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Structure and Bonding of Adsorbates: Investigations with Synchrotron Radiation
C. Morterra, A. Zecchina and G. Costa (Editors),Structure and Reactivity ofSurfuces 01989 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
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STRUCTURE AND BONDING OF ADSORBATES: INVESTIGATIONS WITH SYNCHROTRON RADIATION A. M. BRADSHAW Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-1000 Berlin 33, Germany ABSTRACT Using x-ray absorption, photoemission and photoelectron diffraction i t is possible to obtain important information on the structure and bonding of molecules adsorbed on single crystal metal surfaces. These spectroscopic techniques become particularly powerful when used in conjunction with synchrotron radiation. INTRODUCTION Fundamental studies in surface chemistry are necessarily concentrated in three main areas. Firstly, the elucidation of the structural details of the adsorbed layer plays a n important role, in particular, the determination of bonding site and molecular orientation. Related problems such as adlayer growth, order-disorder transitions and adsorbate-induced reconstructions also receive considerable attention. Secondly, methods are available to probe the electronic energy levels of the adsorbatekubstrate system in an attempt to obtain a better understanding of the chemisorption bond. Concomitant advances in theory have enabled molecular orbital energy diagrams in the so-called cluster approximation and, in the case of ordered overlayers, surface band structures to be calculated. Thirdly, the investigation of the dynamics of adsorption and desorption processes has recently become a highly topical area. The aim here is to elucidate the energy exchange processes occurring between particle and surface and, in the long term, a t deriving empirically the corresponding potential energy surface. The investigatory techniques used in all these studies rely heavily on spectroscopies and scattering experiments involving electrons, photons, ions or atoms. In this article we concentrate specifically on the use of photons in the vacuum UV and soft x-ray regions (XUV) for solving problems in two of these areas, namely, in studies of the structure and bonding of molecules and molecular fragments on metal surfaces. The experiments that will be described - x-ray absorption, photoemission and photoelectron diffraction are based on the phenomena ofphotoabsorption and photoionisation. X-ray absorption spectroscopy (ref. 1) and energy-scanned photoelectron diffraction (ref, 2) have recently become important structural tools in adsorption studies because, unlike low energy electron diffraction (LEED), they do not rely on the presence of long-range order in the adlayer. Since both techniques require a
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continuously tunable source of soft x-rays, usually in the region 200 - 1000 eV photon energy, their use is dependent on the availabability of synchrotron radiation a s well as on suitable "grazing incidence" grating monochromators. (This spectral range covers the IS levels of C, N and 0, which tend to feature in most adsorbate studies.) The measurement of the polarisation dependence of the x-ray absorption spectrum of a n adsorbed species near a n absorption edge (NEXAFS = near edge x-ray absorption fine structure) is a particularly straightforward technique when applied correctly. The most direct technique for probing surface electronic structure is undoubtedly photoemission, or photoelectron spectroscopy (ref. 3). In a reasonable approximation, referred to as Koopmans' theorem, the measured ionisation potentials can be equated with the ground state orbital energies calculated in the IIartree-Fock approximation. When the molecule is only weakly adsorbed a comparison with the photoelectron spectrum of the corresponding gas phase species normally suffices to assign t h e various adsorbate-induced features. For this purpose, laboratory line sources (HeI, etc.) a r e sometimes adequate. Where a strong perturbation of the energy levels of the molecule takes place, assignment can only be carried out using selection rules preferably in conjunction with polarised light. This is particularly true of molecular fragments for which no comparison with gas phase species is possible. For the application of photoemission selection rules prior knowledge of the orientation of the molecule is necessary. Even when the molecule or molecular fragment possesses no symmetry (meaning t h a t the selection rules cannot be applied), a mere change in photon energy often helps in distinguishing, or even assigning, adsorbate-induced features. In the x-ray absorption and photoelectron emission experiments described in this article, i t is always the excited photoelectron, a n Auger electron or secondary electrons which are detected. On account of the low inelastic mean free path for electrons in solids, only the first few atomic layers are generally sampled and the techniques are very surface-sensitive. (This may not be the case in x-ray absorption spectroscopy where the use of total electron yield detection can lead to high bulk sensitivity.) The inelastic mean free path is strongly energy-dependent (ref. 4)so t h a t in photoemission the surface sensitivity can in fact be maximised by choosing a photon energy which produces a photoelectron kinetic energy corresponding to the shortest mean free path (between 50 and 100 eV for most solids - see ref. 4). In the following section the specific properties of synchrotron radiation are described. I then go on to discuss the three experiments in more detail, giving a n example of the application of each to a particular adsorbate system. Lastly, their combined application is demonstrated in a study of the surface formate species which is formed as a n intermediate in the catalytic decomposition of formic acid. SYNCHROTRON RADIATION The generation, properties and uses of synchrotron radiation have been reviewed in many places. The reader is therefore referred only to a very recent book by
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Margaritondo (ref. 5)which serves as a useful introduction to the subject. Synchrotron radiation (SR) is produced when relativistic electrons (or positrons) are centripetally accelerated by the bending magnets of an accelerator or storage ring. It is emitted tangentially to the electron orbit as shown in Fig. 1. In recent years a new generation of storage rings has been built whose sole function is to serve as synchrotron radiation sources. Probably the most striking property of SR is the quasi-continuous spectrum extending from the infrared into the x-ray region. The characteristic spectral distribution curve for synchrotron radiation is shown in Fig. 2. An SR source is characterised by its critical wavelength, A,, which corresponds approximately to the position of the maximum of this curve. A, is inversely proportional to the third power of the electron energy and directly proportional to the radius of curvature of the orbit. Thus, a t the German XUV source BESSY in Berlin A, is 20 A, whereas for the DORISII storage ring (E = 3.7 GeV) in Hamburg (HASYLAB) Ac = 1.4 A. Experiments with hard x-rays are thus also possible on a high energy ring such a s DORIS. Synchrotron radiation is also characterised by its small source size and strongly directional emission. Both properties lead to a source of high spectral brilliance, a quantity which is defined as the number of photons per second in unit bandwidth, per unit solid angle and per unit area of source. Other important properties are the high degree of linear polarisation (exactly 100 % in the plane of the storage ring), the time
1
0.1
I
1.0
I
10
I
100 hl'h,
I
1000
I
10000
Fig. 1. (left) Schematic representations of the spatial distribution of electromagnetic radiation emitted by centripetally accelerated charged particles. Above: low energies; below: relativistic energies (for electrons > 40 MeV). Fig. 2. (right) The characteristic spectral distribution curve for synchrotron radiation.
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structure (useful for certain time-resolved experiments) and the fact t h a t its properties are entirely calculable via the Schwinger equations (see ref. 5). For the purposes of the surface chemical investigations described in this article photon energy tunability and polarisation are the main assets of synchrotron radiation. SR is, however, a source of "white" light and must be used in conjunction with a suitable monochromator. The so-called normal incidence region extends up to about 40 eV photon energy (- 300 A); a s the name suggests, reflections a t the optical components (gratings and mirrors) of the monochromator take place a t normal, or near to normal, incidence. At higher photon energies the reflectivity at normal incidence falls drastically and i t is necessary t h a t reflections from the optical components take place a t angles near to grazing. As a result, grating monochromators for the range 40 1000 eV are difficult to construct and the aberrations produced by the optical components are high. (Crystal monochromators can generally only be used above 800 eV.) These and other problems result in a resolution ( M A ) much lower than t h a t in the normal incidence region. Nonetheless, the advent of new designs of monochromator has recently given considerable impetus to the use of synchrotron radiation in this spectral region. Before concluding this brief account i t should be mentioned t h a t undulators, produced by causing the electron beam to oscillate transversally in a periodic magnet structure inserted into the straight section of a storage ring, can further increase the intensity of synchrotron radiation. These devices produce pseudo-monochromatic r a diation due to constructive interference between the light emitted at successive oscil lations. The intensity can be as high as a factor 10 and the brilliance a factor 103 greater than that of the radiation emitted from the bending magnets on the same storage ring. Already storage rings are being planned or constructed where the main sources of radiation are provided by undulators. XUV facilities of this type - the ALS in Berkeley, Sincrotrone Trieste and BESSY II - will provide considerable impetus in surface science. X-RAY ABSORFTION SPECTROSCOPY Below the photoionisation threshold a core electron in a free molecule can be excited into empty anti-bonding molecular orbitals (m.o.'s) as well as into Rydberg states. These transitions are observable as sharp features directly below the corresponding absorption edge (carbon K, oxygen K etc.) Above the photoionisation threshold further transitions into higher-lying anti-bonding virtual m.o.'s will occur, their broad spectral features being superimposed on the background continuum absorption. This i s shown schematically for a diatomic molecule in Fig. 3. All these features associated with the existence of the molecular potentia.1 constitute t h e near edge x-ray absorption fine structure (NEXAFS) or x-ray absorption near edge structure (XANES). At higher energies above the edge further very weak structure may be visible due to a scattering phenomenon. This is the extended x-ray absorption fine
t
electron yield (-absorption)
..
