al. [51].

K at 1064 nm. As

for N O on Pt foil [234], the s p i n - o r b i t populations are inverted: = 4.3 + 0.6 at 355 nm, 3.3 + 0.5 at 532 n m and 3.1 _+ 1.6 at 1064 nm, higher than the value of 0.6 expected from a T m a x = 340 K B o l t z m a n n distribution. The kinetic energy distributions for the u = 0 and v = 1 levels are essentially the same. The vibrational population ratio of (v = 1 ) / ( v = 0) is 0.04 (corresponding to Tvib = 840 K) at 355 and 532 nm. The v = 1 population is below the detection limit at 1064 nm. The photodesorption q u a n t u m yield is 5 × 10 -8, 4 x 10 -8 and 5 X 10 - 9 at 355, 532 and 1064 nm. The yield is m u c h lower than on N i O / N i ( 1 0 0 ) ( 1 0 - 4 ) and is attributed to m o r e efficient excitation energy dissipation on metal surfaces. The photodesorption yield versus laser power is linear at 355 and 532 n m and slightly non-linear at 1064 nm. The kinetic energy decreases with increasing desorption angle. Unlike on Pt foil [234], the kinetic energy and internal state distribution depend strongly on the initial surface temperature while the (v = 1 ) / ( v = 0) population ratio is weakly temperature-dependent. W h e n the surface temperature increased from 117 to 273 K, ( E t. . . . ) / 2 k of N O ( v = 0, J = 9.5, 2[I3/2) increased by 300 K, the corresponding T,ot increased from 200 + 25 K to 355 + 20 K, and the desorption yield increased by a factor of 4. The surface temperature d e p e n d e n c e is explained in terms of temperature-dependent increases of the m e a n displacement of the P t - N O bond and of the m e a n square angle between the N - O bond axis and the surface

Nj(zII3/z)/Nj(2I]l/2)

188

X.-L. Zhou, X.-Y. Zhu andJ.M. White

normal. These tend to free the frustrated translational and hindered rotational motions of the initially bound NO, and result in higher kinetic and rotational energies of the desorbing N O molecules as well as a higher desorption yield. By studying the photodesorption yield as a function of laser polarization and incidence angle, Buntin et al. [51] determined that the initial excitation is in the substrate. Thus, the photodesorption is the result of attachment of excited subvacuum level substrate electrons (photon energies are all lower than the work function so that no photoelectrons are produced) to adsorbed NO to form a transient N O - species which desorbs according to the description summarized in fig. 18b. Using this model the wavelength dependence can be accounted for as follows: The binding energy of N O on Pt(111) is - 1.08 eV. At 1907 nm the available energy, hu = 0.65 eV, is lower than the binding energy, so no photodesorption is found in the fast channel. The photon energy at 1064 nm, 1.17 eV, is higher than the binding energy; thus, photodesorption is observed. However, since it is lower than the energy (1.32 eV) required, no desorption of N O in u = 1 is observed at 1064 nm. Fig. 9 shows the calculated nascent hot electron distributions at 1064, 532 and 355 nm. The thresholds for desorption in the u = 0 and u = 1 states are marked. The differences between kinetic energies, yields and vibrational-state distributions for desorption with 355 and 532 nm excitations versus 1064 nm excitation are attributed to the threshold nature of the desorption at 1064 rim. The average total energy of desorbing NO at 1064 nm is 0.11 eV (sum of translational, 0.07 eV, and rotational, 0.04 eV, energies). Thus, at 1064 nm, complete transfer of the photon energy to the desorption coordinate is required. Resonant excited electron attachment can account for the indistinguishability of the results at 355 and 532 nm. The similar results observed on the Pt foil indicate, not surprisingly, that the same electron attachment mechanism dominates. Mase et al. [236] studied, with a 193 nm laser, the photostimulated desorption of NO chemisorbed on Pt(100) at 80 and 300 K. The ion NO + was the only species observed desorbing during irradiation of an NO-saturated surface and its intensity was proportional to the third power of laser flux. Unlike NiO/Ni(100) [103], Pt foil [234], and Pt(111) [51], where spectra of NO are bimodal, the TOF spectrum of N O + from Pt(100) can be fit by a single Maxwellian distribution with a characteristic temperature of 3200 K. The translational energy distribution of NO + peaked at ~ 0.25 eV and was independent of laser flux. A two-step N O + formation mechanism was proposed - the first step, a desorption of neutral N O induced by a single-photon valence-electron excitation in the chemisorbed NO and the second, a two-photon non-resonant ionization of the desorbed N O in the vicinity of the surface. With an increasing number of laser shots, the NO + photodesorption yield shows an initial fast decay followed by a slow decay. At 80 and 300 K, the fast decay corresponds to a cross section of 3.5 x 10 -1~ and 1.4 x 10 18 cm 2 and the slow decay to 1 × 10 19 and 1.6 x 10-19 cm 2, respectively. Dynamical studies of photon-induced desorption of N O from condensed thin films of N O on metal [237] and insulator [238] substrates have also been reported. Natzle et al. [237] studied N O films (1-2000 ML thick) condensed on A g ( l l l ) at 25-50 K with 220-270 nm laser pulses. They observed two desorption peaks in TOF, a slow one and a fast one. While the fast peak is present in all cases, the slow one appears only for thick NO layers and high laser fluences. The slow peak, with a mean translational energy of __<0.06 eV, was Maxwellian with temperatures between 160 and 280 K and has a cos 0 angular distribution. Its desorption yield increases exponentially with laser energy, and increases rapidly with NO thickness at high coverages and also rapidly with substrate temperature. Thus, the slow channel was attributed to thermal desorption resulting from bulk heating of the NO layer. For the fast peak, the total yield increases linearly with laser energy and with N O layer thickness from 1 to 50 ML but saturates at higher coverages; the TOF distribution is non-Maxwellian and non-cosine with a mean

Photochemistry at adsorbate / metal interfaces

189

translational energy of 0.22 eV; the T O F spectrum is independent of laser wavelength and energy. Below 10 ML, the T O F peak shifts to lower energy. Raising the substrate temperature from 28 to 67 K increases the yield only slightly, but moves the T O F peak to higher energy and sharpens the angular distribution (from cos40 at 28 K to cosT0 at 67 K). The fast channel is highly vibrationally excited with u = 0 - 3 states almost equally populated. The average vibrational energy is < 0.4 eV. The rotational energy in u = 2 corresponds to a temperature of 2500 K. The desorption yield increases with Ar spacer layer thickness but is attenuated when Ar is deposited on top of NO. The wavelength-dependent desorption yield follows the solid N O optical absorption spectrum; the fast peak was attributed to non-thermal photodesorption involving single-photon electronic excitation of condensed N O dimers. Cousins et al. [238] studied laser (193 nm) induced desorption of N O from N O films condensed on MgF 2 at 30 K. They found that the average translational energy ( ( E t . . . . ) ) and the T O F maximum shift to higher energy as either the film thickness or laser flux increases and that the rotational energy is more than - 10-fold smaller than E t r a n s for both 1, = 0 and 1. For example, for E t.... ( p = 0 ) from 0.14 to 0.71 eV, {Erot) ranges from 0.009 to 0.024 eV (corresponding to 105 to 220 K). For desorbing N O molecules with low translational energies ( < 0.22 eV), the spin-orbit population ratio is comparable with Trot. For those with higher Etrans, the spin-orbit population exceeds that expected from Trot. For molecules with E t r a n s = 0.6 eV, the vibrational distribution of (u --- 1)/(~ = 0) is 3 + 1%. They attribute the photodesorption to "explosive" ejection involving an initial electronic excitation followed by rapid local conversion to heat. The explanation of translationally fast but rotationally cold molecules is given in terms of collisional cooling following desorption (as in supersonic gas expansions) or rotationally constrained desorption. The above photodynamic studies used nanosecond laser pulses. A novel dynamic study of photodesorption of N O from P d ( l l l ) was performed recently by Prybyla et al. [62] using 200 fs pulses of 620 nm (2.0 eV) laser photons. In their experiments, N O was adsorbed at 300 K with only fl and "r states (bridge-bonded with a binding energy of - 1 eV) populated. Fig. 49 shows the N O photodesorption yield versus laser fluence. The non-linear relationship is fit by an intensity power law with an exponent, n = 3.3. At 3 m J / c m 2, the photodesorption quantum

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X.-L. Zhou, X.- Y. Zhu and J.M, White

190

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efficiency is 3 x 10 - 4 . This is comparable to a one-photon cross section of 4 x 10 19 cm 2, 10 4 higher than for N O photodesorption on P t ( l l l ) using nanosecond laser pulses [51]. The translational energy distribution corresponds to a temperature of 600 K. The velocity distribution of N O depends only modestly on rotational energy. Desorbing N O is vibrationally excited with a population ratio of (ul)/(%)= - 0 , 3 , corresponding t o Tvi b = 2200 K. Fig. 50 shows rotational-state distributions f o r 2i-[3/2 a n d 2 I I 1 / 2 spin-orbit levels in both u = 0 and v = 1. The distributions are non-Boltzmann and can be described by two rotational temperatures, 400 and 2600 K, for low and high rotational quanta, respectively. The spin-orbit levels are about equally populated in both the ground and first vibrationally excited states. These observations are distinct from conventional thermal desorption. The unusual laser-fluence-dependent photodesorption yield, the higher cross section compared to nanosecond laser pulses, and the higher vibrational temperature than translational temperature are all distinctive for femtosecond photoprocesses. In another very interesting observation involving longer, i.e., nanosecond, laser pulses, the upper limit of the photodesorption cross section is 10 2~ cm 2, two orders of magnitude lower than for the 200 fs pulses [62]. One novel desorption mechanism proposed by the authors differs from conventional thermal and photochemical processes. The idea relies on an "electronic" resorption; i.e. the surface-molecule vibration is strongly coupled to the electronic excitation in the substrate. In this mechanism, desorption is a consequence of the highly non-equilibrium conditions, on a femtosecond time scale, in the solid and adsorbate. For excitation within the substrate, they estimate a peak electronic temperature of 3000 K for a 200 fs pulse with 3 m J / c m 2 laser fluence. The electronic excitation requires - 1 ps to equilibrate with the lattice. At an electronic temperature of 3000 K, the desorbing N O becomes vibrationally hot, but has lower average translational and rotational energies due to dynamical effects like those observed in conventional thermal desorption. As an alternative to the "electronic" desorption mechanism, the authors [62] also consider excitation through hot electron and hole capture by the P d - N O complex. The higher cross section observed for femtosecond pulses is

