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Structural studies of adsorbed and coadsorbed molecular species
<.
Surface Science 283 (1993) 309-318 North-Holland
surface
science
Structural studies of adsorbed and coadsorbed molecular species D.P. Woodruff Physics Department, University of Wmwick, Coventry CV4 7AL, UK Received
21 April
1992; accepted
for publication
23 May 1992
A short review is presented of the techniques of backscattering photoelectron diffraction and normal incidence standing X-ray wavefield absorption in the particular context of applications to the study of the local adsorption structure of molecular species on surfaces, including cases of coadsorption. Specific examples of NO and NO/O coadsorption on Nitlll), methoxy on Al(111) and PF, on Nitlll) are used to illustrate the ability of these methods to obtain local structural information in situations of especial interest in understanding structural effects in surface reactions. Further developments of these methods to the study of vibrational anisotropies of adsorbed species (such as that associated with the wagging vibrational mode of atop adsorbed NH, and PF, on Ni(lll)), and in the use of photoemission core level (“chemical”) shifts to study more complex problems, are described.
1. Introduction
Quantitative investigations of the static structure of surfaces, and particularly of the local structural environment of adsorbed atoms, molecules, and molecular fragments, have an important role in aiding understanding of both static and dynamic properties of surfaces including surface reaction dynamics. In particular, structural sensitivity is known to characterise many reactions in heterogeneous catalysis, and the nature of “active sites” is commonly discussed, as is the role of adjacent sites in permitting molecules to fragment or interact on surfaces, and of site blocking as a route to a loss of surface reactivity 111. In studying structural problems of greatest relevance to effects in surface reactions, the requirement of long range order in the surface overlayer which typifies conventional diffraction methods (notably conventional low energy electron diffraction - LEED) is a considerable constraint. One would like to investigate the local structural environment without the need for this order, and ideally this local structure should be determinable in the presence of two or more coexistent adsorbed species such as two reactant molecules or fragments, or a single species to0039-6028/93/$06.00
0 1993 - Elsevier
Science
Publishers
gether with an adsorbed surface modifier. For this reason, true local probes which gain their local as well as surface specificity by probing an element-specific property, are particularly valuable. Element specific structural probes also have the virtue that the structural environment of different atoms within an adsorbed molecular species may be investigated independently, providing added direct information on the extent to which such species may have their internal structure modified by the surface bonding. In this paper the potential of two particular techniques of this type, (scanned energy mode) photoelectron diffraction (PhD), and (normal incidence) standing X-ray wavefield absorption (NISXW), in some cases used in conjunction with surface extended and near edge X-ray absorption fine structure (SEXAFS and NEXAFS) are described and illustrated with examples of recent results. All of these techniques necessitate the use of synchrotron radiation, a resource for which cost-effective exploitation demands collaborative efforts. The results described here derive from collaborations of this kind, and as such many individual scientists have contributed; these collaborators are explicitly acknowledged through references to the appropriate original papers
B.V. All rights reserved
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which have been, or are currently in process of being, published. The remainder of this paper is organised as follows. In the next two sections, the two main techniques are introduced and their utility illustrated with specific examples. In section 4 further examples are described which illustrate the potential for the study of coadsorption systems using both techniques, and identity some of the important routes ahead to exploit this potential further. Section 5 describes some results which illustrate how these methods can only provide information on adsorbate vibrational anisotropies. Finally in section 6, some general conclusions are drawn.
I 1s normal emission cu(100) +
I
2. Photoelectron diffraction Fig. 1 illustrates the basic physical process which is used in photoelectron diffraction, namely the coherent interference of the directly emitted photoelectron wavefield from an adsorbed atom with components elastically scattered by surrounding atoms. In order to obtain sensitivity to the location of an atom adsorbed OIZa surface (either as a chemisorbed atom or within a molecular adsorbate), it is necessary to exploit scattering events which are predominantly backscattering (i.e., involving scattering angles between 90
0
,
I
,
,
,
,
i
100 300 200 photoelectron energy (eV)
0
Fig. 1. Schematic figure showing the components of the photoelectron wavefield directly emitted from an adsorbed atom (large circle) and backscattered from surrounding substrate atoms (small circles) which interfere to produce the photoelectron diffraction process described here.
Fig. 2. 0 1s normal emission experimental photoelectron diffraction spectra from chemisorbed oxygen, formate (HCOO) and methoxy (CHsO-) on Cu(100). The dashed lines are the results of extremely simple model calculations as described in the text. The intensity normalised experimental spectra are adapted from refs. 13-51.