.. .- *.. .-.we
hv
Fig. 3. Left: schematic representation of the transitions observed in the x-ray absorption spectrum of a diatomic molecule. Right: typical near edge spectrum (actually for CO on a Ni(100) surface). structure (EXAFS) resulting from interference between the emitted photoelectron wave and waves backscattered from other atoms in the molecule. (The excitation into anti-bonding m.0. above the threshold may also be viewed as an EXAFS-type scattering resonance. Hence the term "shape" resonance: the energy and width of the features depend on the shape of the molecular potential.) When the molecule is placed on the surface, several factors come into play. Firstly, the experiment has to be performed quite differently. Since the substrate is usually a compact solid surface, a measurement of the absorption spectrum in transmission is no longer feasible. As shown schematically in Fig. 4a, the yield of Auger electrons, or the high energy portion of the secondary electrons (partial electron yield) are measured, both signals being proportional to the number of excited core electrons in the immediate surface region. Upon adsorption the near edge resonances may be shifted and their degeneracy lifted or they may even disappear as a result of the formation of the bond to the substrate. New resonances (so-called substrate resonances) may be observed. Since the chemisorption process invariably results in a molecule with a single, fixed orientation and since the excitations are subject to dipole selection rules, the transitions in NEXAFS are polarised. Thus, in the case of the diatomic of Fig. 3 the transition 1s + u is polarised along the molecular axis, i.e. its intensity in the absorption spectrum is given [E.M,]2 = cosea, where a is the angle between the E vector and the molecby .I ular axis and M, in a Cartesian component of the dipole matrix element. Similarly, the transition 1s + 'II is polarised perpendicular to the internuclear axis: I, [E.MXy]2= sin2a. These equations form the basis for the determination of molecular orientation, although care must be taken in their application to polyatomic molecules (ref. 7). As an example, Fig. 5 shows N 1s near edge x-ray absorption spectra of the HCN molecule adsorbed on Pd(ll1) for two orientations of the E vector relative to the surface normal (ref. 8). It is immediately clear that the 'II and u resonances behave very
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-
a) X - r a y absorptioP
T
core level
threshold
bl Photoemission
,L E L
Eco
Fig. 4. Schematic representation of the. three. experiments described in this article: (a) x-ray photoabsorption (NEXAFSISEXAFS), (b) photoelectron spectroscopy (photoemission)and (c) photoelectron diffraction.
cI Photoelectron diffraction
similarly a s the angle BE is changed. In fact, a plot of the uln intensity ratio as a function of 0~ (Fig. 5, bottom) shows that it remains essentially constant. Assuming random azimuthal orientation and a lowering of the C-N bond order to two (as confirmed by vibrational spectroscopy - see ref. 9). this result is only possible if the C-N axis lies parallel to the surface (6 = 90"). The calculated intensity ratios for other angles of inclination 6 = 85,70" and 45" are also shown. (This analysis assumes t h a t the hydrogen atom does not lower the effective symmetry.) Although the result shown in Fig. 5 looks quite definitive, closer examination shows that the accuracy of such determinations cannot be much better than about 10". If the experiment is performed for CO on the same surface, the u and H resonances have exactly the opposite polarisation dependences and show that the molecular axis is perpendicular to the surface. Returning to the general case, we note that surface atoms are likely to be relatively strong backscatterers and that the extended fine structure will be dominated by scattering from the substrate. These relatively weak modulations at higher energy are referred to as the surface EXAFS, or SEXAFS. An analysis of the SEXAFS can give
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further structural information, in particular, on the adsorption site. The technique has often been used in recent years and reviewed extensively by several authors. The reader is referred to ref. 1. PHOTOEMISSION In the photoelectron spectroscopy experiment shown schematically in Fig. 4b, the electrons emitted as a result of their excitation into states above the photoionisation threshold are analysed according to their kinetic energy. The energy balance is given simply by hv = EB E K Cp ,where EBis the ionisation (or bonding) energy of a bound level, EKthe kinetic energy of the photoemitted electron and the work function. The technique is often referred to as XPS (x-ray photoelectron spectroscopy) or ESCA (electron spectroscopy for chemical analysis), the latter being the original acronym proposed by K. Siegbahn. If the photon energy is low (hw < 50 eV), such that essentially only valence electrons are excited, then we normally refer to UPS (ultraviolet photoelectron spectroscopy) or simply photoemission. At these low photon energies the photoionisation cross-section for valence (or outer shell) electrons is also highest. For a molecule on a surface the primary excitation is a dipole transition from a molecular orbital, modified by the interaction with the substrate, into a continuum level (Fig. 6). The same photoabsorption selection rules thus apply as in NEXAFS. Whereas in NEXAFS, however, the final state is a bound or continuum level associated
+
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......... . . . e E = 90' . . . . . . .................... ........
... ......._.
.
..... . . ............ ..................... .. ..
20°
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390 1
1
400 1
1
410 1
1
420 1
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Photon energy ( eV 1 Fig. 5. Left: X-ray absorption spectra of HCN adsorbed on Pd(ll1) a t the N 1s edge. Right: Intensity ratio of the u and 'II resonances as a function of the angle between the E vector and the surface normal, BE. After ref. 8.
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nw
Fig. 6. The photoemission process in the single particle picture for a n adsorbate level on a metal surface.
metal
adsorbate
Energy balance. E~
0
electron energy distributian
-nw -.
E,,#"- a~
with the molecule itself and thus of (potentially) known symmetry, the final state continuum wavefunction in photoemission must be specifically selected in a n angleresolved experiment in order to define its symmetry. When the molecular orientation is also known, the symmetry of the initial state can then be determined. How this procedure can be used in assignment will become clear in the following example. Fig. 7 shows photoemission data from the adsorption systems Cu{ lOO}-CO and Cu{1OO}-COiK (ref. 10). It is known from NEXAFS studies (ref. 11) t h a t the C - 0 axis is perpendicular to the surface in both the pure CO layer and the coadsorption system. For most experimental geometries - meaning E vector orientation and photoelectron emission direction - a spectrum such as that of Fig. 7a is obtained. The two main features at 8.7 eV and 11.5 eV are due to the 4 0 orbital and to the overlapping 5 a a n d 171 orbitals, respectively. The shoulder a t 13 eV is a satellite indicating that charge transfer screening plays a role in this system (see ref. 12). The adsorbate-induced features are generally referred to using the same nomenclature a s the orbital in the free molecule but it should be remembered that they will be altered by the interaction with the substrate. The two overlapping features have been assigned by choosing a particular geometry for which emission from levels of o symmetry i s forbidden (Fig. 7b). This is often referred to as the "forbidden geometry": the E vector is parallel to the surface and orthogonal to the emission plane for a n adsorbate with a symmetry axis. Taken together, the two spectra show that the I n level occurs at a binding energy of 8.4 eV relative to EI: and the 50 level at about 8.7 eV. In the corresponding gas phase spectrum the (vertical) ionisation energy of 5 0 is 3 eV lower than that (if 1 TI. 'I'he increase in binding energy of the 50 level is largely due to the formation of the chemi
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sorption bond: the 50 orbital is located mainly a t the C end of the molecule and interacts most strongly with the metal states. In the co-adsorption system Cu{lOO}-CO/K a strong interaction between the species occurs a t high K:CO concentration ratios. This is characterised by a considerably lowered C-0 stretching frequency (ref. 13) and is probably indicative of a shortrange effect rather than a substrate-mediated, long range interaction. The forbidden geometry experiment for this case is shown in Fig. 7c . Immediately apparent is the splitting of the I n level due to the lower symmetry of the CO/K adsorption complex. Either a direct bonding interaction takes place with one or more K atoms or, alternatively, an electrostatic effect is responsible (the K atoms may be partially ionised). Notice, however, that the 40 feature is also absent (or very nearly absent) which indicates that the forbidden geometry experiment still works, i.e. as far as the 40 orbital is concerned, the rotational axis of the CO molecule still remains an effective symmetry element of the system. Two further remarks should be made before concluding this section. The first concerns Koopmans' theorem which turns out in some cases not to be such a trivial approximation. In most photoemission studies to date i t has simply been assumed that the relative ionisation energies can be compared directly with calculation, i.e. that relaxation indeed plays a role but the shift is the same for all orbitals. When intramolecular relaxation and image charge screening are dominant this may be a reasonable approximation. In the presence of charge transfer screening (the "unfilled
CU I1001 - CO/K L;
Fig. 7. Photoelectron spectra from the adsor tion systems Cu{lOO}-CO and Cu(OO}-CO/K. a) CO in the ( d 2 x d 2 ) R 45" structure; b) as (a) but "forbidden" geometry; c) CO/K coadsorption a t high WCO concentration ratio, also in the forbidden geometry. After ref. 10.