Photochemistry at adsorbate / metal interfaces

191

attributed to: (1) an enhanced flux of energetic electrons or holes at the surface for each absorbed photon or (2) multiple excitation of the adsorbed N O molecules. There are also several studies which focus on the kinetics and mechanisms, rather than the dynamics, of photon-induced reactions of chemisorbed NO. Ho and co-workers studied photon-induced reactions of N O on Ag(111) [230], Cu(111) [289], Si(111) [52,232] and GaAs(110) [56]. They used Xe and H g - X e arc lamps (250-800 nm) for N O on A g ( l l l ) , C u ( l l l ) and S i ( l l l ) and a CW laser radiation (457-900 nm) for N O on G a A s ( l l 0 ) . On these surfaces, photoeffects were observed from UV to IR, and these are briefly described here For adsorption of N O on A g ( l l l ) [230], it was found that, at low coverages, N O is adsorbed both dissociatively (to N and O) and molecularly in two different three-fold bridge sites with a desorption peak temperature of 110 K. At higher coverages, additional N O is adsorbed molecularly in atop sites with a desorption peak temperature of 90 K, and at saturation there is a new bent or tilted N O species in atop sites. Some N20 is formed and remains adsorbed on the surface. Upon irradiation of the saturated N O overlayer on A g ( l l 1), they observed desorption of N O and a small amount of N20 (one tenth of the N O intensity). The source of N20 was not identified but could be due to one or more of the following: (1) direct photodesorption of surface N20, (2) indirect coupling of N20 to excited NO, (3) photosynthesis of N 2 0 from photoinduced reaction of two NO, and (4) reaction of N O with coadsorbed N. They found two molecular N O photodesorption channels, one with a high cross section, of, leading to an exponential decay of N O signal, and another one with a small cross section, a S, leading to a constant N O signal. The latter was attributed to readsorption of N O from the background during photodesorption. At 280 nm, of is 5.4 x 10 -]8 cm 2 and 2.1 × 10 -18 cm 2 for N O and N20 photodesorption, respectively. Post-irradiation T P D and H R E E L S results indicate that the photodesorbing N O is from atop sites, not three-fold sites. They speculate that the excitation of N O adsorbed in three-fold bridge sites is quenched very efficiently due to stronger coupling to Ag. As shown in fig. 51, the maximum photodesorption rate increases slowly from 500 to 360

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192

X.-L. Zhou, X.-Y. Zhu andJ.M. White

nm and then rapidly from 360 to 280 nm. From the measured small surface temperature rise (1.5 K) during irradiation, they conclude that the desorption is non-thermal. They attribute photodesorption to both direct excitation from 2 ~ c to 2-%no~c (2~ro~c and 2"~u,o~c are occupied and unoccupied orbitals resulting from interactions of N O 24* and Ag orbitals) and charge transfer excitation from Ag4s and 3d bands to 2,~uno~c. For hv < 4 eV, excitation from 2"%~,~ and Ag4s is the driving force, while for hv > 4 eV excitation from Ag 3d also contributes. On C u ( l l l ) [289], N O adsorbs molecularly in both atop and bridge sites at 90 K. Irradiation results in desorption of N O and synthesis and subsequent desorption of N20. As on A g ( l l 1), only atop-bound N O was photoactive. The wavelength-dependent photodesorption of N O is almost the same as on A g ( l l l ) . The N O photodesorption signal could be fit by a single exponential decay. The desorption cross section at 320 nm is 2.2 × 10 ~ cm 2 for N O and 1.9 × 10 -z8 cm 2 for N20. The results were interpreted as for Ag(111). After a saturation exposure at 90 K on Si(111) [52-55], the surface contains molecular N O bound in atop and bridge sites, dissociated N and O, and the reactive adsorption product N20. Unlike on A g ( l l l ) and Cu(111), both atop- and bridge-bonded N O on Si(111) are photoactive. During irradiation, N O desorbs and dissociates, and additional N20 is synthesized, which desorbs molecularly and also dissociates to N 2 and O. In this case, N20 synthesis was attributed either to the reaction of N O with hot N atoms from photodissociation of N O or to collisional reaction of photodesorbing N O with surface N O or N. The N20 formed during adsorption undergoes mainly photodissociation. For low incident power ( < 100 m W / c m 2 at 514 nm), they found a square-root power dependence of the m a x i m u m N O photodesorption intensity and a linear dependence for high incident power. Wavelength-dependent results show increased N O photodesorption with decreasing wavelength reaching saturation for ?~ < 350 nm. For ?~ > 350 nm, the wavelength-dependent photodesorption intensity follows closely the square root of the optical absorption of Si(111). The cross sections at 330 nm for photodesorption of NO and N20 were found to be the same - 1.1 × 10-~Tcm -~. Since the lowest electronic excitation energy and the dissociation energy of NO in the gas phase are 5.5 and 5.3 eV, respectively, whereas the photoreaction of N O on S i ( l l l ) is driven with photon energies as low as 2.4 eV, they attribute the results to photogenerated hot holes in the substrate. For photodesorption of NO, they suppose the hole migrates toward the N O - S i complex and annihilates one of the bonding electrons. Since N O is negatively charged on S i ( l l l ) , the excited state after hole capture is unstable and relaxes via desorption. For dissociation of NO, they suppose the hole annihilates an electron in the Si-Si surface bond, causing it to break. The result is a Si radical (defect) which reacts with NO. They found n-type more photoreactive toward N O than p-type Si, which they believe supports the hole-mediated desorption and dissociation mechanism, since holes experience a surface barrier for p-type Si but are accelerated toward the surface in n-type Si. They estimated that hot carriers created within 400 A of the surface can reach the surface to interact with NO. They also estimate that at most 1.3% of the total number of hot carriers actually induce reactions of NO. The saturation of the photodesorption intensity for X < 350 nm is attributed to the fact that only hot holes created at distances shorter than 400 .~ from the surface can interact with adsorbed NO. After adsorption of N O in the presence of K, the concentration of N20 increases while that of N O decreases [232]. As on K-free Si(lll), irradiation results in desorption of N O and synthesis of N20. They make a very important observation and one that confirms substrate excitation; the desorption intensities of NO, N20 and N 2 decrease exponentially with increasing K coverage (or work function decrease). At 0.6 ML K, there is no desorption of NO, but reaction to form N20 still occurs. The branching ratio, dissociation/desorption, of N20 increases with K coverage. On G a A s ( l l 0 ) [56], N O adsorbs at 90 K mostly in a molecular state, but with some

193

Photochemistry at adsorbate / metal interfaces

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decomposition and reaction to form N20. The effect of irradiation is similar to that on Si(lll), both involving photogenerated hot carriers. Upon laser irradiation, N O desorbs and dissociates, leaving behind a disordered and partially oxidized surface, and, in addition, N20 desorbs, along with its dissociation product, N 2. The N O desorption signal is proportional to the square root of the incident laser power at < 10 W / c m 2. The N O photodesorption rate decreases with increasing wavelength and follows the square root of the optical absorbance of GaAs for h < 620 nm. The observed downward deviation above 620 nm was attributed to a decrease in the number of photogenerated carriers that have sufficient energy to overcome the reaction barrier. The quantum yield was estimated to be 4 x 10 -4 molecules per absorbed photon. In another study focused on kinetics, Yoshinobu et al. [231] studied the photodesorption of N O adsorbed at 87 K on clean, oxygen-covered, oxidized and sulfur-saturated Ni(111) in the wavelength range between 325 and 827 nm (Hg-arc lamp). They found neither photodesorption nor photodissociation of N O on clean Ni(111) and a small rate of NO photodesorption on oxygen-covered Ni(111) (o = 7 x 10-19 c m 2 at 325 nm). In contrast, enhanced single-photon-induced non-thermal desorption (no photodissociation) of NO was found on oxidized and sulfur-saturated Ni(111). The photodesorption was first-order in N O coverage. On oxidized and sulfur-covered Ni(111), photodesorption occurs from weakly bound NO states. On partially oxidized Ni(111), there is one dominant weakly bound state ( - 210 K), and, accordingly, there is one photodesorption channel. On fully oxidized Ni(111), there are two photodesorption channels attributed to two distinct weakly bound states ( - 2 0 5 and 290 K). The high cross section channel (o = 3.3 x 10 -18 cm 2 at 325 nm) corresponds to the lower temperature T P D state and the low cross section one (o = 5 x 10-19 cm 2 at 325 nm) to the high temperature T P D state. Fig. 52 shows the photodesorption cross sections from partially and fully oxidized Ni(111) (higher cross section channel) and sulfur-covered Ni(111) versus photon energy and the optical absorption coefficient of NiO. The photon energy thresholds on the three surfaces are the same, 1.5 eV, and much lower than optical absorption of NiO involving interband transition. The authors excluded the possibility of O z- 2p--, Ni 2+ 3d interband transition, which is important for photodesorption of CO on NiO [24]. They favor direct excitation, from

194

X.-L Zhou, X.- Y. Zhu andJ.M. White

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100 200 300 400 Time-of-Flight [~ts] Fig. 53. TOF spectra, obtained with the mass spectrometer, of NO from photodissociation of NO/on NO/Pd(lll). The lines represent least-squares fits using a superposition of two modified Maxwell-Boltzmann distributions. The inset shows the cross sections for the formation of both channels as a function of excitation energy. After Hasselbrink et al. [154]. filled 2 , ~ * ( N O ) - d ( N i ) b o n d i n g orbitals (below the F e r m i level) to e m p t y a n t i b o n d i n g o r b i t a l s 2 " ~ * ( N O ) - d ( N i ) ( a b o v e the F e r m i level), of the N i - N O complex. T h e y believe that the h o t electron a t t a c h m e n t m e c h a n i s m m a y also c o n t r i b u t e but, b e c a u s e of the high cross section, is less i m p o r t a n t t h a n the direct excitation. 4.6. Nitrogen dioxide