and lSW), and these events only have significant cross section if the photoelectron energy is less than N 500 eV (cf. LEED). Information about the scattering path lengths (and hence the structural environment of the emitter) can be obtained from the variation in the detected photoelectron signal as a function of either photoelectron energy or emission angle. So far the angular variation has not been used in backscattering to solve any unknown structures (but see ref. [21 and references cited therein), and we concentrate here on the energy dependence - i.e. the scanned energy mode. Representative data from this type of experiment are shown in fig. 2 which comprises 0 IS photoemission intensity spectra, recorded as the photon energy and thus the photoelectron energy is varied, from three different O-containing adsorbed species on Cu(100). In all cases the emission is recorded along the surface normal. The three spectra correspond to results from chemisorbed oxygen on Cu(100) (in an ordered
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D.P. Woodruff / Structural studies of adsorbed and coaakorbed molecular species
catalytic intermediates formate, HCOO- [4], and methoxy, CH,O- [5], the latter two adsorbates being produced by deprotonation of formic acid and methyl alcohol, respectively, and showing no long range order. Although all of the spectra show some fine structure and noise, each contains a single rather clear dominant periodicity to the oscillatory structure (as reproduced in the dashed curves) which can be attributed to a single scattering path length which gives rise to alternate constructive and destructive inteference as the electron wavelength (energy) is scanned. In each case this single scattering path is associated with a scattering event occurring at a scattering angle close to 180”, for which the scattering cross-section peaks and a near-neighbour scattering event is strongly favoured. These dominant scattering paths therefore involve a nearneighbour Cu (substrate) atom directly “behind” the emitter relative to the detector. As the actual nearest-neighbour Cu-0 distance differs very little between these three adsorption systems, it is clear that these different dominant scattering path lengths arise mainly because the adsorption sites differ. The dashed lines are the results of very simple scattering calculations including only one (or in the case of methoxy, five) Cu backscattering atoms and assuming specific structures. In the case of methoxy, the model involves the 0 atom (through which this species bonds to the surface) being close to, but some 0.4 A off, a hollow site on the surface [51 such that the dominant scatterer is the second layer Cu atom almost directly below; the fact that this path length is much longer than the nearestneighbour scattering path length accounts for the shorter periodicity of the oscillations in the energy spectrum. By contrast formate appears to occupy a site in which the 0 atoms lie almost directly above a top layer Cu atom (the formate bridges two nearest-neighbour Cu atoms but aligns the O-O axis along the Cu-Cu axis [4]), and in the oxygen chemisorption case the 0 effectively occupies an almost coplanar hollow site (so that the dominant Cu scatterer is a second layer atom) although one of its four nearestneighbour coplanar Cu atoms is missing in this missing row reconstruction 131. Fig. 3 shows the
normal emission WC,s
Cu(100) + HCOO
/Pd/-hs +.
Cls
-fY.
I
,
,
,
,
,
,
,
100 200 300 photoelectron energy (eV) Fig. 3. Comparison of experimental (full lines) and theoretical (dotted lines) normal emission photoelectron diffraction spectra from chemisorbed oxygen and formate on Cu(100). The structural models used in the calculations are described in the text.
more complete theoretical fits (using a larger cluster and a rather more exact treatment of the scattering) to the formate and chemisorbed oxygen photoelectron diffraction spectra. These data therefore serve to illustrate a number of features of the photoelectron diffraction method; in particular, the data show strong site sensitivity, are applicable to systems showing no long range order, and are capable of quantitative structure analysis (fig. 3). Notice that the fact that the technique uses core level photoemission as the probe means that it is possible to investigate the structural environment of the C atoms as well as the 0 atoms in the two molecular adsorbate species, and this can lead to further information. This is illustrated for formate in fig. 3 which shows the C 1s normal emission photoelectron diffraction spectrum compared with the results of a theoretical simulation; in this case the 0 atom sites have already been determined by the 01s PhD, and adjusting the C atom site to optimise the fit to the C 1s data has the effect of modifying the internal structure of the formate. The best fit is obtained by assuming that the O-C-O bond
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D.P. Woodruff / Structural studies of adsorbed and coadsorbed molecular species
angle in the adsorbed formate is 134” [4], slightly larger than the 124” of the free formate ion. Similar analysis of the C 1s normal emission data from methoxy on Cu(100) leads to the conclusion that this spectrum is mainly sensitive to the angle of the O-C axis relative to the surface normal, and that the best fit is obtained with this axis essentially perpendicular to the surface [5]. We should note that many of the characteristics of the photoelectron diffraction method in the scanned energy mode are similar to those of the SEXAFS technique [6,71 in which the total, rather than angle derivative, photoionisation cross section is measured as a function of photoelectron kinetic energy. The main difference, however, is that the angle derivative photoelectron diffraction method allows the real space direction of substrate scattering atoms to be obtained far more directly, thus offering much more direct site sensitivity; an added experimental advantage is that the angle-resolved photoelectron diffraction method leads to diffraction modulations typically one order of magnitude larger than those found in the angle-averaged SEXAFS method.
3. Normal incidence
standing
X-ray wavefield
standing x-ray wavefield
Bragg
amplitude
intensity
condition reflectivity absorption - on scatterer planes
--I-
- mid-way scatterer
between planes
0;
Fig. 4. Schematic diagram showing the principle of the SXW method. In the upper part of the figure the interfering incident and scattered X-ray wavefronts are shown, together with the amplitude and intensity distributions of the resultant standing X-ray wavefield. In the lower part, reflectivity and absorption profiles are shown around the nominal Bragg condition.