8.600
\a=o: € 1 kll I
1s
I
I
I
10 5 EF Energy below EF (eV)
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orbital” mechanism) this may not be the case. The second point concerns dispersion. Due to lateral interactions in ordered overlayers, overlap of orbitals on adjacent molecules leads to band formation. Energy dispersion E,(kll) is then observable in the experimental spectrum: as the polar emission angle is varied, and thus the component of photoelectron momentum parallel to the surface, kll, the binding energy E, of a feature will change. This effect can be very pronounced in strongly-bound atomic overlayers with bandwidths of 2 eV or more, but is also observed for molecular adsorbates. I t was first discovered in CO overlayers some years ago (ref. 14). PHOTOELECTRON DIFFRACTION In photoemission there is a n interesting experimental variation known a s constant initial state (CIS) spectroscopy, in which the photon energy and the analysed kinetic energy are scanned simultaneously. For the excitation out of a given occupied level, a range of final states is then sampled. This approach is particularly useful in band structure studies of solids when the momentum, or wavevector, of the final state is also known, i.e. when the emission direction is also defined. In such a n experiment on a free molecule a differential partial cross-section is measured. If the photoelectron current is angle-integrated, meaning that all the photoemitted electrons are collected, then the photoabsorption spectrum, or partial photoionisation cross-section, would be obtained. Putting the molecule (or a n atom) on a surface and taking a CIS spectrum of a core level introduces, in the same way a s in SEXAFS, scattering from the substrate (Fig. 412). The resulting interferences, which depend on the adsorption site and orientation modulate the differential cross-section; the phenomenon is normally referred to a s photoelectron diffraction (PED). I t has recently been shown that the modulations in intensity are of the order of 50 90of the background signal in experiments on the C and 0 1s levels (ref. 15). (Photoelectron diffraction effects are also observed when the photoelectron emission angle is varied a t fixed photon energy (ref. 16). I t is t h u s sensible to distinguish between energy-scanned and angle-scanned PED.) By performing model calculations for a given structure and emission direction and comparing with experiment, information on adsorption site and orientation is obtained. The data are collected by measuring a short (- 30 eV) photoelectron spectrum around the appropriate core level peak a t a particular emission angle for a series of photon energies at typically 1 - 3 eV intervals. The peak areas are integrated and normalised to the background on the high kinetic energy side to give a plot of intensity against photoelectron energy. One of the main problems is that substantial changes in the background (inelastic) photoemission signal may occur when the photon energy passes through an absorption edge of the substrate or when Auger electron features and photoemission peaks (generated by second-order light from the monochromator) are superimposed. One way of recognising these features is to record the core level peaks in the form of a group plot as shown in Fig. 8 (ref. 17). Here, the 0 1s level from the methoxy species on Cu{lOO}has been investigated in normal emission. The feature
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in the background a t 270 eV is due to carbon K W Auger electrons. We note in these raw data the strong intensity modulations referred to above which are due to backscattering from surface Cu atoms. The desired structural information is obtained by calculating such photoelectron intensitylenergy curves for various geometries and then comparing theory with experiment. There is still some controversy about the level of sophistication required in these calculations, in particular as to whether a full multiple scattering treatment is necessary. How does the technique of photoelectron diffraction relate to SEXAFS? By varying the photon energy in PED the scattered electron intensity is distributed between the various final state manifolds corresponding to different emission directions. If i t were possible to angle-integrate over the whole photoelectron current, then we would recover the SEXAFS signal. Again, we note the difference between a partial crosssection and a differential partial cross-section. In practical terms, however, i t means that in PED the interference between the directly emitted and scattered waves occurs at the detector and is determined by the path length differences. These in turn depend on the direction and separation of emitter and neighbouring scatterers. PED is thus particularly sensitive to the adsorption site of the emitter. In SEXAFS this real space information is only obtainable via the polarisation dependence of the modulation amplitude. Note too that PED modulations are typically a factor ten larger than those in SEXAFS.
0
1
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100
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300
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Photoelectron kinetic energy (eV)
Fig. 8. Raw 0 1s photoelectron diffraction data a t normal emission from the system Cu(100)-methoxy. The monochromator has been ramped in steps of 3 eV. Note the feature in the background a t 270 eV due to the carbon K W emission. After ref. 17.
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Before concluding this section i t should perhaps be noted t h a t SEXAFS h a s so far been used more widely in surface structural studies than photoelectron diffraction although only with mixed success in the soft x-ray region. On the other hand, t h e potential of PED has not yet been fully realised; the PED analysis for the surface formate species described below illustrates this point. THE SURFACE FORMATE SPECIES ON C u ( l l 0 ) The decomposition of formic acid on metal and oxide surfaces i s a model heterogeneous reaction. Many studies have shown that i t proceeds via a well-defined reaction intermediate - the surface formate species - which can be isolated and investigated with spectroscopic techniques. Adsorption of formic acid at low temperatures gives rise to a molecular species on the surface which deprotonates on warming. On a Cut1 10) surface thisoccurs at 270 K, giving the formate species, which in turn decomposes a t 450 K with evolution of H2 and CO (Ref. 18).Various studies, in particular with vibrational spectroscopy, have indicated that the two C - 0 bonds a r e equivalent and that the symmetry is CB,(ref. 19). In a n x-ray absorption study Puschmann et al. (ref. 20) have recently shown t h a t the molecular plane of the surface formate species on Cu{llO} is aligned along the < 110> azimuth (the direction of the close-packed rows) and oriented perpendicular to the surface. Fig. 9 shows NEXAFS spectra at the oxygen edge with the E vector aligned in ( a ) the < 110> azimuth and (b) the < 100> azimuth. The n-type resonance corresponds to excitation into the 2b2 orbital and is expected to be polarised perpendicular to the molecular plane. There are actually two o-type resonances, corresponding to 7al and 5bl; a t the C edge they can actually be resolved (ref. 21). I n the azimuth (Fig. 9b) we note that the intensity of the A resonance varies drastically a s the angle between the E vector and the surface normal is changed. In <110> this effect is hardly observed, indicating that the molecular plane i s perpendicular to the surface and aligned in the < 110> azimuth, i.e. parallel to the close packed rows. (The measurable intensity in the TI resonance when the E vector is oriented in the <110> azimuth is due to incomplete polarisation of the incident radiation.) This qualitative conclusion is supported by comparing the BE = 90" spectra in Figs. 9a and 9b (full lines). Here we observe a drastic change i n the d o intensity ratio as the E vector (parallel to the surface) is moved around from the < 110 > to the azimuth. A quantitative analysis of the polar angular dependence in the < 100 > azimuth indicates that the molecular plane i s indeed perpendicular to the surface, the accuracy of the determination being f 10" (ref. 20). The corresponding SEXAFS analysis of Puschmann et al. (ref. 20) gave t h e socalled aligned atop site shown as inset A in Fig. 10. The adsorption site of the formate species on both Cu {llO} and Cu{lOO} has proved, however, to be controversial (refs. 22,231. The recent photoelectron diffraction data of Woodruff et al. (ref. 24) h a s indicated t h a t the same local geometry pertains on both surfaces a n d that the aligned
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bridge site (B) is actually occupied. The 0 1s data for both surfaces in normal emission are shown in Fig. 10. It is immediately apparent that the two curves are very similar. The modulation frequency is identical and turns out to be due to backascattering from Cu atoms almost directly "behind" the oxygen atom. The calculated curves for the locally equivalent site B (using C u - 0 distances of 1.99 A for (100) and 1.94 A for (110) are also shown in Fig. 10 and provide the best fit to the experimental data. No satisfactory fits were obtained for other sites; in particular, the aligned atop site (A) can be definitely ruled out. The calculations of Woodruff et al. (ref. 24) used curved wave double scattering with clusters of typically 500 copper atoms. Although agreement between theory and experiment is not perfect (some higher order scattering and
B
A
(110) azimuth
cu I1001 (100) azimuth
Tc
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b) I
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v
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550 560 Photon energy lev) 5LO
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100
Fig. 9. (left) 0 1s x-ray absorption spectra from the surface formate species on Cu(llO}. The E vector is aligned in (a) the <110> azimuth and in (b) the <110> azimuth. After ref. 20. Fig. 10. (right) Photoelectron diffraction data (normal emission) for the surface formate mecies on Culllo) together with the calculated curves for the aliened bridge site. The insets A and B vdesignate the aligned atop and aligned bridge suites, respectively. After ref. 24. I
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angle averaging may have to be considered), the main features of the PED spectra are reproduced. Having established the orientation of the formate species on the Cu{llO} surface we can proceed to examine the photoemission data. Fig. 11 shows the effect of deprotonation of adsorbed formic acid which occurs on warming the surface to above 270 K. Spectrum (a)can be assigned by comparison with the photoelectron spectrum of the free molecule. The formate species also gives rise to four spectral bands and, since the number of expected orbitals is the same, i t is tempting to assume a one-to-one correspondence, allowing of course for the change in symmetry from C, to Czv. The application of selection rules proves, however, that such a n assignment is incorrect (ref. 25). Fig. 12 shows three spectra a t hv = 25 eV with the E vector parallel to the surface and aligned along the <110> azimuth, i.e. oriented in the molecular plane of the formate species. Spectrum (b) was obtained a t normal photoelectron emission, for which the selection rules tell us that only levels belonging to bl in Czv will be observed. This immediately assigns two features in the spectrum a t 4.8 eV and 9.6 eV below EF. By moving the detector off-normal into the < 100> azimuth (spectrum (a), E kll) emisssion from a2 states should be observed as well. Whereas peak 3 remains in the same place peak 1 shifts slightly to lower binding energy indicating t h a t it also contains a level of a2 symmetry. Similarly, by moving the detector off-normal into the <110> azimuth (spectrum (c) Ellkll a , and bl states are expected. Under these conditions peak 1 moves up in binding energy, as does peak 3. In addition, peak 4 is observed. Thus, three a1 states are also present. Peak 2 is only visible with E l k 8 for
-
a)
--
Fig. 11. ( a ) Photoelectron spectrum of adsorbed formic acid on Cu{llO} and (b) correspondin spectrum after formation of the ormate species above 270 K. h v = 25 eV. After ref. 25.