NO2, which tends to dimerize readily, is a very interesting molecule with c o m p l e x p h o t o c h e m i s t r y a n d p h o t o p h y s i c s . U s i n g p u l s e d excimer laser (3.5, 4.0, 5.0 a n d 6.4 eV) e x c i t a t i o n a n d T O F , L I F a n d T P D detection, b o t h the kinetics a n d the d y n a m i c s of d i m e r i z e d N O 2 p h o t o c h e m i s t r y on N O - c o v e r e d P d ( l l l ) at 105 K were s t u d i e d b y Ertl a n d co-workers [154]. A n N O p r e c o v e r e d surface was used to simplify the e x p e r i m e n t s b y e l i m i n a t i n g a p h o t o l y s i s c h a n n e l that p r o d u c e s N O that is r e t a i n e d b y the substrate. P h o t o d i s s o c i a t i o n of N O 2 a n d c o n c u r r e n t N O d e s o r p t i o n were observed. T h e p h o t o d e s o r p t i o n cross section was 3 × 10-18 c m 2 at 6.4 eV a n d d e c r e a s e d r a p i d l y with decreasing p h o t o n energy, e.g. it is an o r d e r of m a g n i t u d e lower at 4 eV. T h e d y n a m i c s of d e s o r b i n g N O were q u a n t i t a t i v e l y d e t e r m i n e d b y T O F a n d L I F . T w o N O d e s o r p t i o n channels were identified, as shown in fig. 53. F o r the faster channel, N O d e s o r p t i o n is n o n - t h e r m a l , has a m e a n t r a n s l a t i o n a l energy, ( E t. . . . ) / 2 k = 800 K, i n d e p e n d e n t of p h o t o n energy, a n d is highly r o t a t i o n a l l y excited. T h e t r a n s l a t i o n a l a n d r o t a t i o n a l energies for the fast c h a n n e l were positively correlated; (Etra,~s)/2k increases linearly from 680 to 1900 K for J from 1.5 to 34.5. T h e B o l t z m a n n p l o t of the r o t a t i o n a l d i s t r i b u t i o n for N O d e s o r b e d in u = 0 in the fast channel (velocity = 1100 m / s ) is linear a n d yields a r o t a t i o n a l t e m p e r a t u r e of - 900 K.

Photochemistry at adsorbate / metal interfaces

195

B o t h 21-[3/2 a n d 2HI/2 manifolds are equally populated. At velocities of 1470 and 890 m / s , the corresponding rotational temperatures are 1050 and 710 K, respectively. The population of NO in u = 1 is estimated on the order of a few percent. The angular distribution was strongly peaked, cos46, toward the surface normal. For the slow channel, both the translational ( - 135 K) and rotational energies of N O were accommodated to the surface temperature (120 K during irradiation); ( E t r a n s ) / 2 k is independent of the internal energy of NO. The Boltzmann plot of the rotational distribution for NO desorbed in the slow channel (velocity = 250 m / s ) can only be fit in two separate regimes by rotational temperatures of 115 and 470 K for internal energies below and above - 250 cm-1, respectively. The non-thermal and thermal character of the fast and slow channels, respectively, are also reflected in their wavelength dependences (inset of fig. 53); the fast channel shows a rapid rate increase above 4 eV, whereas the slow one shows practically no dependence on wavelength. The N O desorbed in the slow channel was ascribed to NO molecules which, after excitation, underwent a trapping-desorption process. The photon excitation mechanism was determined by carefully measuring the N O desorption yield as functions of angle of incidence and polarization. For both s- and p-polarized light, the angular dependence is nicely fit by the bulk Pd absorptivity at both 5 and 6.4 eV. Attempts to fit the data using the direct adsorbate-substrate excitation model were unsuccessful. Therefore, the photochemistry of N O 2 on NO-covered Pd(111) was attributed to substrate excitation involving excited electrons with energies both above and below the vacuum level. The intermediate was proposed to be N204- which dissociates to NO 3 and NO. Using 193 nm laser radiation, Mull et al. [103b] studied the photochemistry of NO 2 adsorbed on epitaxially grown oxide NiO(100) on Ni(100). At 90 K, NO2 adsorbs as a monomer at low coverages with the dimer, N204, growing in at high coverages. During laser irradiation, NO is the main desorption species. The photolysis cross section for the monomer is 3 X 10 -19 c m 2 (compared to 6 × 10 -19 c m 2 for gas-phase NO 2 absorption) and 4 × 10 -18 cm 2 for the dimer.

4. Z Ketene

Another interesting kind of adsorbate is ketene, C H 2 C O , and its photochemistry on P t ( l l l ) was the first adsorbate-metal system successfully studied in White's lab [213]. CH2CO has a well-established photochemistry in the gas phase [20], where it photodissociates to methylene (CH2) and carbon monoxide (CO). Its surface photochemistry was clearly demonstrated by T P D and SIMS spectra, which differed significantly depending on whether the system was illuminated. However, the lack of vibrational spectroscopy and the interference, in TPD, of the thermal decomposition of CH2CO made interpretation of the photochemistry difficult. Nevertheless, this early study showed that the photochemistry of CH2CO on P t ( l l l ) is competitive with quenching by the metal and the authors speculated that photodissociation leads to C - C bond cleavage to give adsorbed CH 2 and CO. This system deserves further attention; not only is the photochemistry itself of interest, but the potential for preparing methylene groups in the presence of CO makes it a valuable model system for investigating certain aspects of hydrocarbon synthesis reactions. 4.8. Azomethane

Motivated in part by the desire to prepare adsorbed methyl fragments, Hanley et al. [228], using TPD, studied the photolysis of chernisorbed and physisorbed (CH3)N 2 on P d ( l l l ) at 87 K continuously irradiated with light from a Hg-arc lamp (hu < 5.4 eV). The gas-phase optical

196

X.-L. Zhou, X.- Y. Zhu and J, M. White

absorption by ( C H 3 ) 2 N 2 includes a broad resonant transition, from the N lone-pair electrons to N = N e * . The onset is at - 3.0 eV, the peak at 3.6 eV (a = 2 x 10 .20 cm 2) and the fall-off near 4.5 eV. Between 4.5 and 5.5 eV, the absorption is very weak. The (CH3)2N2 chemisorbs through the N = N bond and in TPD, mostly decomposes to H C N and H 2 above 250 K. Physisorbed (CH3)2N 2 desorbs molecularly between 106 and 114 K. N o photolysis was evident in the chemisorbed monolayer but in the multilayer, analogous to the gas phase, photolysis breaks the C - N bond, producing C H 3 radicals and N 2. The N 2 desorbs during photolysis at 87 K; a fraction of the C H 3 radicals scavenge H atoms and desorb as methane; the other C H 3 remains on the surface. The multilayer photolysis starts at - 3.1 eV and the yield increases with photon energy up to 5.4 eV, the highest photon energy available. This does not mimic the gas-phase optical absorption spectrum and points to an important contribution from substrate excitation, particularly above 4.0 eV. The absence of photolysis in the monolayer is striking and was attributed to structural and electronic modification of (CH3)2N2 upon chemisorption, which alters the optical response a n d / o r enhances the relative importance of quenching. From the perspective of preparing interesting intermediates, relevant to catalytic hydrocarbon synthesis, this system is very attractive.

4.9. SO, and H2S Castro and White [152] studied the photochemistry of SO 2 adsorbed on A g ( l l l ) at 100 K with bandpass-filtered h > 250 nm CW radiation. The thermal adsorption and desorption of SO 2 on Ag(111) is completely reversible, with desorption peaks at 180 and 130 K for monolayer and multilayer, respectively. In the gas phase, the optical absorption of SO 2 has a m a x i m u m around 280 nm and is non-dissociative [20]. Likewise, irradiation of adsorbed SO 2 shows no photodissociation; only photodesorption was observed. Between 0.1 and 1 ML, the photodesorption rate at 313 nm increases linearly with coverage. Below 0.1 ML, most SO 2 bonds to defect sites and yields a higher photodesorption cross section than for SO 2 bound to ordered terrace sites. Above 1 ML, the rate drops sharply. The wavelength-dependent results are very interesting. As shown in fig. 54, the cross section for 1 ML decreases from 254 to 290 nm,

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Photochemistry at adsorbate / metal interfaces

197

increases and reaches a maximum at 330 nm, and then drops sharply above 330 nm (the cross section at 313 nm is 2.8 × 10 -20 cm2). Photodesorption is negligible above 360 nm. For 2 ML, the cross section is at least 10 times smaller than for 1 ML, at all wavelengths. As for H 2 0 / P d ( 1 1 1 ) [155], all of the photon-driven desorption originates in the excited species formed within the first monolayer. The second and higher layers of SO 2 inhibit the photodesorption pathway via a momentum transfer effect; i.e., the kinetic energies of the photodesorbing molecules are shared with molecules above the first layer. The resonant behavior in SO 2 photodesorption above 290 nm tracks the optical absorbance of Ag. It is thus concluded that the observed resonance is due to a bulk plasmon excitation which peaks at 326 nm [290] and which presumably scatters into single-electron-excited states that can couple to surface-bound SO 2. The Ag surface plasmon excitation, which peaks at 344 nm, cannot be excited on flat surfaces [38,290], and d-band excitation should not have a resonance of this width in this spectral region. The photodesorption mechanism is attributed to attachment of subvacuum electrons forming SO2-, which desorbs following the Antoniewicz model [108]. Harrison et al. [159,160] studied the photon-driven dynamics of H2S adsorbed on LiF(001) at 114 K. H2S weakly physisorbs on LiF(001) and desorbs with a broad peak extending from 114 to 140 K. In the gas phase, H2S is photodissociated to H and HS at 193 and 222 nm. Some of the HS radicals are vibrationally excited, a fact reflected in the H translational energy distribution. The ratio of HS*(u > 1 ) / H S 0 , = 0) depends on wavelength, but there are some variations from lab to lab [291-293]. On LiF, photodissociation to HS(a) and H(g) occurs from submonolayer to multilayer coverages of H2S. The T O F of H is bimodal, consistent with production of ground and vibrationally excited HS fragments. The photodissociation yield increases with dose and saturates at >_ - 1 L dose ( - 1 ML). Fig. 55 shows the translational energy distribution of H photodissociated from H2S in the gas phase and adsorbed on LiF(001) at 222 nm. The narrow high-energy peak is due to HS in u -- 0 and the broad low-energy peak to HS in p > 1. The H atoms produced from adsorbed H2S have nearly the same kinetic energies as those produced in the gas phase. However, the H S * / H S ratio (1.6 and 1.0 for 0.11 and 1.1 L doses, respectively) is much higher than in the gas phase. When H2S is dosed at 150 K, adsorption occurs principally on defect sites, and the photodissociation produces only H atoms corresponding to vibrationally excited HS. It is proposed that breaking the symmetry of the U2S molecule upon adsorption would lead to increased predissociation of excited H2S from a distorted configuration that yields HS*. Similar results are observed at 193 nm; the maximum H kinetic energy is 2.5 eV and the H S * / H S ratio is 1.4 for a 0.55 L dose. No photolysis of adsorbed H2S is found at 248 nm. The angular distribution of H atoms also shows bimodality, the extent decreasing with the reduction in HaS coverage and becoming - cos 8 at very low doses (0.04 L). In addition to photodissociation to produce H atoms, an interesting photoreaction to produce H 2 occurs for coverages higher than 0.2 ML ( - 0 . 2 L dose) [160]. At 222 nm, the translational energy distribution of H 2 is also bimodal, a narrow low-energy peak (with a peak energy of 0.017 eV and a F W H M of 0.061 eV) and a broad high-energy peak (see below for peak energy and FWHM). The yields of both slow and fast H 2 increase nearly linearly with doses below 2.2 L but approaches saturation at higher doses. The ratio of fast-Hz/slow-H 2 decreases from - 5 for 0.2 L to - 2 for multilayer doses. Both slow and fast H 2 production yields increase linearly with laser fluence, indicating a single-photon process. For a multilayer coverage, the fast H 2 follows cos28 distribution and the slow H2, COS40.The fast-H2/slow-H 2 ratio increases from - 2 at 50 o (detection angle with respect to the surface normal) to - 5.5 at 60 o. The peak energy of the fast H 2 increases from - 0.45 eV at 5 o to - 0.6 eV above 20 o while the F W H M is relatively constant ( - 1 eV) below 40 o and decreases to - 0.8 eV at 60 o.