absorption
A quite different structural technique which nevertheless also relies on photoelectric absorption of incident soft X-rays is NISXW [8,9]. The standing X-ray wavefield (SXW) method exploits a particular feature of X-ray diffraction, that when a Bragg scattering condition is met the coherent interference of the incident and diffracted X-ray wavefields is such as to establish an X-ray standing wave in the solid (and beyond the surface where the two waves overlap) which has a periodicity which is equal to, or an integral submultiple of, the subtrate scattering atom periodicity. The nodal (plane) surfaces of this standing wave lie parallel to the scatterer planes of the appropriate Bragg condition. A proper description of this X-ray scattering process requires application of dynamical (multiple scattering) X-ray diffraction theory (without this the scattered intensity could exceed that of the incident intensity) which leads
to the conclusion that the X-rays have a finite penetration (“extinction depth”) into the crystal, and that the total reflectivity condition therefore has a finite range in angle or energy. Within this range the phase of the standing X-ray wave is found to be displaced in a systematic and predictable fashion relative to the location of the scatterer atoms. This means that if we investigate the absorption of this standing X-ray wavefield by an adsorbed atom as a function of the scattering condition within the range of its existence, the location of the adsorbed atom can be found relative to the location of the (extended) substrate scatterer atom planes. This is illustrated schematically in fig. 4. Notice that at one end of the total reflectivity range the nodes of the standing wave lie on the scatterer planes and at the other end they lie midway
D.P. Woodruff / Structural studies of adsorbed and coadsorbed molecular species
between these planes; the X-ray adsorption profiles as a function of scattering condition for atoms located on, or midway between, these (extended) atom plane locations are therefore almost exactly inverted, highlighting the very great sensitivity to a change in layer spacing of hoalfthe bulk layer spacing value, typically l-l.5 A. The purpose of the normal incidence SXW experiment is to extend the range of materials to which the technique can be applied. In general, the total reflectivity range in angle or energy is small; in particular, the angular range may be only a few seconds of arc. This means that the experiment is only viable if the substrate to be studied has a crystalline mosaicity which is better than this small angular range, a condition which is typically met only by extremely perfect crystals of Si or some other semiconductors. However, if one chooses to work at normal incidence to the scatterer planes then the usual Bragg equation has a turning point in angle at this condition (2d sin 0 = nh at 0 = 900) and the dependence on the crystal mosaicity is weak [8,9]. This means that normal metal single crystals can be used routinely for the experiment. An example of the application of this method to a molecular adsorbate, methoxy on Al(111) 1101 is shown in fig. 5. We should note that the X-ray energies involved in normal incidence fee (111) Bragg conditions are typically 2.6-3.2 keV, an energy which is large relative to the deepest electronic binding energies of first row elements of the periodic table, so NISXW has limited applicability to these elements due to the low photoabsorption cross section. Nevertheless, in the case of AK1111 at least, it is possible to monitor the absorption of the standing wave at the oxygen atoms by measuring the 0 1s photoemission yield as the photon energy is scanned through the Bragg condition at 2660 eV (the photoelectron kinetic energy being much too high for significant photoelectron diffraction modulation of the signal). This 0 absorption profile is compared with that obtained for the substrate Al atoms by monitoring the yield of AlKLL Auger electrons which provides an absolute energy reference and an experimental curve for which the structural parameters are known and thus from
5r%-emrelative
‘,-0 photon energy
313
I -4 (eV)
0
4
8
Fig. 5. NISXW absorption profiles at 0 and Al atoms for both the (111) and (ill) Bragg conditions from methoxy and from chemisorbed oxygen on Al(111). The dashed lines show theoretical fits to the Al absorption profiles, whilst the lines marked with squares and circles show similar fits to the 0 absorption profiles.
which the non-structural parameters can be obtained. The figure also shows the best fit theoretical curve for the 0 absorption profile in which the O-Al layer spacing perpendicular to the surface has been adjusted to a value of 0.70 + 0.10 A to optimise the fit. Also shown in fig. 5 are similar data collected from chemisorbed atomic oxygen on the same AK1111 surface 1111;this is a well-known precursor phase to surface oxidation on Al which forms no ordered superlattice structure although LEED analysis of this structure has been achieved using an apparent (1 x 1) saturation state. The NISXW data were collected from a surface for which the average coverage was below 1 monolayer, so neither the oxygen nor the methoxy form obvious long range ordered structures. As may be seen from fig. 5, the (111) standing wave data ar,e very similar for the two species, and the 0.70 A top layer spacing is consistent with that deduced from LEED [12,13] and SEXAFS 1141for the atomic oxygen phase. In order to obtain the adsorption site, however, a single layer spacing is insufficient, but this information is obtainable from a second standing wave experiment using scatterer planes at an
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D.P. Woodruff / Structural studies of adsorbed and coadsorbed molecular species
angle to the crystal surface. In this example NISXW were recorded normal to the (ill) planes at an angle of 70.5” to the surface normal. The combination of the two non-parallel layer spacings then permits a determination of the absolute adsorption site by simple real space triangulation of these two values. A particularly interesting feature of this experiment for the methoxy and atomic oxygen phases on Al(111) is that although the O-Al layer spacings perpendicular to the surface are the same, the layer spacings perpendicular to the (ill) planes are quite different (see fig. 51, indicating the two adsorption sites differ. Simple application of the triangulation establishes that the atomic oxygen occupies fee hollow sites (above an Al atom in the third layer of the substrate) also found in LEED studies of this phase [12,13], but the 0 atom in methoxy occupies the hcp hollow directly above an Al atom in the second layer. This distinction is actually a very interesting one; despite the fact that the two hollow sites may be expected to differ in binding energy by a very small amount, there are rather few cases for which adsorbates adopt both sites, and there appear to be no other examples of any adsorbate actually preferring the hcp site [151. Indeed, the only previous example of uniquely hcp site occupation which has been identified is for ethylidyne on Rh(lll), but in this case the molecule is co-adsorbed with atomic hydrogen and the unusual site has been suggested to be due to the fact that the fee sites are blocked by the adsorbed H atoms [16]. This example clearly serves to illustrate the potential of the NISXW method to determine the local adsorption site of specific elemental species within an adsorbate without the need for long range order. In principle, at least, it is also possible to use this method to probe the site of different elemental species within the same molecular adsorbate but in this particular example the photoabsorption cross section of the other principle atom, carbon, is too low to permit this additional information to be obtained. An alternative example in which this additional information has been obtained by NISXW to clarify the internal structure of the molecular adsorbate is given below in section 6.