A 270K V
b)
HC006- (ad)
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Energy betow EF (eV)
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non-normal incidence (not shown in Fig. 12), indicating that i t belongs to b2. By performing further confirmatory experiments a t other orientations of the E vectpr, in particular when it is aligned in the <110> azimuth, a complete assignment is possible. Peak 1contains three bands due to la2(11),4bl(o) and 6al(a) a t 4.7, 4.8 and 5.1 eV below EF, whereas peak 2 consists only of lb2(n) at 7.8 eV. Peak 3 contains 3bl((I) and %'il((I)at 9.6 and 9.7 eV; peak 4 is due to 4al((I) a t 13.0 eV. These measured ionisation energies have been compared with HF-SCF calculations for the formate ion (ref. 26) as well as with an INDO Cu(llO}-HCOO cluster calculation (ref. 27). The relative orbital energies from the latter, semi-empirical treatment are in reasonable agreement with the measured binding energies although the absolute values, as expected, are way out. The important result from this calculation is the correct assignment of the photoelectron spectrum (via Koopmans' theorem), in particular, the prediction that three levels are present in the first band and only one in the second. An analysis of the percentage formate character in the adsorbate-derived orbitals reveals that the la2, 4bl and 6al orbitals are most strongly involved in the chemisorption bond. Relative to the formate ion, surface formate has both lower u and II populations but the (I population difference is the greater. The 'TI donation occurs mainly via the la2 orbital; the strongest (I donor is the 4bl orbital. Back-donation from the metal into the anti-bonding n*(2b2) orbital is neglible because, unlike the situation in adsorbed CO, the latter is too high in energy.
I
I
HCOO/Cu1110) > w = ~ e5 ~in ITIOI azimuth
Fig. 12. Angle-resolved photoelectron spectra from the system Cu(ll0)HCOO for three different emission an les. hv = 25 eV. After re&25.
9 = 60'
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I
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5 Energy below EF (ev)
I
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[iiol
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SOME COSCLUSIONS I have attempted in this short review article to describe some of the experiments that can be performed with synchrotron radiation in order to gain a better understanding of the structure and bonding of molecules and molecular fragments adsorbed on metal surfaces. In structural studies the usefulness of x-ray absorption and photoelectron diffraction has been demonstrated, particularly in situations where the adlayer is not ordered and conventional diffraction techniques cannot be applied. In fact, most molecular adsorbates show little sign of long-range order (CO and C&fi tend to be exceptions) and, for molecular fragments formed in simple heterogeneous reactions, i t almost never occurs. The ability to independently determine the orientation of adsorbed species opens up the possibility of applying selection rules to the assignment of the adsorbate-induced features in photoelectron spectra. Photoemission studies of adsorbed molecules in recent years have tended to be more diagnostic in nature, meaning that they are largely concerned with general characterisation rather than with bonding aspects. The combination of photoemission studies, where the emphasis is on accurate determination of ionisation energy and symmetry, with theory will hopefully change this situation in future. For this purpose, more cluster calcu lations a t higher levels of sophistication will be necessary; attempts to solve some of the very difficult problems caused by the present use of Koopmans' theorem would also be highly desirable. ACKNOWLEDGEMENTS This work has been supported financially by the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 6-81 and by the Fonds der Chemischen Industrie. I also acknowledge the considerable contributions of my colleagues H. Conrad, A . L. D. Kilcoyne, M. E. Kordesch, Th. Lindner, C. F. McConville, G. Paolucci, K. C. Prince, J. Somers, L. Sorba, M. Surman, G . P. Williams and D. P. Woodruff to various parts of the work reviewed in this article. REFERENCES
1
J. Stbhr, in V. R. Vanselow and R. Howe (Editors), Chemistry and Physics of Solid Surfaces, Springer, Berlin, 1984, p. 231; J. Haase, Appl. Phys. A38 (1985) 181; J. Haase and A. M. Bradshaw, in R. J. Bachrach (Editor), Synchrotron Radiation
2
J. J. Barton, C. C. Bahr, S. Robey, J. G. Tobin, L. E. Klebanoff and D. A. Shirley, Phys. Rev. Lett. 51 (1983) 272; M. Sagurton, E. L. Bullock and C. S. Fadley, Phys. Rev. B 30 (1984) 7332; D. P. Woodruff, Surface Sci. 166 (1986) 377. N. V. Richardson and A. M. Bradshaw, in A. Baker and C. R. Brundle (Editors),
3
4
Research: Physics of Low Dimensional Systems, Plenum, New York, in press.
Electron Spectroscopy: Theory, Techniques and Applications, Academic Press, London, 1982, p. 154; M. Schemer and A. M. Bradshaw, in D. A. King and D. P. Woodruff (Editors), The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Elsevier, Amsterdam, 1983, Vol. 2, p. 165. M. P. Seah and W. Dench, J. Surf. Interf. Anal. l(1979) 1.
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5. 6
7 8 9 10 11 12 13 14
15 16 17 18 19
G. Margaritondo, Introduction to Synchrotron Radiation, Oxford University Press, New York, Oxford, 1988. J. Stbhr and R. Jaeger, Phys. Rev. B 26 (1982) 4111. (The derivation of these expressions is somewhat laborious in this first paper on x-ray absorption spectroscopy of adsorbed molecules. See Somers et al., Surface Sci. 183 (1987) 576, for a simpler approach.) A. M. Bradshaw, J. Somers and Th. Lindner, Proc. of Solvay Conference on the Physics and Chemistry of Surfaces, Springer, Berlin, in press. M. E. Kordesch, Th. Lindner, J. Somers, W. Stenzel, H. Conrad, A. M. Bradshaw and G. P. Williams, Spectrochim. Acta 43A (1987) 1561. M. E. Kordesch, W. Stenzel and H. Conrad, Surface Sci. 205 (1988) 100. M. Surman, K. C. Prince, L. Sorba and A. M. Bradshaw, Surface Sci., in press. G. Paolucci , M. Surman, K. C. Prince, L. Sorba, A. M. Bradshaw, C. F. McConville and D. P. Woodruff, Phys. Rev. B 34 (1986) 1340. D. Heskett and E. W. Plummer, Phys. Rev. B 33 (1986) 2322. L. H. Dubois, B. R. Zagorski and H. S. Luftman, J. Chem. Phys. 87 (1987) 1367. K. Horn, A. M. Bradshaw and K. Jacobi, Surface Sci. 72 (1978) 719; K. Horn, A. M. Bradshaw, K. Hermann and I. P. Batra, Solid State Commun. 31 (1979) 257. C. F. McConville, D. P. Woodruff, K. C. Prince, G. Paolucci, V. Chab, M. Surman and A. M. Bradshaw, Surface Sci. 166 (1986) 221. E. G. C. S. Fadley, Phys. ScriptaT 17 (1987) 39. Th. Lindner, J. Somers, A. M. Bradshaw, A. L. D. Kilcoyne and D. P. Woodruff, Surface Sci. 203 (1988) 333. D. H. S. Ying and R. Madix, J. Catalysis 6 1 (1980) 48. B. E. Hayden, K. C. Prince, D. P. Woodruff and A. M. Bradshaw, Surface Sci. 133
(1983) 589. 20 A. Puschmann, J. Haase, M. D. Crapper, C. E. Riley and D. P. Woodruff, Phys. Rev. Lett. 54 (1985) 2250. 2 1 J. Somers, A. W. Robinson, Th. Lindner and A. M. Bradshaw, to be published. 22 D. Outka, R. J. Madix and J. Stiihr, Surface Sci. 164 (1985) 235. 23 M. D. Crapper, C. E. Riley and D. P. Woodruff, Surface Sci. 184 (1987) 121. 24 D. P. Woodruff, C. F. McConville, A. L. D. Kilcoyne, Th. Lindner, J. Somers, M. Surman, G: Paolucci and A. M. Bradshaw, Surface Sci. 201 (1988) 228. 25 Th. Lindner, J. Somers, A. M. Bradshaw, G. P. Williams, Surface Sci. 185 (1987) 75; P. Hofmann and D. Menzel, Surface Sci. 191 (1987) 353. 26 S. D. Peyerimhoff, J. Chem. Phys. 47 (1967) 349. 27 J. A. Rodriguez and C. T. Campbell, Surface Sci. 183 (1987) 449.