198

X.-L. Zhou, X.- Y. Zhu and J.M. White

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For a detection angle of 5 °, the peak energy of the fast H 2 drops from - 0.67 eV at < 1 ML to - 0.5 eV at multilayer coverages while the F W H M ( - 1 eV) is invariant. The formation of H 2 is attributed to reaction of photogenerated H atoms with adjacent H2S molecules, i.e. H + H 2 S - * H 2 + HS. This is supported by the fact that the relative H 2 / H yield increases markedly with coverage; H production saturates at - 1 L dose while H 2 yield increases above 1 L dose. The maximum kinetic energy of H 2 at 222 nm is 2.1 + 0.2 eV, also consistent with the sum of the energy released by the reaction (0.53 eV) and the maximum H kinetic energy ( - 1.7 eV). The fast H 2 is attributed to reaction with HES of those H atoms that recoil with a significant component of momentum directed away from the surface. The slow H 2 is attributed to reaction of H atoms moving parallel to the surface (formed from S - H bonds lying in the surface plane); the resulting H E suffers a large number of collisions before escaping from the surface. The photoreaction at 193 nm is interesting. Although the H atoms have higher kinetic energies than at 222 nm, the T O F shows a H 2 peak with a lower translational energy, 0.032 eV. Molecular H2S desorption is also observed in TOF at 193, 222 and 248 nm [160]. For doses < 1 L ( ~ 1 ML), there is only one broad and slow HES TOF peak. For doses > 1 L, an additional narrow and fast peak appears. The slow peak is attributed to photoacoustic

Photochemistry at adsorbate / metal interfaces

199

desorption mediated by photon absorption of color centers in the crystal [158]. The fast peak is attributed to quenching of electronically excited H2S by neighboring molecules; the result is a transfer of electronic energy into translational energy [161]. Both peaks can be characterized by Maxwellian distributions with superimposed stream velocities. At 222 nm, the slow peak increases nearly linearly for doses ranging from 10 -2 to 102 L, while the fast peak shows a threshold at - 1 L dose and increases at large doses. For the translational energy distribution of the slow peak, both the peak energy (45-95 meV) and the F W H M (110-250 meV) increase with H2S dose; the former levels off for doses above a few L, while the latter does not. The photodesorption yield also increases linearly with laser fluence. The angular distribution depends on H2S coverage: - cos40 for 0.045 L dose and - cos110 for doses > 0.45 L. Both the translational energy and the yield of this slow peak decrease in the order 222 > 248 > 193 nm. For doses between 1 and 36 L, the translational energy distribution of the fast peak at 222 nm is characterized by a peak energy of 0.78 + 0.05 eV and a F W H M of 0.6 + 0.2 eV, suggesting that it occurs predominantly from the uppermost layer. In an interesting comparison, for 1 : 1 mixed thick solid of D20 and N H 3 on a quartz plate, pulsed 193 nm laser irradiation produced H 2 and HD, but a negligible amount of D 2 [241]. The mean translational temperature was, however, only 160 K, close to the surface temperature ( - 130 K), and there was no high kinetic energy hydrogen as observed from H2S on LiF(001). In addition to H a and H D in the gas phase, N2H 4, N2H 2 and N H 2 O D were the main products retained on the surface. The photoreaction is believed to be initialized by photodissociation of NI-I3, producing active H atoms.

4.10. Carbonyl sulfide The near-UV optical spectrum of gaseous OCS starts at - 2 5 5 nm, is continuous and maximizes at 225 nm with a Omax = 1 × 10-19 cm2; the photolysis products are CO and S, the latter reacting with another OCS to form CO and S2 [20]. The quantum yield of CO formation is 1.81 at both 253.7 and 228.8 nm [20]. In recent studies, electronically excited sulfur, S*, dominated; the quantum yield of ground-state S was < 0.02 [294]. In both liquid and solid phases, the S* yield is much lower [295]. On A g ( l l l ) at 100 K, Zhou and White [229], using XPS, TPD, UPS and A~, studied the photodissociation of OCS with broad-band radiation ( > 230 nm) from a Hg-arc lamp. OCS adsorbs molecularly on A g ( l l l ) at 100 K with little distortion of its gas-phase molecular structure, it lowers the work function by 0.6 eV at 1 ML coverage, and it desorbs intact at 128 K with no thermal decomposition. Photolysis produces CO(g) and 5(a); there is no photoinduced molecular OCS desorption. The dissociation extends to - 420 nm, significantly different from the gas-phase optical absorption (255 nm), and was attributed to a substrate-mediated dissociative electron attachment (DEA) mechanism with, perhaps, some direct excitation of an O C S - A g complex involving charge transfer from Ag to OCS(a). The photodissociation cross section is - 4 . 4 × 10 -20 cm 2 at 254 nm and decreases monotonically with increasing wavelength. On a wide-band-gap insulator, LiF(001) at 116 K, Polanyi and co-workers [161] studied the photolysis of OCS(a) with a 222 nm pulsed laser. The OCS adsorbs with its molecular axis perpendicular to the surface and equal probability of S and O towards the surface. Its desorption temperature shifts from 134 to 124 K as the coverage increases to 1 ML. They observed photodissociation, with desorption of energetic CO(g) and S(g) fragments, photoejection and photodesorption of molecular OCS and, significantly, photoreaction between photolytically generated S* with OCS(a) to form both excited and ground state S2(g), CO(g) and

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CO(a). There was a significant dependence on surface preparation. At 222 nm, the photodissociation cross section on an unannealed surface decreased monotonically from 3.3 × 10-16 cm 2 at 2 x 1 0 - 4 L to 2 x 1 0 - ] 7 c m 2 at 0.11 L dose; on an annealed surface, the cross section was 10 times lower. Both are much higher than for gas-phase photolysis (10-19 cm 2) [161a,b]. Direct contact between OCS and LiF was required for the enhancement since interposing a H 2 0 spacer layer led to undetectable photodissociation yields [161a,b]. Surface and bulk defect F-centers, i.e., a substrate-mediated effect, was proposed to account for the enhancement. According to this model, these centers absorb photons and, by long-range electronic-to-electronic energy transfer, activate adsorbed OCS. Photolysis at 308 nm was below the detection limits. The translational energy distribution, P(trans), of the S fragments in the adsorbate phase shows a single peak with a peak translational energy, Tp(S), of 0.13-0.18 eV and F W H M of 0.38-0.44 eV, depending on coverage. This is compared with a bimodal distribution in the gas phase with To(S) = 0.27 eV ( F W H M = 0.12 eV) and - 0.9 eV, respectively. Interestingly, 50% of the S products from adsorbed OCS photolysis have translational energies in excess of the maximum translational energy observed in the gas phase and the maximum translational energy for S photoproducts from adsorbed OCS reaches the thermodynamic limit ( - 1.15 eV) for a collinear gas-phase dissociation to ground-state S(3p) and C O ( J = 0). With increasing OCS(a) coverage from 1 0 - 4 to 0.1 ML, the peak energy in the translational energy distribution, Tp(S), increases from 0.13 to 0.18 eV while the F W H M of P(trans) increases from 0.38 to 0.44 eV. The angular distribution of the S fragment followed a cos 0 dependence. For the CO photofragment derived from gas-phase photolysis, the P(trans) is bimodal: To(CO)= 0.3 eV with F W H M = 0.12 eV for C O ( J = 55) and T0(CO ) = 0.13 eV for C O ( J = 66). For OCS on LiF(001), P(trans) shows a single peak with Tp(CO) = 0.08 eV and F W H M = 0.23 eV. Unlike S atoms, P(trans), for CO does not extend to the thermodynamic limit for a collinear dissociation, a fact attributed to internal excitation of CO. In contrast to the gas phase, where S / S * ~ 0, for photodissociation of OCS(a) on LiF this ratio was no less than 0.33. In addition to photodissociation, photoejection (PEJ) of molecular OCS occurred for coverages above 0.5 ML and at 222 nm [161c]. The translational energy distribution is non-Boltzmann and has a peak at 0.28 eV and a F W H M of 0.21 eV. The angular distribution follows a cos180 dependence, very sharply peaked toward the surface normal. At 308 nm, gaseous OCS is transparent and no PEJ was found; direct absorption of OCS(a) appears to be required. The proposed mechanism involves self-quenching, a process in which the energy of an electronically excited OCS* is transferred, in part, to a neighboring OCS, This involves relaxation to the electronic ground state and concurrent electronic-to-translational and internal energy conversion. The resulting redistribution leaves large amounts of energy in coordinates that are effective for desorption; thus, OCS is ejected. The PEJ yield increased rapidly between 0.5 and 2 ME and saturated above 2 ML. Saturation indicates the formation of ejected products only from the topmost two OCS layers. The estimated PEJ cross section was 4.7 X 10-19 cm 2 for a 2 ML coverage. This is comparable to the optical absorption cross section of solid OCS (1.3 x 10 -18 cm2), but is - 12 times larger than in the gas phase. A second admolecule desorption process with different dynamical properties, denoted photodesorption (PDES) and ascribed to processes mediated by LiF, was observed [161c]. In contrast with PEJ, PDES occurred at both 222 and 308 nm with the same translational energy distribution and is, thus, believed to be mediated by substrate absorption. It is characterized as non-thermal since the calculated temperature rise during the laser pulse is too low to drive it. This desorption was interpreted in terms of defect centers in LiF which absorb radiation. Subsequently, the absorbed energy is converted into phonons which, following propagation to the surface, like a shock wave, induce desorption of OCS(a). The PDES quantum efficiency was -