4. Coadsorption structures and chemical site distinctions As surface science experiments on well-characterised surfaces move closer to those of relevance to surface chemical reactions (including those in commercial catalysts), the ability to investigate coadsorption systems becomes crucial. Any local element-specific structural probe has the potential to be applied to problems of this kind, and a recent photoelectron diffraction study [17] serves to illustrate this potential; the results of this study also highlight the limited understanding which we have of such problems due to the relative dearth of this type of structural investigation. The system investigated was NO adsorbed on Ni(lll), both with, and without, coadsorbed atomic oxygen. This adsorption system is of considerable interest because vibrational spectroscopy [18-201 (particularly high resolution vibrational spectroscopy in the form of infrared reflection absorption spectroscopy [19,20]) indicates that there are three distinct states of NO involved which have been attributed to different local adsorption structures. Vibrational spectroscopy has undoubtedly been one of the most successful methods of studying surfaces having two or more coexistent adsorbed species (especially different molecular fragments) due to its high spectral resolution relative to its spectral range. Its power to provide “fingerprinting” characterisation of such problems is therefore not in doubt, but its use as a quantitative structural method is less strongly based. In the NO/Ni(lll) case, three quite distinct N-O vibrational stretch frequencies are seen, the lowest being characteristic of low coverages of NO on bare Ni(lll1, the intermediate value being the only frequency seen at high NO coverages, and the highest frequency being the state seen when coadsorbed with atomic oxygen. These three distinct vibrational states have been proposed to be characteristic of three different adsorption geometries, tilted bridge, bridge, and atop, respectively 119,201. In applying photoelectron diffraction to this problem, we note that the NO bonds to the surface through the N atom, so N 1s photoelectron diffraction spectra should allow quantitative
D.P. Woodruff / Structural studies of adsorbed and coadsorbed molecular species
I
1
*
I
*
1
300 100 200 photoelectron energy
*
1
I
400
(eV)
Fig. 6. Nls normal emission experimental photoelectron diffraction spectra from NO adsorbed on Ni(ll1) in three different states (characterised by different N-O vibrational stretch frequencies), and from NH 3 on Ni(ll1).
determination of the NO adsorption sites (as do the 0 Is spectra for the different oxygen-containing species on Cu(100) - fig. 2), whilst the presence of the coadsorbed atomic oxygen will not influence this measurement. Of course, 0 1s photoelectron diffraction spectra obtained from these systems will be more complex, because in the case of the oxygen coadsorption phase there are two distinct 0 sites associated with the NO and the atomic 0, and the spectra will provide an incoherent sum of the photoelectron diffraction from these two sites). Fig. 6 shows normal emission N Is photoelectron diffraction spectra [17] recorded from the three different adsorption phases which have been found to be characteristic of the three different N-O vibrational frequencies. The three spectra are extremely similar, differing only in the relative intensities of the fine structure, and contrast strongly with the spectra of fig. 2 for which structural changes are known to occur. For comparison, fig. 6 also includes a similar N 1s normal emission photoelectron diffraction spectrum from NH, on Ni(ll1) [21] which is quite different (in this case the molecule occupies an atop site). The qualitative message of fig. 6 is clear; NO occupies essentially the same
315
adsorption site in all three vibrationally distinct phases. Quantitative structural analysis using model scattering calculations reveals that the common local adsorption site in these three phases is actually the hollow 1171, and not the bridge or atop sites, a result confirmed for the high coverage Ni(lll)/NO phase by an independent SEXAFS investigation [22]. The cause of the large vibrational frequency shifts appears to be due to vibrational coupling (in the high coverage NOonly phase) and electronic charge transfer (in the coadsorption phase) rather than site change. It is appropriate to remark that NO, with its singly occupied 25~ antibonding state, is probably particularly susceptible to internal vibrational frequency shifts due to local electronic interactions, but this result also highlights the danger of using internal vibrational frequencies as a signature of specific adsorption sites without proper supporting evidence. It also illustrates the potential of these element specific local structural probes to provide proper quantitative structural analysis of coadsorption phases independent of their long range order. Despite this proven capability, however, element specificity is inadequate to investigate many coadsorption problems of interest because the same elements may be involved in both adsorbates (indeed this is the case for NO/O, but here the N atom is unique to one species, and is also the atom bonding to the surface and so likely to give the clearest site determination). An obvious case of interest is that involving hydrocarbon fragmentation in which the only accessible atom (H does not have any core electrons!) in all the fragments is carbon. One route to studying the structure of these fragments independently is to exploit the different phoemission core level binding energies associated with these different chemical states. Photoelectron diffraction is clearly ideally suited to exploiting these “chemical shifts” in the measured binding energies because it is intrinsically a core level photoemission technique, but the NISXW method could also be used in the photoemission absorption detection mode as used in the AK1111 example given above to provide similar site specificity. Clearly the key to the