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STRUCTURE AND BONDING OF ADSORBATES: INVESTIGATIONS WITH SYNCHROTRON RADIATION A. M. BRADSHAW Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-1000 Berlin 33, Germany ABSTRACT Using x-ray absorption, photoemission and photoelectron diffraction i t is possible to obtain important information on the structure and bonding of molecules adsorbed on single crystal metal surfaces. These spectroscopic techniques become particularly powerful when used in conjunction with synchrotron radiation. INTRODUCTION Fundamental studies in surface chemistry are necessarily concentrated in three main areas. Firstly, the elucidation of the structural details of the adsorbed layer plays a n important role, in particular, the determination of bonding site and molecular orientation. Related problems such as adlayer growth, order-disorder transitions and adsorbate-induced reconstructions also receive considerable attention. Secondly, methods are available to probe the electronic energy levels of the adsorbatekubstrate system in an attempt to obtain a better understanding of the chemisorption bond. Concomitant advances in theory have enabled molecular orbital energy diagrams in the so-called cluster approximation and, in the case of ordered overlayers, surface band structures to be calculated. Thirdly, the investigation of the dynamics of adsorption and desorption processes has recently become a highly topical area. The aim here is to elucidate the energy exchange processes occurring between particle and surface and, in the long term, a t deriving empirically the corresponding potential energy surface. The investigatory techniques used in all these studies rely heavily on spectroscopies and scattering experiments involving electrons, photons, ions or atoms. In this article we concentrate specifically on the use of photons in the vacuum UV and soft x-ray regions (XUV) for solving problems in two of these areas, namely, in studies of the structure and bonding of molecules and molecular fragments on metal surfaces. The experiments that will be described - x-ray absorption, photoemission and photoelectron diffraction are based on the phenomena ofphotoabsorption and photoionisation. X-ray absorption spectroscopy (ref. 1) and energy-scanned photoelectron diffraction (ref, 2) have recently become important structural tools in adsorption studies because, unlike low energy electron diffraction (LEED), they do not rely on the presence of long-range order in the adlayer. Since both techniques require a
202
continuously tunable source of soft x-rays, usually in the region 200 - 1000 eV photon energy, their use is dependent on the availabability of synchrotron radiation a s well as on suitable "grazing incidence" grating monochromators. (This spectral range covers the IS levels of C, N and 0, which tend to feature in most adsorbate studies.) The measurement of the polarisation dependence of the x-ray absorption spectrum of a n adsorbed species near a n absorption edge (NEXAFS = near edge x-ray absorption fine structure) is a particularly straightforward technique when applied correctly. The most direct technique for probing surface electronic structure is undoubtedly photoemission, or photoelectron spectroscopy (ref. 3). In a reasonable approximation, referred to as Koopmans' theorem, the measured ionisation potentials can be equated with the ground state orbital energies calculated in the IIartree-Fock approximation. When the molecule is only weakly adsorbed a comparison with the photoelectron spectrum of the corresponding gas phase species normally suffices to assign t h e various adsorbate-induced features. For this purpose, laboratory line sources (HeI, etc.) a r e sometimes adequate. Where a strong perturbation of the energy levels of the molecule takes place, assignment can only be carried out using selection rules preferably in conjunction with polarised light. This is particularly true of molecular fragments for which no comparison with gas phase species is possible. For the application of photoemission selection rules prior knowledge of the orientation of the molecule is necessary. Even when the molecule or molecular fragment possesses no symmetry (meaning t h a t the selection rules cannot be applied), a mere change in photon energy often helps in distinguishing, or even assigning, adsorbate-induced features. In the x-ray absorption and photoelectron emission experiments described in this article, i t is always the excited photoelectron, a n Auger electron or secondary electrons which are detected. On account of the low inelastic mean free path for electrons in solids, only the first few atomic layers are generally sampled and the techniques are very surface-sensitive. (This may not be the case in x-ray absorption spectroscopy where the use of total electron yield detection can lead to high bulk sensitivity.) The inelastic mean free path is strongly energy-dependent (ref. 4)so t h a t in photoemission the surface sensitivity can in fact be maximised by choosing a photon energy which produces a photoelectron kinetic energy corresponding to the shortest mean free path (between 50 and 100 eV for most solids - see ref. 4). In the following section the specific properties of synchrotron radiation are described. I then go on to discuss the three experiments in more detail, giving a n example of the application of each to a particular adsorbate system. Lastly, their combined application is demonstrated in a study of the surface formate species which is formed as a n intermediate in the catalytic decomposition of formic acid. SYNCHROTRON RADIATION The generation, properties and uses of synchrotron radiation have been reviewed in many places. The reader is therefore referred only to a very recent book by
203
Margaritondo (ref. 5)which serves as a useful introduction to the subject. Synchrotron radiation (SR) is produced when relativistic electrons (or positrons) are centripetally accelerated by the bending magnets of an accelerator or storage ring. It is emitted tangentially to the electron orbit as shown in Fig. 1. In recent years a new generation of storage rings has been built whose sole function is to serve as synchrotron radiation sources. Probably the most striking property of SR is the quasi-continuous spectrum extending from the infrared into the x-ray region. The characteristic spectral distribution curve for synchrotron radiation is shown in Fig. 2. An SR source is characterised by its critical wavelength, A,, which corresponds approximately to the position of the maximum of this curve. A, is inversely proportional to the third power of the electron energy and directly proportional to the radius of curvature of the orbit. Thus, a t the German XUV source BESSY in Berlin A, is 20 A, whereas for the DORISII storage ring (E = 3.7 GeV) in Hamburg (HASYLAB) Ac = 1.4 A. Experiments with hard x-rays are thus also possible on a high energy ring such a s DORIS. Synchrotron radiation is also characterised by its small source size and strongly directional emission. Both properties lead to a source of high spectral brilliance, a quantity which is defined as the number of photons per second in unit bandwidth, per unit solid angle and per unit area of source. Other important properties are the high degree of linear polarisation (exactly 100 % in the plane of the storage ring), the time
1
0.1
I
1.0
I
10
I
100 hl'h,
I
1000
I
10000
Fig. 1. (left) Schematic representations of the spatial distribution of electromagnetic radiation emitted by centripetally accelerated charged particles. Above: low energies; below: relativistic energies (for electrons > 40 MeV). Fig. 2. (right) The characteristic spectral distribution curve for synchrotron radiation.
204
structure (useful for certain time-resolved experiments) and the fact t h a t its properties are entirely calculable via the Schwinger equations (see ref. 5). For the purposes of the surface chemical investigations described in this article photon energy tunability and polarisation are the main assets of synchrotron radiation. SR is, however, a source of "white" light and must be used in conjunction with a suitable monochromator. The so-called normal incidence region extends up to about 40 eV photon energy (- 300 A); a s the name suggests, reflections a t the optical components (gratings and mirrors) of the monochromator take place a t normal, or near to normal, incidence. At higher photon energies the reflectivity at normal incidence falls drastically and i t is necessary t h a t reflections from the optical components take place a t angles near to grazing. As a result, grating monochromators for the range 40 1000 eV are difficult to construct and the aberrations produced by the optical components are high. (Crystal monochromators can generally only be used above 800 eV.) These and other problems result in a resolution ( M A ) much lower than t h a t in the normal incidence region. Nonetheless, the advent of new designs of monochromator has recently given considerable impetus to the use of synchrotron radiation in this spectral region. Before concluding this brief account i t should be mentioned t h a t undulators, produced by causing the electron beam to oscillate transversally in a periodic magnet structure inserted into the straight section of a storage ring, can further increase the intensity of synchrotron radiation. These devices produce pseudo-monochromatic r a diation due to constructive interference between the light emitted at successive oscil lations. The intensity can be as high as a factor 10 and the brilliance a factor 103 greater than that of the radiation emitted from the bending magnets on the same storage ring. Already storage rings are being planned or constructed where the main sources of radiation are provided by undulators. XUV facilities of this type - the ALS in Berkeley, Sincrotrone Trieste and BESSY II - will provide considerable impetus in surface science. X-RAY ABSORFTION SPECTROSCOPY Below the photoionisation threshold a core electron in a free molecule can be excited into empty anti-bonding molecular orbitals (m.o.'s) as well as into Rydberg states. These transitions are observable as sharp features directly below the corresponding absorption edge (carbon K, oxygen K etc.) Above the photoionisation threshold further transitions into higher-lying anti-bonding virtual m.o.'s will occur, their broad spectral features being superimposed on the background continuum absorption. This i s shown schematically for a diatomic molecule in Fig. 3. All these features associated with the existence of the molecular potentia.1 constitute t h e near edge x-ray absorption fine structure (NEXAFS) or x-ray absorption near edge structure (XANES). At higher energies above the edge further very weak structure may be visible due to a scattering phenomenon. This is the extended x-ray absorption fine
t
electron yield (-absorption)
..