Photochemistry at adsorbate / metal interfaces

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quite high, estimated to be - 0.1. The PDES translational energy distribution varied with OCS coverage, detection angle, and laser pulse energy. For a detection angle of 5 ° off the surface normal, and a laser pulse energy of 8.7 mJ/pulse, Tp increased from 0.018 to 0.051 eV and the F W H M increased from 0.06 to 0.185 eV over the dose range from 6 × 10 -3 to 23 L. For a 2.1 L dose, a detection angle of 5 o, and increasing laser energy from 0.8 to 14.6 m J/pulse, To varied from 0.045 to 0.065 eV and F W H M from 0.14 to 0.25 eV. For a fixed dose and laser energy, both T v and F W H M decrease by - f o u r - f o l d for detection angle varying from 0 to 5 °. The PDES angular distribution narrows continuously from - cos 0 at - 3 × 10 -~2 ML to - cos110 at > 1 ML coverages. One of the important aspects of this work, was reaction to form S2 in what Polanyi and co-workers denote as a surface-aligned photoreaction (PRXN) [161d]. There were two, direct and indirect, PRXN pathways with different dynamics. For OCS coverages, based on a sticking coefficient of unity, below 0.01 ML, the direct PRXN dominates. For coverages varying from 0.0001 to 0.01 ML, the P(trans) of S2 is invariant, with a Tp of 0.09 eV, a F W H M of 0.25 eV and a maximum translational energy (0.9 eV) corresponding to the thermodynamic limit for formation of ground-state S2 and CO from S* and OCS. Over this coverage range, the angular distribution narrows from ~ cos 0 to - cosS0 which, the authors believe, suggests a change in alignment and hence in PRXN dynamics. The Tp is lower than that found in the gas phase (0.25 eV), which is ascribed to significant energy transfer into internal excitation of the PRXN product and the substrate. The P(trans) is invariant with detection angle for low coverages ( < 0.01 ML). For coverages higher than 0.01 ML, both direct and indirect P R X N ' s take place. For direct PRXN, both Tp and F W H M are higher and the angular distribution is narrower at high coverages. For example, at 1 ML, Tp = 0.16 eV, F W H M = 0.42 eV and the S2 follows a cosT0 distribution. This change was attributed to the onset of reaction between molecules in adjacent layers of adsorbate above a critical coverage. The indirect PRXN, with a Tp = 0.01 eV and a F W H M = 0.05 eV at 1 ML, is believed to be a trapping-desorption process because interposing HzO layers on top of OCS(a) eliminates this channel. The P(trans) of the direct P R X N was not perturbed by adding water. 4.11. M e t a l c a r b o n y l s

Metal carbonyls have a rich photochemistry in the gas and liquid solution phases [87]. The photochemical studies of metal carbonyls adsorbed on solid surfaces are mostly related to the deposition of metal films using carbonyls as precursors. To date, three carbonyls, Fe(CO)5, Mo(CO)6 and W(CO)6, adsorbed on metals, semiconductors and insulators, have been studied. In most cases, they adsorb molecularly and weakly at low temperatures. Photodissociation results in the retention of M(CO) x (x 4: 0) fragments and the desorption of CO in all cases, except one study which reported that Fe atoms were deposited on Si(111)-(7 × 7) by UV photodissociation of adsorbed Fe(CO)5 [255]. Because of their intrinsic organometallic character, we do not limit this part of the literature review to metals. In an early study, Celii et al. [166,167] studied the photodissociation of Fe(CO) 5 adsorbed on A g ( l l 0 ) at 120 K using 337 nm laser radiation. During irradiation, they observed desorption of CO but not Fe-containing fragments. They interpreted these results in terms of the photodissociation of an F e - C O bond with the release of CO into the gas phase (CO does not stick to Ag(110) at 120 K). Although no post-irradiation surface analysis was done, they proposed retention of Fe(CO)x (x < 5) fragments. They also studied photodissociation of Fe(CO)~ on Si(100) and AlzO 3, finding similar results. For the same Fe(CO)5 coverage, particularly around one ML, and the same laser irradiation condition, the CO desorption yield

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was independent of the substrate (metal, semiconductor and insulator). They concluded that photodissociation was the result of direct absorption of the UV fight by adsorbed Fe(CO)5 and that the dissociation rate of the resulting excited state was faster than any quenching mechanism. In the gas phase, Fe(CO)5 exhibits a weak absorption near 330 nm, ascribed to a ligand field transition [167]. Using classical energy transfer theory [117] and assuming the dipole is located 4.5 ]~ (the van der Waals radius of Fe(CO)5 ) from Ag(ll0), they estimated a quenching lifetime between 0.3 and 1.4 ps and inferred an upper limit for the dissociation lifetime of Fe(CO)5 of 0.3 ps. Recently, Henderson et al. [253] studied photodissociation of monolayer and multilayer Fe(CO)5 adsorbed on A g ( l l l ) at 90 K with 256 and 365 nm radiations. Photolysis breaks a fraction of the F e - C O bonds in Fe(CO)5 and produces a proposed Fex(CO)y cluster which is less susceptible to photon irradiation than Fe(CO)5 and thermally decomposes at 330 K with the desorption of CO's and the deposition of clean Fe. The photodissociation cross sections for 1 and 5 ML Fe(CO)s, respectively, are 9 . 2 × 10 -~8 and 9 . 4 × 1 0 - t 8 c m 2 at 256 nm, and 5.5 × 10 -20 and 3.3 × 10 -20 cm 2 at 365 nm. For 1 ML coverage, on average, 2.4 CO's for each Fe(CO)5 are retained in Fe~(CO)y at 365 nm and 3.7 CO's are retained at 256 nm. For 5 ML coverage, the average number of CO for each Fe(CO)5 retained is 2.1 and 2.3 at 365 and 256 nm, respectively. These results are taken as indicating that quenching by A g ( l l l ) is greater for 256 nm than for 365 nm photons and that quenching is greater for excited Fex(CO)y species in the first monolayer than in the multilayer. Several studies of photolysis of Fe(CO)5 adsorbed on Si have also been reported. After irradiating Fe(CO)5 adsorbed on Si(100) at 77 K using a high-pressure Hg-arc lamp, Jackman and Foord [256] found an appreciable amount of C and O, besides Fe, in AES. U p o n heating the surface to 300 K prior to recording AES, the C and O signals were reduced to a few atomic percent relative to Fe. Since Fe(CO)5 did not thermally decompose on Si(100), they believed that Fe(CO)x fragments were produced upon UV irradiation of Fe(CO) 5 and decomposed thermally, producing gas-phase CO and surface Fe-containing fragments. Bartosch et al. [296], in studying photodecomposition of Fe(CO)5 on Si(100), observed CO-containing Fe carbonyl fragments. In contrast, photodeposition of clean Fe on Si(lll)-(7 × 7) from Fe(CO)5 was reported by Swanson et al. [255]. For Fe(CO)5 adsorbed on S i ( l l l ) at 120 K, desorption of CO, but no Fe(CO)x, was observed in desorption during UV laser irradiation (193, 249, 308 and 337 nm). After irradiation, no partially decarbonylated Fe(CO)~ (x < 5) fragments were detected using IR. In addition, T P D showed no new features after laser photolysis; thermal decomposition of Fe(CO)~ would give new CO T P D peaks. The deposited Fe was clearly evident in AES. At 720 nm, irradiation showed no photoeffects even though absorption by Si is strong. Thus, the authors concluded that photodissociation in the UV resulted from direct absorption of light by Fe(CO)5 , which induces metal-to-ligand charge transfer (d ~ 2,~ *) and ligand field excitations, just as in the gas phase. They also concluded that the photodissociation was a two-photon process. Between 337 and 193 nm, two photons provide enough energy (6 eV in the gas phase) to eject all five CO ligands from Fe(CO)5. A very different picture for F e ( C O ) 5 / S i ( l l l ) was reported by Gluck et al. [254]. First, they observed a mixture of molecularly and dissociatively adsorbed Fe(CO)5 on S i ( l l l ) at 90 K. Second, laser irradiation at 257 nm did not give clean Fe, but Fe(CO)x fragments. Third, irradiation at 514 nm did not cause decomposition but did cause bonding changes. Photolysis of W(CO)6 on S i ( l l l ) was studied by Ho and co-workers [254,262] and by Friend and co-workers [267]. UV laser irradiation resulted in desorption of CO and retention of W(CO)x fragments. The photodissociation is attributed to direct excitation (metal-to-ligand