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viability of structure determination of chemically distinct atoms through their photoemission chemical shifts lies in the comparative sizes of the chemical shifts to be exploited and the limits of detectability set by the instrumental resolution and by the intrinsic photoemission linewidths. However, the fact that X-ray photoelectron spectroscopy (XPS) has proved so succesful as a laboratory method for studying surface chemistry bears witness to the potential of this route for investigating many chemically important coadsorption problems if instrumental resolution comparable to that obtained in laboratory XPS instruments can be obtained. This instrumental target is certainly achievable, and we have already made measurements on two relevant model systems. In one of these we have tackled a slightly different aspect of the problem of determining the adsorption site of atoms of the same element but different chemical environments which is the case of such atoms within a single (larger) adsorbed molecular species. Specifically we have determined the adsorption site of the two different C atoms within surface acetate (CH,COO-) and trifluoroacetate (CF,COO-) species on Cu(ll0). A substantial (m 3 eV> chemical shift is seen between the C 1s photoemission peak from the methyl carbon and the carboxyl carbon in the acetate, whilst in the trifluoroacetate a chemical shift of a similar magnitude but opposite sign is seen in the CF, carbon relative to that in the carboxyl group. We find that the photoelectron diffraction spectra of the carboxyl and methyl (or trifluoromethyl) groups are quite different and can be reconciled, with the aid of model calculations, with their different sites above the surface [231. A second rather different example concerns the adsorption of PF, on Ni(ll1). This species is extremely sensitive to electron or ultraviolet photon irradiation, and is known to dissociate, apparently into PF,, PF and P fragments. As has been shown in similar core level photoemission experiments of fragmented PF, on Ru(OOOl), there are substantial chemical shifts in the P2p photoemission peak between these different components [24], so that chemical shift photoelectron diffraction offers the possibility of determining the local
adsorption site of each fragment whilst they are coexisting on the surface. We have collected data for this system and are currently in the process of analysing them [25]. Although the chemical shifts between these fragments are quite large (almost 2 eV) this demonstration experiment is important in establishing what should be possible with many more coadsorption systems following the appropriate instrumental development.
5. Vibrational adsorbed
anisotropies species
and amplitudes
of
So far the role of these structural techniques in determining the static local structure of adsorbed (and coadsorbed) atomic and molecular species has been stressed. These methods are also capable, however, of providing information on the vibrational amplitudes of the adsorbed species in a directionally sensitive fashion. In diffraction (interference) phenomena generally, atomic vibrations enter into the physics because such vibrations produce an uncertainty in the atomic positions and hence in the interference path lengths; this is manifest by a Debye-Waller factor which attenuates the amplitude of a giving scattering interference term. This effect, of course, appears in LEED and the vibrational amplitude of the scattering atoms (including the adsorbates) appear as a fitting parameter in the theory. In LEED, however, the vibrations are generally assumed to be isotropic and any anisotropy would be difficult to extract, even if inserted into the theory, because multiple scattering introduces several different scattering vectors, and thus vibrational amplitudes in different directions enter into each scattered beam intensity in a complex beam and energy dependent fashion. In the case of photoelectron diffraction the fact that different collection geometries can be chosen to optimise the role of different individual subtrate backscatterer atoms means that it is significantly easier to obtain vibrational amplitudes, in specific directions. The first evidence of this effect was in a photoelectron diffraction study of NH, on Ni(ll1) [21,26] at a fixed temperature (N 110 K) in which it was found that the ampli-
D.P. Woodruff / Structural studies of adsorbed and coadsorbed molecular species
tude of the photoelectron diffraction modulations were strongly suppressed in off-normal emission geometries relative to normal emission. Theoretical simulation reveals that the ammonia occupies atop sites so that this effect is in part due to the strong nearest-neighbour backscattering obtained in normal emission, but the effect is too pronounced to be explained by this alone, and it appears to be due to a much larger vibrational amplitude parallel to the surface than perpendicular to the surface. This is entirely reasonable; for an atop adsorbed species there is a soft wagging vibration which is expected to have a large amplitude. A very similar situation appears to occur in the case of PF, adsorption which is also found to adsorb in atop sites on the basis of both NISXW measurements (supported by SEXAFS) [27] and photoelectron diffraction [251. In the case of NISXW the information on vibrational amplitudes [28] derives from one of the structural fitting parameters, the so-called coherent fraction. If the X-ray absorbing atoms are partially disordered, either statically or dynamically, the SXW absorption profile is modified in that it becomes a sum of the ideal profile for atoms at the mean layer spacing, attenuated by a DebyeWaller term, and a matching incoherent term which simply has the form of the reflectivity curve. The coherent fraction therefore provides a measure of the mean square displacement of the absorber atoms in the direction perpendicular to the scatterer planes. In the case of (111) and (ill) NISXW from PF, on Ni(lll), this means that the (111) profile provides a measure of the mean square displacements perpendicular to the surface, whilst the (ill) profile provides a value for the displacement almost parallel to (actually 20” from) the surface. The fits to the P absorption profiles for the (111) and (ill) NISXW, shown in fig. 7 do, indeed show a substantially reduced coherent fraction for the (ill> NISXW which we interpret as being due to a similar large vibration wagging vibration. Very recently we have also investigated this same system by photoelectron diffraction at different temperatures [25] and find clear evidence that the origin of the enhanced mean square
317
1.4 1.2 g 1.0 ‘0 $j 0.8 2 1.2 E
1.0
relative photon energy WI
Fig. 7. NISXW absorption profiles at both P and F atoms (as well as the substrate Ni atoms) recorded at the (111) and (ill) nominal Bragg conditions for PF, adsorbed on Ni(ll1). The dashed lines are fits to the substrate (Ni) absorption profiles, whilst the lines marked with squares and circles are the fits to the adsorbate atom absorption profiles.