.. .- *.. .-.we
hv
Fig. 3. Left: schematic representation of the transitions observed in the x-ray absorption spectrum of a diatomic molecule. Right: typical near edge spectrum (actually for CO on a Ni(100) surface). structure (EXAFS) resulting from interference between the emitted photoelectron wave and waves backscattered from other atoms in the molecule. (The excitation into anti-bonding m.0. above the threshold may also be viewed as an EXAFS-type scattering resonance. Hence the term "shape" resonance: the energy and width of the features depend on the shape of the molecular potential.) When the molecule is placed on the surface, several factors come into play. Firstly, the experiment has to be performed quite differently. Since the substrate is usually a compact solid surface, a measurement of the absorption spectrum in transmission is no longer feasible. As shown schematically in Fig. 4a, the yield of Auger electrons, or the high energy portion of the secondary electrons (partial electron yield) are measured, both signals being proportional to the number of excited core electrons in the immediate surface region. Upon adsorption the near edge resonances may be shifted and their degeneracy lifted or they may even disappear as a result of the formation of the bond to the substrate. New resonances (so-called substrate resonances) may be observed. Since the chemisorption process invariably results in a molecule with a single, fixed orientation and since the excitations are subject to dipole selection rules, the transitions in NEXAFS are polarised. Thus, in the case of the diatomic of Fig. 3 the transition 1s + u is polarised along the molecular axis, i.e. its intensity in the absorption spectrum is given [E.M,]2 = cosea, where a is the angle between the E vector and the molecby .I ular axis and M, in a Cartesian component of the dipole matrix element. Similarly, the transition 1s + 'II is polarised perpendicular to the internuclear axis: I, [E.MXy]2= sin2a. These equations form the basis for the determination of molecular orientation, although care must be taken in their application to polyatomic molecules (ref. 7). As an example, Fig. 5 shows N 1s near edge x-ray absorption spectra of the HCN molecule adsorbed on Pd(ll1) for two orientations of the E vector relative to the surface normal (ref. 8). It is immediately clear that the 'II and u resonances behave very
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-
a) X - r a y absorptioP
T
core level
threshold
bl Photoemission
,L E L
Eco
Fig. 4. Schematic representation of the. three. experiments described in this article: (a) x-ray photoabsorption (NEXAFSISEXAFS), (b) photoelectron spectroscopy (photoemission)and (c) photoelectron diffraction.
cI Photoelectron diffraction
similarly a s the angle BE is changed. In fact, a plot of the uln intensity ratio as a function of 0~ (Fig. 5, bottom) shows that it remains essentially constant. Assuming random azimuthal orientation and a lowering of the C-N bond order to two (as confirmed by vibrational spectroscopy - see ref. 9). this result is only possible if the C-N axis lies parallel to the surface (6 = 90"). The calculated intensity ratios for other angles of inclination 6 = 85,70" and 45" are also shown. (This analysis assumes t h a t the hydrogen atom does not lower the effective symmetry.) Although the result shown in Fig. 5 looks quite definitive, closer examination shows that the accuracy of such determinations cannot be much better than about 10". If the experiment is performed for CO on the same surface, the u and H resonances have exactly the opposite polarisation dependences and show that the molecular axis is perpendicular to the surface. Returning to the general case, we note that surface atoms are likely to be relatively strong backscatterers and that the extended fine structure will be dominated by scattering from the substrate. These relatively weak modulations at higher energy are referred to as the surface EXAFS, or SEXAFS. An analysis of the SEXAFS can give
207
further structural information, in particular, on the adsorption site. The technique has often been used in recent years and reviewed extensively by several authors. The reader is referred to ref. 1. PHOTOEMISSION In the photoelectron spectroscopy experiment shown schematically in Fig. 4b, the electrons emitted as a result of their excitation into states above the photoionisation threshold are analysed according to their kinetic energy. The energy balance is given simply by hv = EB E K Cp ,where EBis the ionisation (or bonding) energy of a bound level, EKthe kinetic energy of the photoemitted electron and the work function. The technique is often referred to as XPS (x-ray photoelectron spectroscopy) or ESCA (electron spectroscopy for chemical analysis), the latter being the original acronym proposed by K. Siegbahn. If the photon energy is low (hw < 50 eV), such that essentially only valence electrons are excited, then we normally refer to UPS (ultraviolet photoelectron spectroscopy) or simply photoemission. At these low photon energies the photoionisation cross-section for valence (or outer shell) electrons is also highest. For a molecule on a surface the primary excitation is a dipole transition from a molecular orbital, modified by the interaction with the substrate, into a continuum level (Fig. 6). The same photoabsorption selection rules thus apply as in NEXAFS. Whereas in NEXAFS, however, the final state is a bound or continuum level associated
+
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.
..... . . ............ ..................... .. ..
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390 1
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400 1
1
410 1
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Photon energy ( eV 1 Fig. 5. Left: X-ray absorption spectra of HCN adsorbed on Pd(ll1) a t the N 1s edge. Right: Intensity ratio of the u and 'II resonances as a function of the angle between the E vector and the surface normal, BE. After ref. 8.
208
nw
Fig. 6. The photoemission process in the single particle picture for a n adsorbate level on a metal surface.
metal
adsorbate
Energy balance. E~
0
electron energy distributian
-nw -.
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with the molecule itself and thus of (potentially) known symmetry, the final state continuum wavefunction in photoemission must be specifically selected in a n angleresolved experiment in order to define its symmetry. When the molecular orientation is also known, the symmetry of the initial state can then be determined. How this procedure can be used in assignment will become clear in the following example. Fig. 7 shows photoemission data from the adsorption systems Cu{ lOO}-CO and Cu{1OO}-COiK (ref. 10). It is known from NEXAFS studies (ref. 11) t h a t the C - 0 axis is perpendicular to the surface in both the pure CO layer and the coadsorption system. For most experimental geometries - meaning E vector orientation and photoelectron emission direction - a spectrum such as that of Fig. 7a is obtained. The two main features at 8.7 eV and 11.5 eV are due to the 4 0 orbital and to the overlapping 5 a a n d 171 orbitals, respectively. The shoulder a t 13 eV is a satellite indicating that charge transfer screening plays a role in this system (see ref. 12). The adsorbate-induced features are generally referred to using the same nomenclature a s the orbital in the free molecule but it should be remembered that they will be altered by the interaction with the substrate. The two overlapping features have been assigned by choosing a particular geometry for which emission from levels of o symmetry i s forbidden (Fig. 7b). This is often referred to as the "forbidden geometry": the E vector is parallel to the surface and orthogonal to the emission plane for a n adsorbate with a symmetry axis. Taken together, the two spectra show that the I n level occurs at a binding energy of 8.4 eV relative to EI: and the 50 level at about 8.7 eV. In the corresponding gas phase spectrum the (vertical) ionisation energy of 5 0 is 3 eV lower than that (if 1 TI. 'I'he increase in binding energy of the 50 level is largely due to the formation of the chemi
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209
sorption bond: the 50 orbital is located mainly a t the C end of the molecule and interacts most strongly with the metal states. In the co-adsorption system Cu{lOO}-CO/K a strong interaction between the species occurs a t high K:CO concentration ratios. This is characterised by a considerably lowered C-0 stretching frequency (ref. 13) and is probably indicative of a shortrange effect rather than a substrate-mediated, long range interaction. The forbidden geometry experiment for this case is shown in Fig. 7c . Immediately apparent is the splitting of the I n level due to the lower symmetry of the CO/K adsorption complex. Either a direct bonding interaction takes place with one or more K atoms or, alternatively, an electrostatic effect is responsible (the K atoms may be partially ionised). Notice, however, that the 40 feature is also absent (or very nearly absent) which indicates that the forbidden geometry experiment still works, i.e. as far as the 40 orbital is concerned, the rotational axis of the CO molecule still remains an effective symmetry element of the system. Two further remarks should be made before concluding this section. The first concerns Koopmans' theorem which turns out in some cases not to be such a trivial approximation. In most photoemission studies to date i t has simply been assumed that the relative ionisation energies can be compared directly with calculation, i.e. that relaxation indeed plays a role but the shift is the same for all orbitals. When intramolecular relaxation and image charge screening are dominant this may be a reasonable approximation. In the presence of charge transfer screening (the "unfilled
CU I1001 - CO/K L;
Fig. 7. Photoelectron spectra from the adsor tion systems Cu{lOO}-CO and Cu(OO}-CO/K. a) CO in the ( d 2 x d 2 ) R 45" structure; b) as (a) but "forbidden" geometry; c) CO/K coadsorption a t high WCO concentration ratio, also in the forbidden geometry. After ref. 10.