Photochemistry at adsorbate / metal interfaces

203

charge transfer and ligand field excitation) as in the gas phase. Ho and co-workers [254,262] reported that irradiation at 257 nm caused at least two CO molecules from each W(CO)6 to desorb and that the resulting fragments decomposed thermally, yielding CO at 350, 420 and 465 K in subsequent TPD. Irradiation at 514 nm did not decompose W(CO)6 but caused a bonding change which was attributed to either band-gap excitation or red-shifted direct excitation induced by adsorption. Friend and co-workers [267] studied this system in more detail using 193, 248, 308, 351 and 720 nm laser radiation. They found that both monolayer and multilayer W(CO)6 were dissociated by UV, but not visible, light and that the photodissociation of multilayers was a wavelength-dependent single-photon step. The wavelength dependence is similar to the absorption spectrum of W(CO)6 in a hydrocarbon solution at 77 K. Compared to multilayers, the photon-induced chemistry of monolayer W(CO)6 was significantly quenched and showed almost no wavelength dependence, pointing to the involvement of photon-excited substrate electrons and holes. The isolated W(CO)x photofragments were photoinactive upon further UV irradiation. They proposed that, in the submonolayer, the fragments could coordinate to the substrate and that the suhstrate quenched any further excitation of the fragments on a very rapid time-scale. For multilayer coverages, the photofragments were unreactive toward CO and C6H 6 yet formed metal carbonyl cluster compounds. Photolysis of Mo(CO)6 adsorbed on several metal surfaces and the (100) and (111) faces of Si has been reported. On Rh(100), Mo(CO)6 chemisorbed dissociatively to form disordered CO and Mo(CO)x mixtures in the first layer at 80 K; physisorbed molecular layers formed above the first layer [259]. Irradiating the multilayer at 325 nm caused partial decarbonylation of Mo(CO)6 (at least two M o - C O bonds break) and concurrent desorption of CO. The resulting Mo(CO)x fragments were stable and inactive to further irradiation. No photodesorption was observed by irradiating the first layer, though changes in surface bonding, composition and structure were indicated in HREELS. The photodissociation cross section for physisorbed Mo(CO)6 was (1.5 + 0.1) × 10 -18 cm 2, compared to 4.4 × 10 -17 cm 2 in the gas phase. The quantum yield was - 5%, indicating strong quenching of excited Mo(CO)6. On CO-saturated Rh(100), Mo(CO)6 adsorbed molecularly and underwent photodissociation more readily, because of reduced quenching efficiency, than Mo(CO)6 adsorbed on clean Rh(100). On Mo(100), UV photodissociation of Mo(CO)6 to Mo(CO)x occurred [266]. Detailed mechanistic studies of Mo(CO)6 photolysis have been performed on Si(100), Si(lll), A g ( l l l ) , C u ( l l l ) and graphite. Fig. 56 shows typical Mo(CO)6 photodissociation spectra for monolayer and multilayer (3.5 ML) coverages on A g ( l l l ) [7]. The desorbed CO was monitored as a function of irradiation time by QMS. At t = 0 s, the irradiation (1000 W xenon-arc lamp) started and the CO signal increased promptly. This rise was followed by a monotonic decay, which could be fit with the sum of two or three exponentials (a fast initial decay followed by slower ones). This kind of time-dependent photodissociation is characteristic of metal carbonyls on different suhstrate surfaces [267,259,260]. For the case shown here, the initial rise of the CO signal from the monolayer was seven times smaller than from 3.5 ML. The reduction, beyond that expected on the basis of coverage, was attributed to a faster quenching rate for adsorbates closer to the surface. On Si(100) [265], Creighton reported photodissociation of Mo(CO)6(a ) to CO(g) and Mo(CO)x(a ) (x = 5) with a cross section of (5 + 3) × 10-17cm 2 at 248 nm. This is comparable to the absorption cross section of gas-phase Mo(CO)6. Mo(CO)x(a ) decomposed thermally at 335 K to form CO(g) and surface Mo, C and O in a stoichiometry of MoCO0.3. The dissociation was a resonant process - no photodissociation at 351 nm and a smaller cross section at 193 than at 248 nm laser radiations. This resonant single-photon behavior was also found on Si(lll), C u ( l l l ) , A g ( l l l ) and graphite as shown clearly in fig. 57 [7,41]. The Mo(CO)6

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Mo(CO)6/Ag(1 1 1) 2 8 5 ± 5 . 5 nrn c

_d t_. (3

-6 c03

I

I

I

1

I

o

50 lOO 15o Time (s) Fig. tiff.CO mass signal from photodissociation of monolayerand multilayer Mo(CO)6 on Ag(lll) at 80 K. Irradiation is with a 1000 W Xe-arc lamp monochromaticsystem. After Ho [7].

exposure was 1 L, corresponding to approximately 1 ML, and the light source was a 1000 W xenon-arc l a m p / m o n o c h r o m a t o r system. The resonant photodissociation can be fit nicely by the absorption spectrum of Mo(CO)6. The experimental data shown in fig. 57 are calculated from the initial rise of the CO signal, I, divided by photon flux, F, and were scaled to the absorption spectrum of Mo(CO) 6. The photodissociation cross section does not vary significantly with the substrate: 6.0 × 10 -~9 cm 2 at 330 nm on Si(111), 1.8 x 10 -17 and 7.7 x 10 -~8 cmz at 285 and 325 rim, respectively, on Ag(111), 1.7 x 10 -17 and 2.2 x 10 -~8 cmz at 290 and 325 nm, respectively on Cu(111) [7]. For Ag(111), there is a small resonance at - 330 rim, attributed to a substrate excitation contribution, namely, an interband transition of Ag [41]. However, as discussed in section 3.4, bulk Ag plasmon excitation cannot be ruled out [230]. The substrate-independent resonant photodissociation of Mo(CO)6 indicates strongly that the primary step is absorption of photons by adsorbed Mo(CO)6. This conclusion was confirmed by studying the variation with polarization and angle of incidence [41]. The resonance at 290 nm is ascribed to Mo(4d) to CO(2¢r*) charge transfer and the shoulder at - 3 2 5 nm to the ligand field transition (Mo(4d --* 4d* )). Post-irradiation surface analysis has been reported for Mo(CO)6 on Si(lll). Ho and co-workers [254,260] reported that Mo(CO)6 adsorbed molecularly on Si(111) at 90 K, that it desorbed at - 2 0 0 K without thermal decomposition, and that laser irradiation at 257 nm caused at least two, and probably up to four, M o - C O bonds to cleave. H R E E L S showed that the resulting Mo-carbonyl fragments were stable upon further irradiation. T P D showed three CO desorption peaks (306, 366 and 439 K) after irradiation. Based on the kinetic analysis, only 10% of the excited Mo(CO)6 decomposed by releasing a CO ligand. Zanoni et al. [264] found, however, that dissociative adsorption occurred for low exposures ( < 0.5 L) of Mo(CO)6 on S i ( l l l ) at < 100 K, and that photoinduced metallization (complete dissociation) of Mo(CO)6 on Si(111) occurred only with irradiation at hu > 8 eV. Ho and co-workers found that significant changes in photodissociation of Mo(CO)6 took place when it was adsorbed on K-covered C u ( l l l ) and Si(111) [7,41,54]. The nature of

Gr/

Photochemistry at adsorbate / metal interfaces

5 o

~.~ ~/~. Ag(111)

205

(a) K/Cu 260

300

340 260 Wavelength (nm)

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03 E

340

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l

•

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I

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II i

250

300

350

400

Wavelength (rim) Fig. 57. Wavelength dependences of the initial photodissociation rate (measured as the maximum CO desorption signal as shown in fig. 56) for 1 ML Mo(CO)6 adsorbed on A g ( l l l ) , graphite, C u ( l l l ) and S i ( l l l ) at 80-90 K. The dashed curves for A g ( l l l ) and graphite and the solid curves for C u ( l l l ) and S i ( l l l ) are the optical absorption spectrum of Mo(CO)6 measured in cyclohexane solution. After Ha [7] and Ying and Ha

300

I

xlO

I

II

i

I

I

40O 500 600 700 Wavelength (rim)

800

Fig. 58. Wavelength dependences of the photoyield for 1 ML Mo(CO)6 adsorbed on K-preadsorbed surfaces. The linear Y scale is in arbitrary units for the different panels. The K coverages for the data shown in panels (a), (b), and (c) are 0.15, 0.5 and 0.2 ML, respectively. After Ying and Ha [41b].

[41b].

Mo(CO)6 adsorbed on K-covered surfaces does not differ significantly from K-free surfaces. As shown in fig. 58, for that K coverage (0.15 ML on Cu(111) and 0.5 ML on Si(111)) which provided the maximum decrease in work function, the photodissociation of Mo(CO)6 (1 L exposure) occurred for wavelengths longer than 800 nm, significantly red-shifted from K-free surfaces ( - 3 5 0 nm). In the UV region, the cross section increases significantly and the resonant photodissociation from direct absorption makes a much smaller fractional contribution, especially on Si(111). The change in the wavelength dependence of the photodissociation was attributed to a new photodissociation channel introduced by a work function decrease associated with adsorbed K. This new channel does not involve direct excitation of Mo(CO)6. The polarization and incidence angle dependence and the wavelength dependence in the red-shifted region demonstrate that the new channel is due to substrate excitation [41]. Fig. 59 shows the energy levels of Mo(CO)6 adsorbed on Si(111) and K/Si(111). The energy levels of

X.-L. Zhou, X.-Y. Zhu a n d J . l ~ White

206

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level

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Fermi level

4d' (1T19)

-6 +d (1Alg) -8

-10 -12

5~/1~

-14 -16

4~

Fig. 59. Schematic diagram of the electronic levels of physisorbed Mo(CO)6 molecules and Fermi levels at K-free and K-preadsorbed S i ( l l l ) and C u ( l l l ) . The energy levels are approximate and the energy scale is different below and above ~ 8 eV. After Ying and Ho [41b].