displacements parallel to the surface is indeed thermal in origin; i.e. they are due to dynamic rather than static displacements. Notice that this adsorption system, in which we are able to measure both the P and F NISXW absorption profiles, illustrates the potential of the NISXW technique to determine, independently, the location of of atoms of different elemental species within an adsorbed molecule. The results provide us with clear information that the molecule is adsorbed in a highly symmetric configuration, with its C,, axis perpendicular to the surface, and the internal bondlengths and bond angles show no sign of distortion due to the metal substrate interaction.
6. Conclusions In summary, this short paper has provided experimental demonstrations of the utility of the photoelectron diffraction and NISXW methods to determine the local adsorption structure of adsorbed atoms and molecules in the absence of long range order, and in some cases in the presence of coadsorbed species. The potential to extend these methods through the use of chemical shifts in photoemission binding energies has also been described. Finally, examples have also been
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given of how these techniques can provide information on the magnitude and anisotropy of the vibrational states of these adsorbates. These achievements and potentials take surface structural methods further into the range of complexity of most relevance to understanding problems in surface reactions and catalysis.
Acknowledgements
As has already been mentioned, the work described here is the results of a series of major collaborations as indicated in the references cited, and without the efforts of these collaborators none of this work would have been possible. Much of this work has stemmed from longstanding and fruitful interactions with the groups of Professor Alex Bradshaw of the Fritz Haber Institute using the BESSY storage ring, and Dr. Rob Jones of Nottingham University using the Daresbury SRS. The financial assistance of the Science and Engineering Research Council, and of the European Commission under the SCIENCE and Large Scale Facilities programmes, is also gratefully acknowledged.
References [l] D.A. King and D.P. Woodruff, Eds., The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 4, Fundamental Studies of Heterogeneous Catalysis (Elsevier, Amsterdam, 19821. [2] R. Dipped, D.P. Woodruff, X-M. Hu, M.C. Asensio, A.W. Robinson, K-M. Schindler, K-U. Weiss, P. Gardner and A.M. Bradshaw, Phys. Rev. Lett. 68 (1992) 1543. [3] M.C. Asensio, M.J. Ashwin, A.L.D. Kilcoyne, D.P. Woodruff, A.W. Robinson, Th. Lindner, J.S. Somers, D.E. Ricken and A.M. Bradshaw, Surf. Sci. 236 (1990) 1. [4] D.P. Woodruff, C.F. McConville, A.L.D. Kilcoyne, Th. Lindner, J. Somers, M. Surman, G. Paolucci and A.M. Bradshaw, Surf. Sci. 201 (1988) 228. [5] Th. Lindner, J. Somers, A.M. Bradshaw, A.L.D. Kilcoyne and D.P. Woodruff, Surf. Sci. 203 (1988) 333. [6] J. Stdhr, in: X-ray Absorption: Principles, Techniques and Applications of EXAFS, SEXAFS and XANES, Eds. R. Prins and D.C. Kongsberger (Wiley, New York, 1988) p. 443. [7] D.P. Woodruff, Rep. Prog. Phys. 49 (1986) 683.
181D.P.