8.600
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1s
I
I
I
10 5 EF Energy below EF (eV)
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orbital” mechanism) this may not be the case. The second point concerns dispersion. Due to lateral interactions in ordered overlayers, overlap of orbitals on adjacent molecules leads to band formation. Energy dispersion E,(kll) is then observable in the experimental spectrum: as the polar emission angle is varied, and thus the component of photoelectron momentum parallel to the surface, kll, the binding energy E, of a feature will change. This effect can be very pronounced in strongly-bound atomic overlayers with bandwidths of 2 eV or more, but is also observed for molecular adsorbates. I t was first discovered in CO overlayers some years ago (ref. 14). PHOTOELECTRON DIFFRACTION In photoemission there is a n interesting experimental variation known a s constant initial state (CIS) spectroscopy, in which the photon energy and the analysed kinetic energy are scanned simultaneously. For the excitation out of a given occupied level, a range of final states is then sampled. This approach is particularly useful in band structure studies of solids when the momentum, or wavevector, of the final state is also known, i.e. when the emission direction is also defined. In such a n experiment on a free molecule a differential partial cross-section is measured. If the photoelectron current is angle-integrated, meaning that all the photoemitted electrons are collected, then the photoabsorption spectrum, or partial photoionisation cross-section, would be obtained. Putting the molecule (or a n atom) on a surface and taking a CIS spectrum of a core level introduces, in the same way a s in SEXAFS, scattering from the substrate (Fig. 412). The resulting interferences, which depend on the adsorption site and orientation modulate the differential cross-section; the phenomenon is normally referred to a s photoelectron diffraction (PED). I t has recently been shown that the modulations in intensity are of the order of 50 90of the background signal in experiments on the C and 0 1s levels (ref. 15). (Photoelectron diffraction effects are also observed when the photoelectron emission angle is varied a t fixed photon energy (ref. 16). I t is t h u s sensible to distinguish between energy-scanned and angle-scanned PED.) By performing model calculations for a given structure and emission direction and comparing with experiment, information on adsorption site and orientation is obtained. The data are collected by measuring a short (- 30 eV) photoelectron spectrum around the appropriate core level peak a t a particular emission angle for a series of photon energies at typically 1 - 3 eV intervals. The peak areas are integrated and normalised to the background on the high kinetic energy side to give a plot of intensity against photoelectron energy. One of the main problems is that substantial changes in the background (inelastic) photoemission signal may occur when the photon energy passes through an absorption edge of the substrate or when Auger electron features and photoemission peaks (generated by second-order light from the monochromator) are superimposed. One way of recognising these features is to record the core level peaks in the form of a group plot as shown in Fig. 8 (ref. 17). Here, the 0 1s level from the methoxy species on Cu{lOO}has been investigated in normal emission. The feature
211
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in the background a t 270 eV is due to carbon K W Auger electrons. We note in these raw data the strong intensity modulations referred to above which are due to backscattering from surface Cu atoms. The desired structural information is obtained by calculating such photoelectron intensitylenergy curves for various geometries and then comparing theory with experiment. There is still some controversy about the level of sophistication required in these calculations, in particular as to whether a full multiple scattering treatment is necessary. How does the technique of photoelectron diffraction relate to SEXAFS? By varying the photon energy in PED the scattered electron intensity is distributed between the various final state manifolds corresponding to different emission directions. If i t were possible to angle-integrate over the whole photoelectron current, then we would recover the SEXAFS signal. Again, we note the difference between a partial crosssection and a differential partial cross-section. In practical terms, however, i t means that in PED the interference between the directly emitted and scattered waves occurs at the detector and is determined by the path length differences. These in turn depend on the direction and separation of emitter and neighbouring scatterers. PED is thus particularly sensitive to the adsorption site of the emitter. In SEXAFS this real space information is only obtainable via the polarisation dependence of the modulation amplitude. Note too that PED modulations are typically a factor ten larger than those in SEXAFS.
0
1
I
100
200
I
300
I
400
Photoelectron kinetic energy (eV)
Fig. 8. Raw 0 1s photoelectron diffraction data a t normal emission from the system Cu(100)-methoxy. The monochromator has been ramped in steps of 3 eV. Note the feature in the background a t 270 eV due to the carbon K W emission. After ref. 17.
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Before concluding this section i t should perhaps be noted t h a t SEXAFS h a s so far been used more widely in surface structural studies than photoelectron diffraction although only with mixed success in the soft x-ray region. On the other hand, t h e potential of PED has not yet been fully realised; the PED analysis for the surface formate species described below illustrates this point. THE SURFACE FORMATE SPECIES ON C u ( l l 0 ) The decomposition of formic acid on metal and oxide surfaces i s a model heterogeneous reaction. Many studies have shown that i t proceeds via a well-defined reaction intermediate - the surface formate species - which can be isolated and investigated with spectroscopic techniques. Adsorption of formic acid at low temperatures gives rise to a molecular species on the surface which deprotonates on warming. On a Cut1 10) surface thisoccurs at 270 K, giving the formate species, which in turn decomposes a t 450 K with evolution of H2 and CO (Ref. 18).Various studies, in particular with vibrational spectroscopy, have indicated that the two C - 0 bonds a r e equivalent and that the symmetry is CB,(ref. 19). In a n x-ray absorption study Puschmann et al. (ref. 20) have recently shown t h a t the molecular plane of the surface formate species on Cu{llO} is aligned along the < 110> azimuth (the direction of the close-packed rows) and oriented perpendicular to the surface. Fig. 9 shows NEXAFS spectra at the oxygen edge with the E vector aligned in ( a ) the < 110> azimuth and (b) the < 100> azimuth. The n-type resonance corresponds to excitation into the 2b2 orbital and is expected to be polarised perpendicular to the molecular plane. There are actually two o-type resonances, corresponding to 7al and 5bl; a t the C edge they can actually be resolved (ref. 21). I n the
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bridge site (B) is actually occupied. The 0 1s data for both surfaces in normal emission are shown in Fig. 10. It is immediately apparent that the two curves are very similar. The modulation frequency is identical and turns out to be due to backascattering from Cu atoms almost directly "behind" the oxygen atom. The calculated curves for the locally equivalent site B (using C u - 0 distances of 1.99 A for (100) and 1.94 A for (110) are also shown in Fig. 10 and provide the best fit to the experimental data. No satisfactory fits were obtained for other sites; in particular, the aligned atop site (A) can be definitely ruled out. The calculations of Woodruff et al. (ref. 24) used curved wave double scattering with clusters of typically 500 copper atoms. Although agreement between theory and experiment is not perfect (some higher order scattering and
B
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200 300 LOO Photoelectron kinetic energy (eV1
100
Fig. 9. (left) 0 1s x-ray absorption spectra from the surface formate species on Cu(llO}. The E vector is aligned in (a) the <110> azimuth and in (b) the <110> azimuth. After ref. 20. Fig. 10. (right) Photoelectron diffraction data (normal emission) for the surface formate mecies on Culllo) together with the calculated curves for the aliened bridge site. The insets A and B vdesignate the aligned atop and aligned bridge suites, respectively. After ref. 24. I
214
angle averaging may have to be considered), the main features of the PED spectra are reproduced. Having established the orientation of the formate species on the Cu{llO} surface we can proceed to examine the photoemission data. Fig. 11 shows the effect of deprotonation of adsorbed formic acid which occurs on warming the surface to above 270 K. Spectrum (a)can be assigned by comparison with the photoelectron spectrum of the free molecule. The formate species also gives rise to four spectral bands and, since the number of expected orbitals is the same, i t is tempting to assume a one-to-one correspondence, allowing of course for the change in symmetry from C, to Czv. The application of selection rules proves, however, that such a n assignment is incorrect (ref. 25). Fig. 12 shows three spectra a t hv = 25 eV with the E vector parallel to the surface and aligned along the <110> azimuth, i.e. oriented in the molecular plane of the formate species. Spectrum (b) was obtained a t normal photoelectron emission, for which the selection rules tell us that only levels belonging to bl in Czv will be observed. This immediately assigns two features in the spectrum a t 4.8 eV and 9.6 eV below EF. By moving the detector off-normal into the < 100> azimuth (spectrum (a), E kll) emisssion from a2 states should be observed as well. Whereas peak 3 remains in the same place peak 1 shifts slightly to lower binding energy indicating t h a t it also contains a level of a2 symmetry. Similarly, by moving the detector off-normal into the <110> azimuth (spectrum (c) Ellkll a , and bl states are expected. Under these conditions peak 1 moves up in binding energy, as does peak 3. In addition, peak 4 is observed. Thus, three a1 states are also present. Peak 2 is only visible with E l k 8 for
-
a)
--
Fig. 11. ( a ) Photoelectron spectrum of adsorbed formic acid on Cu{llO} and (b) correspondin spectrum after formation of the ormate species above 270 K. h v = 25 eV. After ref. 25.