Mo(CO)6 , inferred from photoemission and electronic ELS results [297,298], were referred to the vacuum level since the adsorbate was physisorbed. The work function of M o ( C O ) 6 / K / Si(Cu) is - 2.9 eV, - 2.0 eV lower than Mo(CO)6/Si(Cu). The rise of the Fermi level makes excited levels of Mo(CO)6 energetically accessible for photogenerated hot electrons but makes the occupied levels more difficult to reach by photogenerated holes. Therefore, they concluded that hot electrons induced the dissociation. In fig. 59, the lowest unoccupied state, 4d*, is lower than the Fermi level on K / S i . They make the questionable proposal that, even so, 4d* was not fully occupied prior to photon irradiation since there is a potential barrier through which the tunneling of thermally excited electrons is difficult. This proposal will be reasonable if the adsorbate and substrate have negligible coupling. On the other hand, photogenerated hot electrons, having much higher kinetic energy, tunneled easily and attached to antibonding orbitals, forming Mo(CO)6 which, as in the gas phase [299], dissociated. In fig. 58c, where OK (0.2 ML) is lower and the work function is higher than in fig. 58b (0.5 ML), the photoyield at 436 nm is only 2% of that in fig. 58b. This suggests that the local interaction between K and Mo(CO)6 is not responsible for the enhanced photodissociation because otherwise the photoyield would scale with the K coverage [41]. Besides a new channel, there are other changes on K-covered surfaces: (1) the power dependence of the photoyield on K / S i is fractional (0.5-0.7) but remains linear on K / C u ; (2) in the UV region, the cross section increases significantly and the resonant photodissociation from direct absorption of Mo(CO)6 becomes obscure, especially on K / S i . The latter change was attributed to a superposition of both direct and substrate excitations.

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4.12. Metal alkyls Metal alkyls are important precursors for growth of metal and semiconductor films. While most surface photochemical studies of metal alkyls have so far been conducted on semiconductor or insulator surfaces, we review them briefly here for the sake of completeness. There have been numerous engineering studies on laser-assisted deposition of thin films using metal alkyls in an ambient vapor ( > mTorr) [11-13,300]. These studies, which have been reviewed [11-13], yield little information on the mechanism(s) at the molecular levels of photodecomposition at adsorbate/substrate interfaces. Here we focus on photochemical studies conducted under U H V conditions. Nearly ten years ago, Osgood and co-workers showed that irradiation with 257.2 nm photons of an adsorbed first monolayer of trimethylaluminum (TMA) and dimethylcadmium (DMCd) on a quartz surface resulted in photodissociation of the m e t a l - C H 3 bond and deposition of metal atoms [245]. Recently, they studied laser (193 and 248 nm) photolysis of chemisorbed D M C d and dimethylzinc (DMZn) on an OH-containing quartz surface at room temperature [249]. After irradiation at 193 nm of chemisorbed D M C d and DMZn, UV transmission spectroscopy shows a smooth and structureless spectrum which is stable upon further irradiation and which is significantly different from that before irradiation [249]. The results are interpreted in terms of UV photolysis, which releases the methyl groups and forms a metal film. As expected on the basis of gas-phase absorption, at 248 nm, the photolysis rate of adsorbed D M C d is much slower than at 193 nm and there is no photolysis for adsorbed DMZn. The photodissociation cross section for adsorbed DMZn at 193 nm is - 10-18 cm 2 with a quantum yield of - 0.1. In another study [250], D M C d was chemisorbed on an oxidized Si surface which also contained OH groups. Using total internal reflection IR spectroscopy, Osgood et al. found a loss of CH-containing species at 193 nm but not at 248 nm. Post-irradiation XPS shows the loss of both surface Cd and C. A fraction of the loss of C - H IR absorption intensity during 193 nm laser irradiation was attributed to non-thermal photodesorption of chemisorbed D M C d molecules. The photolysis cross section at 193 nm for adsorbed D M C d is - 4 x 10-18 cm 2, smaller than the gas-phase absorption cross section (2 × 10 -17 cm2), a property attributed to substrate quenching. Villa et al. [247], using TOF and REMPI, studied the photolysis of In(CH3) 3 (TMIn) adsorbed on a quartz surface at 150 K (estimated maximum coverage of 10 ~4 molecules/cm 2) with 222 nm pulsed-laser radiation. In the gas phase, TMIn absorbs strongly between 200 and 250 with a maximum at 213 nm [301]. Photolysis of adsorbed TMIn results in formation of a film of In metal on the surface with desorption of C H 3, In, InCH 3 and In(CH3) ~ neutral fragments and In + and I N C H , ionic fragments. The photofragmentation is non-thermal; the kinetic energies of desorbing fragments are independent of laser flux and each fragment has a different translational temperature. The translational energy distributions of the neutral fragments change with TMIn coverage. Doubling T M I n coverage increases the translational energy from 324 to 664 K for C H 3 , from 497 to 917 K for InCH 3 and from 484 to 848 K for In(CH3) 2. This coverage effect is attributed to the formation of TMIn aggregates of increasing size, which presumably weakens the intermolecular and molecule-surface bonds, making more of the photolysis photon energy available for translational excitation. The TOF profiles of the ionic fragments are much narrower than expected for Maxwell-Boltzmann distributions and have translation energies on the order of 2 eV. This is taken as an indication of an electric field created on the surface during the laser pulse. Similarly, NH~- and N H ~ ions originating from a mixture of N H 3 and H 2 0 ice and photoejected, by high intensity ( > 1 J / c m 2 with a pulse width of 10 ns) 193 nm laser radiation, possess much higher translational energies (1-4 eV) than the ejected neutral species ( - 0.1 eV) [241].

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Lubben et al. [242], using HREELS, XPS and AES, studied photolysis of monolayer and multilayer AIz(CH3) 6 (TMA) adsorbed on Si(100) and S i ( l l l ) with 193 and 248 nm pulsed lasers. The absorption cross section of T M A in the gas phase is - 10- iv and 10-19 cm 2 at 193 and 248 nm, respectively. They found no photoeffects at 248 nm. At 193 nm, irradiation with low laser intensities causes monomerization of adsorbed TMA dimers, while irradiation with high laser intensities results in partial decomposition of TMA monomers and desorption of some CH 3 fragments. Since XPS shows no evidence of loss of A1, all AI(CH3) n (n = 1 and 2) fragments are believed to remain on the surface. Some CH 3 remains on the surface and polymerizes to form higher hydrocarbons. Subsequent heating of the resulting irradiated surface leads to evolution of H 2, leaving A1 and C on the surface. In contrast, Orlowski and Mantell [243], using TOF, found desorption of CH 3 and AI(CH3) . (n = 1-3) upon pulsed-laser irradiation at 193 nm of T M A adsorbed on Al-covered SiO2/Si at 298 K. The desorbing AI(CH3) 3 has a translational temperature of 388 K, close to the calculated surface temperature of 380 K during laser irradiation. The photodesorption of AI(CH3) 3 is attributed to photodissociation of the T M A dimer, producing the monomer which then desorbs at nearly the surface temperature. The photodissociation fragments have much higher energies; for 0.2 ML, the translational temperatures are 920 K for AI(CH3) 2, 1290 K for AICH 3 and 990 K for CH 3. As for In(CH3) 3 on quartz [247], the translational temperature of AI(CH3) 2 increases with coverage. There are thresholds for AI(CH3) 2 and A1CH 3 desorption; that for A1CH 3 is considerably larger than for AI(CH3) 2. The authors took this as evidence for a sequential photodecomposition process. For TMA adsorbed at 300 K on hydroxylated A1203 and SiO 2 and pulse-irradiated with a 193 nm laser, Higashi [246] found desorption of CH 3 and but not AI(CH3) .. In this case, T M A is strongly chemisorbed because of surface OH groups. The photodecomposition cross section at 193 nm is - 10 -17 cm 2, close to the gas-phase optical absorption cross section of TMA. Interestingly, the CH~ TOF is characterized by a Maxwell-Boltzmann distribution with a temperature of 150 K, only half of the surface temperature. The explanation, not very satisfying, is that the excited state has a long lifetime to dissipate much of the excitation energy, and desorption of CH 3 occurs from shallow potentials in a weakly bound state. Higashi and Rothberg [244], using pulsed optoacoustic spectroscopy, showed that methyls can be removed by 193 nm laser irradiation from a TMA-saturated sapphire surface containing OH groups, leaving behind high-quality AI films. A 248 nm laser, however, was ineffective. Trimethylgallium (TMG) adsorbed on a quartz surface can be photodissociated by 248 and 355 nm but not by 532 nm lasers [214,248]. Ga atoms are photodesorbed and detected by LIF. The Ga LIF intensity does not vary between 20 and 200°C, but it decreases from 200 to 400°C. This temperature effect is taken to indicate that Ga atoms are generated by thermally assisted photodissociation of the adsorbed TMG. Mantell and Orlowski [251], using TOF, studied 193, 248 and 351 nm laser photolysis of tri-isobutylaluminum (TIBA) adsorbed on A1. They observed, at all three wavelengths, photoninduced non-thermal desorption of "fIBA, rupture of isobutyl groups to create methyl radicals and B-hydride elimination to form isobutylene. The translational temperatures, at 193, 248 and 351 nm, respectively, are 283, 196 and 125 K for isobutylene (monitored at mass 39); 338 and 214 K (no data at 351 nm) for methyl; and 505, 224 and 268 K for TIBA. (Although the surface temperature was not specified in the paper, the authors said that the translational temperature is considerably lower than the surface temperature in a number of the cases, especially at longer wavelengths). The ratio of isobutylene TOF signals without correction for angular distribution is 1 : 0.6 : 0.05 for (193 nm): (248 nm) : (35 1 nm) and the ratio of corresponding TIBA T O F signals is 1 : 0 . 7 : 0 . 4 . In the gas phase, TIBA shows no absorption at 351 nm and negligible

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absorption at 248 nm. The results demonstrate red-shifted photolysis of adsorbed T I B A on the A1 surface as compared to the gas-phase photolysis. Based on the low translational temperatures of photodesorbing species and red-shifted photolysis, they proposed a mechanism for photolysis of surface TIBA involving hot electrons excited in the A1 substrate. 4.13..4 romatics