Woodruff, D.L. Seymour, C.F. McConville, C.E. Riley, M.D. Crapper, N.P. Prince and R.G. Jones, Phys. Rev. Lett. 58 (1987) 1460. D.L. Seymour, CF. McConville, C.E. 191D.P. Woodruff, Riley, M.D. Crapper, N.P. Prince and R.G. Jones, Surf. Sci. 195 (19881 237. no1 M. Kerkar, A.B. Hayden, D.P. Woodruff, M. Kadodwala and R.G. Jones, J. Phys. (Condens. Matter) 4 (19921 5043. illI M. Kerkar, D. Fisher, D.P. Woodruff and B. Cowie, Surf. Sci. 271 (1992) 45. WI V. Martinez, F. Soria, M.C. Mufioz and J.L. Sacedon, Surf. Sci. 128 (1983) 424. and P. Westrin, J. Phys. C (Solid 1131J. Neve, J. Rundgren State Phys.) 15 (1982) 4391. 1141D. Norman. S. Brennan, R. Jaeger and J. Stiihr, Surf. Sci. 105 (1981) L297. J.B. Pendry, P.J. Rous, D.K. Saldin, 1151J.M. MacLaren, G.A. Somojai, M.A. Van Hove, and D.D. Vvedensky, Surface Crystallographic Information Service: A Handbook of Surface Structures (Reidel, Dordrecht, 1987). M.A. Van Hove and G.A. Somojai, Surf. [161R. Koestner, Sci. 121 (1982) 321. D.P. Woodruff, A.W. Robinson, K-M. 1171M.C. Asensio, Schindler, P. Gardner, D. Ricken, A.M. Bradshaw, J.C. Conesa and A.R. Gonzalez-Elipe, Chem. Phys. Lett. 192 (1992) 259. W31 S. Lehwald, J.T. Yates, Jr. and H. Ibach, in: Proc. IVC-8, ICSS-4, ECOSS-3, Cannes, Suppl. Le Vide, Les Couches Minces, No. 201 (1980) 221. 1191W. Erley, Surf. Sci. 205 (19881 L771. DOI J.G. Chen, W. Erley and H. Ibach, Surf. Sci. 224 (1989) 215. [21] K-M. Schindler, V. Fritzsche, M.C. Asensio, P. Gardner, D.E. Ricken, A.W. Robinson, A.M. Bradshaw, D.P. Woodruff, J.C. Conesa and A.R. Gonzalez-Elipe, Phys. Rev. B 46 (1992) 4836. [22] A. Aminpirooz, A. Schmalz, L. Becker and J. Haase, Phys. Rev. B 45 (1992) 6337. [23] K-U. Weiss, R. Dippel, K-M. Schindler, V. Fritzsche, P. Gardner, A.M. Bradshaw, A.L.D. Kilcoyne and D.P. Woodruff, to be published. [24] S.A. Joyce, J.A. Yarmoff and T.E. Madey, Surf. Sci. 254 (1991) 144. [25] R. Dippel, K-U. Weiss, X-M. Hu, K-M. Schindler, P. Gardner, M.C. Asensio, V. Fritzsche, D.P. Woodruff, A.M. Bradshaw, A.R. Gonzalez-Elipe and J.C. Gmesa, to be published. [26] V. Fritzsche, K-M. Schindler, P. Gardner, A.M. Bradshaw, M.C. Asensio and D.P. Woodruff, Surf. Sci. 269/270 (1992) 35. [27] M. Kerkar, D.P. Woodruff, J.A. Sanchez, M.C. Asensio, M. Fernandez-Garcia and J.C. Conesa, to be published. [28] M. Kerkar, D. Fisher, D.P. Woodruff, R.G. Jones, R.D. Diehl, C.F. McConville and B. Cowie, J. Vat. Sci. Technol. A 10 (1992) 2148.
Surface Science 283 (1993) 309-318 North-Holland
surface
science
Structural studies of adsorbed and coadsorbed molecular species D.P. Woodruff Physics Department, University of Wmwick, Coventry CV4 7AL, UK Received
21 April
1992; accepted
for publication
23 May 1992
A short review is presented of the techniques of backscattering photoelectron diffraction and normal incidence standing X-ray wavefield absorption in the particular context of applications to the study of the local adsorption structure of molecular species on surfaces, including cases of coadsorption. Specific examples of NO and NO/O coadsorption on Nitlll), methoxy on Al(111) and PF, on Nitlll) are used to illustrate the ability of these methods to obtain local structural information in situations of especial interest in understanding structural effects in surface reactions. Further developments of these methods to the study of vibrational anisotropies of adsorbed species (such as that associated with the wagging vibrational mode of atop adsorbed NH, and PF, on Ni(lll)), and in the use of photoemission core level (“chemical”) shifts to study more complex problems, are described.
1. Introduction
Quantitative investigations of the static structure of surfaces, and particularly of the local structural environment of adsorbed atoms, molecules, and molecular fragments, have an important role in aiding understanding of both static and dynamic properties of surfaces including surface reaction dynamics. In particular, structural sensitivity is known to characterise many reactions in heterogeneous catalysis, and the nature of “active sites” is commonly discussed, as is the role of adjacent sites in permitting molecules to fragment or interact on surfaces, and of site blocking as a route to a loss of surface reactivity 111. In studying structural problems of greatest relevance to effects in surface reactions, the requirement of long range order in the surface overlayer which typifies conventional diffraction methods (notably conventional low energy electron diffraction - LEED) is a considerable constraint. One would like to investigate the local structural environment without the need for this order, and ideally this local structure should be determinable in the presence of two or more coexistent adsorbed species such as two reactant molecules or fragments, or a single species to0039-6028/93/$06.00
0 1993 - Elsevier
Science
Publishers
gether with an adsorbed surface modifier. For this reason, true local probes which gain their local as well as surface specificity by probing an element-specific property, are particularly valuable. Element specific structural probes also have the virtue that the structural environment of different atoms within an adsorbed molecular species may be investigated independently, providing added direct information on the extent to which such species may have their internal structure modified by the surface bonding. In this paper the potential of two particular techniques of this type, (scanned energy mode) photoelectron diffraction (PhD), and (normal incidence) standing X-ray wavefield absorption (NISXW), in some cases used in conjunction with surface extended and near edge X-ray absorption fine structure (SEXAFS and NEXAFS) are described and illustrated with examples of recent results. All of these techniques necessitate the use of synchrotron radiation, a resource for which cost-effective exploitation demands collaborative efforts. The results described here derive from collaborations of this kind, and as such many individual scientists have contributed; these collaborators are explicitly acknowledged through references to the appropriate original papers
B.V. All rights reserved
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which have been, or are currently in process of being, published. The remainder of this paper is organised as follows. In the next two sections, the two main techniques are introduced and their utility illustrated with specific examples. In section 4 further examples are described which illustrate the potential for the study of coadsorption systems using both techniques, and identity some of the important routes ahead to exploit this potential further. Section 5 describes some results which illustrate how these methods can only provide information on adsorbate vibrational anisotropies. Finally in section 6, some general conclusions are drawn.