A 270K V
b)
HC006- (ad)
I
15
I
10
k
Energy betow EF (eV)
I
0
-
B
215
non-normal incidence (not shown in Fig. 12), indicating that i t belongs to b2. By performing further confirmatory experiments a t other orientations of the E vectpr, in particular when it is aligned in the <110> azimuth, a complete assignment is possible. Peak 1contains three bands due to la2(11),4bl(o) and 6al(a) a t 4.7, 4.8 and 5.1 eV below EF, whereas peak 2 consists only of lb2(n) at 7.8 eV. Peak 3 contains 3bl((I) and %'il((I)at 9.6 and 9.7 eV; peak 4 is due to 4al((I) a t 13.0 eV. These measured ionisation energies have been compared with HF-SCF calculations for the formate ion (ref. 26) as well as with an INDO Cu(llO}-HCOO cluster calculation (ref. 27). The relative orbital energies from the latter, semi-empirical treatment are in reasonable agreement with the measured binding energies although the absolute values, as expected, are way out. The important result from this calculation is the correct assignment of the photoelectron spectrum (via Koopmans' theorem), in particular, the prediction that three levels are present in the first band and only one in the second. An analysis of the percentage formate character in the adsorbate-derived orbitals reveals that the la2, 4bl and 6al orbitals are most strongly involved in the chemisorption bond. Relative to the formate ion, surface formate has both lower u and II populations but the (I population difference is the greater. The 'TI donation occurs mainly via the la2 orbital; the strongest (I donor is the 4bl orbital. Back-donation from the metal into the anti-bonding n*(2b2) orbital is neglible because, unlike the situation in adsorbed CO, the latter is too high in energy.
I
I
HCOO/Cu1110) > w = ~ e5 ~in ITIOI azimuth
Fig. 12. Angle-resolved photoelectron spectra from the system Cu(ll0)HCOO for three different emission an les. hv = 25 eV. After re&25.
9 = 60'
-..
-\--
15
I
10
I
5 Energy below EF (ev)
I
0
[iiol
216
SOME COSCLUSIONS I have attempted in this short review article to describe some of the experiments that can be performed with synchrotron radiation in order to gain a better understanding of the structure and bonding of molecules and molecular fragments adsorbed on metal surfaces. In structural studies the usefulness of x-ray absorption and photoelectron diffraction has been demonstrated, particularly in situations where the adlayer is not ordered and conventional diffraction techniques cannot be applied. In fact, most molecular adsorbates show little sign of long-range order (CO and C&fi tend to be exceptions) and, for molecular fragments formed in simple heterogeneous reactions, i t almost never occurs. The ability to independently determine the orientation of adsorbed species opens up the possibility of applying selection rules to the assignment of the adsorbate-induced features in photoelectron spectra. Photoemission studies of adsorbed molecules in recent years have tended to be more diagnostic in nature, meaning that they are largely concerned with general characterisation rather than with bonding aspects. The combination of photoemission studies, where the emphasis is on accurate determination of ionisation energy and symmetry, with theory will hopefully change this situation in future. For this purpose, more cluster calcu lations a t higher levels of sophistication will be necessary; attempts to solve some of the very difficult problems caused by the present use of Koopmans' theorem would also be highly desirable. ACKNOWLEDGEMENTS This work has been supported financially by the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 6-81 and by the Fonds der Chemischen Industrie. I also acknowledge the considerable contributions of my colleagues H. Conrad, A . L. D. Kilcoyne, M. E. Kordesch, Th. Lindner, C. F. McConville, G. Paolucci, K. C. Prince, J. Somers, L. Sorba, M. Surman, G . P. Williams and D. P. Woodruff to various parts of the work reviewed in this article. REFERENCES
1
J. Stbhr, in V. R. Vanselow and R. Howe (Editors), Chemistry and Physics of Solid Surfaces, Springer, Berlin, 1984, p. 231; J. Haase, Appl. Phys. A38 (1985) 181; J. Haase and A. M. Bradshaw, in R. J. Bachrach (Editor), Synchrotron Radiation
2
J. J. Barton, C. C. Bahr, S. Robey, J. G. Tobin, L. E. Klebanoff and D. A. Shirley, Phys. Rev. Lett. 51 (1983) 272; M. Sagurton, E. L. Bullock and C. S. Fadley, Phys. Rev. B 30 (1984) 7332; D. P. Woodruff, Surface Sci. 166 (1986) 377. N. V. Richardson and A. M. Bradshaw, in A. Baker and C. R. Brundle (Editors),
3
4
Research: Physics of Low Dimensional Systems, Plenum, New York, in press.
Electron Spectroscopy: Theory, Techniques and Applications, Academic Press, London, 1982, p. 154; M. Schemer and A. M. Bradshaw, in D. A. King and D. P. Woodruff (Editors), The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Elsevier, Amsterdam, 1983, Vol. 2, p. 165. M. P. Seah and W. Dench, J. Surf. Interf. Anal. l(1979) 1.
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5. 6
7 8 9 10 11 12 13 14
15 16 17 18 19
G. Margaritondo, Introduction to Synchrotron Radiation, Oxford University Press, New York, Oxford, 1988. J. Stbhr and R. Jaeger, Phys. Rev. B 26 (1982) 4111. (The derivation of these expressions is somewhat laborious in this first paper on x-ray absorption spectroscopy of adsorbed molecules. See Somers et al., Surface Sci. 183 (1987) 576, for a simpler approach.) A. M. Bradshaw, J. Somers and Th. Lindner, Proc. of Solvay Conference on the Physics and Chemistry of Surfaces, Springer, Berlin, in press. M. E. Kordesch, Th. Lindner, J. Somers, W. Stenzel, H. Conrad, A. M. Bradshaw and G. P. Williams, Spectrochim. Acta 43A (1987) 1561. M. E. Kordesch, W. Stenzel and H. Conrad, Surface Sci. 205 (1988) 100. M. Surman, K. C. Prince, L. Sorba and A. M. Bradshaw, Surface Sci., in press. G. Paolucci , M. Surman, K. C. Prince, L. Sorba, A. M. Bradshaw, C. F. McConville and D. P. Woodruff, Phys. Rev. B 34 (1986) 1340. D. Heskett and E. W. Plummer, Phys. Rev. B 33 (1986) 2322. L. H. Dubois, B. R. Zagorski and H. S. Luftman, J. Chem. Phys. 87 (1987) 1367. K. Horn, A. M. Bradshaw and K. Jacobi, Surface Sci. 72 (1978) 719; K. Horn, A. M. Bradshaw, K. Hermann and I. P. Batra, Solid State Commun. 31 (1979) 257. C. F. McConville, D. P. Woodruff, K. C. Prince, G. Paolucci, V. Chab, M. Surman and A. M. Bradshaw, Surface Sci. 166 (1986) 221. E. G. C. S. Fadley, Phys. ScriptaT 17 (1987) 39. Th. Lindner, J. Somers, A. M. Bradshaw, A. L. D. Kilcoyne and D. P. Woodruff, Surface Sci. 203 (1988) 333. D. H. S. Ying and R. Madix, J. Catalysis 6 1 (1980) 48. B. E. Hayden, K. C. Prince, D. P. Woodruff and A. M. Bradshaw, Surface Sci. 133
(1983) 589. 20 A. Puschmann, J. Haase, M. D. Crapper, C. E. Riley and D. P. Woodruff, Phys. Rev. Lett. 54 (1985) 2250. 2 1 J. Somers, A. W. Robinson, Th. Lindner and A. M. Bradshaw, to be published. 22 D. Outka, R. J. Madix and J. Stiihr, Surface Sci. 164 (1985) 235. 23 M. D. Crapper, C. E. Riley and D. P. Woodruff, Surface Sci. 184 (1987) 121. 24 D. P. Woodruff, C. F. McConville, A. L. D. Kilcoyne, Th. Lindner, J. Somers, M. Surman, G: Paolucci and A. M. Bradshaw, Surface Sci. 201 (1988) 228. 25 Th. Lindner, J. Somers, A. M. Bradshaw, G. P. Williams, Surface Sci. 185 (1987) 75; P. Hofmann and D. Menzel, Surface Sci. 191 (1987) 353. 26 S. D. Peyerimhoff, J. Chem. Phys. 47 (1967) 349. 27 J. A. Rodriguez and C. T. Campbell, Surface Sci. 183 (1987) 449.