Goncher et al. [64,268] studied the photochemistry of several aromatic molecules (pyridine, pyrazine, benzaldehyde, acetophenone, aniline, benzene), and cyclohexane and cyclohexene adsorbed on rough Ag surfaces. R a m a n spectroscopy at the 1580 c m - 1 band of surface carbon was used to monitor photolytically produced graphitic carbon from these adsorbed molecules. The light source was a continuous ion laser giving wavelengths between 350 and 410 nm. On rough Ag surfaces, there is an enhanced local surface field [302] confirmed by experiments such as surface-enhanced R a m a n scattering and the observation of efficient second harmonic generation [303]. An enhanced photodecomposition rate is also expected. Among these molecules, they found no photofragmentation for benzene, acetophenone, cyclohexane and cyclohexene under their irradiation conditions (350.7 and 406.7 nm). Photofragmentation was found for the other molecules, with the rate being higher at 406.7 nm than at 350.7 nm, except for benzaldehyde. Based on laser power dependence studies at 406.7 nm, the initial excitation step is a two-photon process. For benzaldehyde, single-photon excitation dominated at 350.7 but two-photon excitation at 406.7 nm. The excitation was interpreted as intra-adsorbate resonant excitation. The wavelength dependence of photofragmentation of pyridine was studied in more detail. There was no photodecomposition at 457.9 and 514.5 nm, attributed to the absence of a resonant two-photon absorption, but, with decreasing wavelength, the decomposition rate increases rapidly to a maximum at - 400 nm and then decreases gradually. For pyridine, the photodecomposition rate at 125 K is - 15 times higher than at 90 K. This suggests that the reaction of intermediate species leading to surface carbon is thermally activated. Using benzene as a spacer layer, the distance dependence of the pyridine photodecomposition rate was studied. With increasing spacer layer thickness, the rate increased, reaching a m a x i m u m at 15-20 ,~, after which it decreased. They interpreted the distance-dependent results as a combination of enhanced molecular absorption and quenching by energy transfer to the surface. The maximum rate was interpreted in terms of different length scales for these two processes; surface-enhanced absorption, longer length scale, is still present when the energy quenching rate has decreased substantially. In a similar experiment, Wolkow and Moskovits [65] studied the photochemistry of pyrazine and triazine on rough Ag with 514.5 nm laser radiation. Graphitic carbon is the major photolysis product. Like Goncher et al. [64], they found that the initial excitation at 514.5 nm is a two-photon process. Their results indicate that enhanced photolysis takes place most efficiently directly at the metal surface rather than a short distance above it, as might be expected by considering the competition between non-radiative relaxation of the excited states and excitation by the surface-enhanced electric field. In addition, they found that the photolysis rate can be increased by overcoating the monolayer pyrazine or triazine with an appropriate thickness of either the same or different molecules. This is because the localized surface plasmon of the rough surface can be tuned by bringing the overlayer thickness into resonance with the photolysis laser. They concluded that, in Goncher's experiment [64], pyridine reaches the Ag surface either by diffusion through the benzene spacer layer at 90 K or by contamination from a previous experiment and suggested that the same mechanism causes the observed spacer layer effect.

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4.14. Reactions among coadsorbed species One very interesting prospect, now beginning to be realized for surface photochemistry on metals, is the possibility of promoting novel reactions and novel reaction dynamics. On insulators, this subject has been pioneered by the Polanyi group and one case, OCS on LiF, was described in the previous section [160,161,215]. On metals, there was some early work, not very well characterized, as discussed in section 1. More recently, Ho and co-workers have taken up this subject and have studied photoinduced reactions on P t ( l l l ) of CO + 02 [36], N O + 0 2 [269] and H + 02 [270]. The coadsorbates were continuously irradiated using a 150 W Xe-arc lamp at a substrate temperature of _< 100 K. It is known that O 2 adsorbs molecularly as a peroxo species on Pt(111) at 100 K with thermal dissociation occurring above 150 K and that photodissociation of O2/Pt(111 ) extends to longer wavelengths than in the gas phase [85]. The CO + 02 system [36] was prepared by saturating Pt(111) with 02 followed by a saturation exposure of CO at 100 K. In subsequent TPD, there were sharp CO 2 production peaks at - 150 and - 320 K. Isotope labelling was important. After adsorbing 13C180 and 1~O2, the ~3C~O2 mass signal promptly rose when the shutter to the light source was opened. The carbon dioxide signal decayed exponentially with irradiation time and dropped to zero when the shutter was closed. When 13C~80 was coadsorbed with atomic 180, no photoproduct of 13C1802 was observed. T h e 13C1802 photoyield from 13C180 + ~802/Pt(111 ) and the photoyield of atomic O from O 2 / P t ( l l l ) have the same wavelength dependence; both decrease monotonically with increasing wavelength and have a threshold of - 450 nm. This threshold for atomic oxygen is, for unknown reasons, at longer wavelengths than that (300 nm) found by Zhu et al. [85]. The photoreaction cross section was (3.3 + 0.5) × 1 0 - ! 0 cm 2 at 338 nm and 2 × 10 -21 cm 2 at 443 nm. The initial step of the oxidation reaction is attributed to the photoelectronic excitation of the adsorbed 1802. The dissociation produces hot O atoms (with high translational energy or possibly electronically excited) which collide and react with neighboring CO. Alternatively, it is possible that, once adsorbed O2 is electronically excited, the O C - O bond formation and the O - O bond dissociation may occur in a concerted fashion. To study N O + O 2 / P t ( l l l ) the substrate was initially saturated with 1802 at 85 K and then dosed with 1 L of 14N160 (70% saturated) [269]. On O2-saturated P t ( l l l ) , N O occupies both normal bridge ( u ( N - O ) = 192 meV) and atop ( u ( N - O ) = 216 meV) sites, but there is also a new state ( u ( N - O ) = 236 meV and p ( P t - N O ) = 49 meV). When irradiating N O + O 2 / P t ( l l l ) , Mieher and Ho found no N O 2 desorption, only N O and 02. The wavelength dependences are different for photoinduced desorption of N O and 02. At 358 nm, the cross section is (8.8 -F 1) × 10 -19 cm 2, - 4 × 103 times larger than that at 355 nm for N O on P t ( l l l ) alone. The cross section for 02 desorption is also - 2 times larger than that for 02 on P t ( l l l ) alone. Interestingly, irradiation of N O + O 2 / P t ( l l l ) leads to no atomic O. Clearly, coadsorption is changing the yields and perhaps the paths to photoproducts. Catalytic water formation continues to be a widely studied topic and the h y d r o g e n oxygen-platinum system is a benchmark in this area. Ho's group [270] has examined the photochemistry in the H + O z / P t ( l l l ) system using a surface saturated with 1802 at 85 K followed by 10 L exposure of H 2. Some dissociation of 1802 and formation of HzlSO occur during H 2 exposure. Upon irradiation, they found very little desorption of H2180 and nearly the same photodesorption rate of 1802 as for a802 on P t ( l l l ) alone. After irradiation at 330 nm, they found H R E E L S evidence for the formation of O H species and increased amounts of H2~80. The O H formation was strongly wavelength-dependent, decreasing from 330 to 600 nm with a cross section at 330 nm of (8.6 + 1.3) × 10 -21 cm 2. The formation of O H and H 2 0 was attributed to a reaction between photogenerated hot O atoms and surface H and OH. As for

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the formation of CO 2 from CO and 0 2, concerted formation of H - O bonds and dissociation of O - O bonds should also be considered. 4.15. Photoinactive adsorbate-metal systems

As shown in table 1, there are many cases where photolysis of adsorbates is not observed. In the gas phase, some of the listed molecules are transparent at the wavelengths employed and this may also be the case for these molecules adsorbed on metal surfaces. Photolysis using higher energy photons will be interesting and we already know that H 2 0 on Pd(111) is readily photolyzed at 193 nm, but appears to involve substrate excitation [155,240]. On the basis of gas-phase data [20,185], we expect direct UV absorption by adsorbed (CH3)2CO and C6H6, but there is no photodissociation [64,268,273,274]. In these cases, electronic excitation, even in close proximity to the metal, may not be sufficient to drive dissociation channels.

5. Summary and prospects As demonstrated in this paper, photodissociation of intra-adsorbate bonds at a d s o r b a t e / metal interfaces often competes strongly with substrate quenching, the traditionally expected channel. While quenching does sometimes dominate, promotion of photodissociation also occurs. Dissociation due to substrate excitation is clearly evident in a number of systems and measurements now exist that demonstrate a strong correlation between photoelectron yields and photon-driven dissociation yields. Direct excitation of the adsorbate or adsorbate-substrate complex is also important, particularly in multilayers, but in a few cases for monolayers as well. The dominant excitation pathway depends on wavelength, on the adsorbate, and on the metal substrate in ways which are not yet generally predictable. As a newly emerging area, surface photochemistry has a bright future, both as a fundamental research area of chemistry, physics and engineering and as an area with important p r a c t i c a l / technological implications. Work on the dynamics and the mechanism(s) of various systems continues vigorously. Dynamic studies using a pulsed laser as the UV source, combined with time-of-flight mass spectrometry, laser-induced fluorescence, and multi-photon ionization, will provide much information on the internal (translational, vibrational and rotational) energy distribution of the desorbing photofragments and on how the photofragments form. Advances in femtosecond excitation and spectroscopy, will make it possible to unravel the short-time-scale dynamics that are important in these systems. All these aspects of surface photochemistry can advance our understanding of adsorbate-surface interactions in general. Although only mentioned in passing here, photochemical effects initiated by core level excitations, both shallow and deep, may hold prospects for very specific bond excitations because of the exceptionally localized nature of the initial excitation. In addition to metal single crystals, supported metal thin films, from submonolayer to multilayer, are of interest. Semiconductor and insulator supports are of interest and such work can build on the growing body of knowledge about photochemical dynamics of adsorbates on the semiconductors and insulators themselves. Since excited substrate electrons play an important role, studying the low-energy electron-induced chemistry of a d s o r b a t e - m e t a l systems, in parallel with the photochemistry, will be interesting and productive. An obvious and practical application of photodissociation is the synthesis of interesting intermediates that are important in heterogeneous catalysis. Many intermediates have been proposed, but few observed, because they are not stable enough to accumulate in large

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concentrations at operating experimental conditions. Photodissociation of adsorbate-metal systems provides the opportunity to prepare large concentrations of those intermediates at low surface temperature and to study their reactivities. Beyond this preparative interest, photondriven processes at metal/adsorbate interfaces may offer unique paths, structures and products, not available via thermal excitation. In this review, we have noted a few photon-driven chemical reactions; this is an area ripe for further investigation.

Acknowledgement We gratefully acknowledge the stimulating contributions of many colleagues to the results reported in this article. We particularly thank Ms. Pare Cook for splendid editing of our English and Drs. A. Cassuto, S.K. Jo and C.R. Flores for useful comments. This research was supported in part by the National Science Foundation, by the US Department of Energy, Office of Basic Energy Sciences, and by the Army Research Office.

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[20]

[21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42]

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