I 1s normal emission cu(100) +
I
2. Photoelectron diffraction Fig. 1 illustrates the basic physical process which is used in photoelectron diffraction, namely the coherent interference of the directly emitted photoelectron wavefield from an adsorbed atom with components elastically scattered by surrounding atoms. In order to obtain sensitivity to the location of an atom adsorbed OIZa surface (either as a chemisorbed atom or within a molecular adsorbate), it is necessary to exploit scattering events which are predominantly backscattering (i.e., involving scattering angles between 90
0
,
I
,
,
,
,
i
100 300 200 photoelectron energy (eV)
0
Fig. 1. Schematic figure showing the components of the photoelectron wavefield directly emitted from an adsorbed atom (large circle) and backscattered from surrounding substrate atoms (small circles) which interfere to produce the photoelectron diffraction process described here.
Fig. 2. 0 1s normal emission experimental photoelectron diffraction spectra from chemisorbed oxygen, formate (HCOO) and methoxy (CHsO-) on Cu(100). The dashed lines are the results of extremely simple model calculations as described in the text. The intensity normalised experimental spectra are adapted from refs. 13-51.
and lSW), and these events only have significant cross section if the photoelectron energy is less than N 500 eV (cf. LEED). Information about the scattering path lengths (and hence the structural environment of the emitter) can be obtained from the variation in the detected photoelectron signal as a function of either photoelectron energy or emission angle. So far the angular variation has not been used in backscattering to solve any unknown structures (but see ref. [21 and references cited therein), and we concentrate here on the energy dependence - i.e. the scanned energy mode. Representative data from this type of experiment are shown in fig. 2 which comprises 0 IS photoemission intensity spectra, recorded as the photon energy and thus the photoelectron energy is varied, from three different O-containing adsorbed species on Cu(100). In all cases the emission is recorded along the surface normal. The three spectra correspond to results from chemisorbed oxygen on Cu(100) (in an ordered
311
D.P. Woodruff / Structural studies of adsorbed and coaakorbed molecular species
catalytic intermediates formate, HCOO- [4], and methoxy, CH,O- [5], the latter two adsorbates being produced by deprotonation of formic acid and methyl alcohol, respectively, and showing no long range order. Although all of the spectra show some fine structure and noise, each contains a single rather clear dominant periodicity to the oscillatory structure (as reproduced in the dashed curves) which can be attributed to a single scattering path length which gives rise to alternate constructive and destructive inteference as the electron wavelength (energy) is scanned. In each case this single scattering path is associated with a scattering event occurring at a scattering angle close to 180”, for which the scattering cross-section peaks and a near-neighbour scattering event is strongly favoured. These dominant scattering paths therefore involve a nearneighbour Cu (substrate) atom directly “behind” the emitter relative to the detector. As the actual nearest-neighbour Cu-0 distance differs very little between these three adsorption systems, it is clear that these different dominant scattering path lengths arise mainly because the adsorption sites differ. The dashed lines are the results of very simple scattering calculations including only one (or in the case of methoxy, five) Cu backscattering atoms and assuming specific structures. In the case of methoxy, the model involves the 0 atom (through which this species bonds to the surface) being close to, but some 0.4 A off, a hollow site on the surface [51 such that the dominant scatterer is the second layer Cu atom almost directly below; the fact that this path length is much longer than the nearestneighbour scattering path length accounts for the shorter periodicity of the oscillations in the energy spectrum. By contrast formate appears to occupy a site in which the 0 atoms lie almost directly above a top layer Cu atom (the formate bridges two nearest-neighbour Cu atoms but aligns the O-O axis along the Cu-Cu axis [4]), and in the oxygen chemisorption case the 0 effectively occupies an almost coplanar hollow site (so that the dominant Cu scatterer is a second layer atom) although one of its four nearestneighbour coplanar Cu atoms is missing in this missing row reconstruction 131. Fig. 3 shows the
normal emission WC,s
Cu(100) + HCOO
/Pd/-hs +.
Cls
-fY.
I
,
,
,
,
,
,
,
100 200 300 photoelectron energy (eV) Fig. 3. Comparison of experimental (full lines) and theoretical (dotted lines) normal emission photoelectron diffraction spectra from chemisorbed oxygen and formate on Cu(100). The structural models used in the calculations are described in the text.
more complete theoretical fits (using a larger cluster and a rather more exact treatment of the scattering) to the formate and chemisorbed oxygen photoelectron diffraction spectra. These data therefore serve to illustrate a number of features of the photoelectron diffraction method; in particular, the data show strong site sensitivity, are applicable to systems showing no long range order, and are capable of quantitative structure analysis (fig. 3). Notice that the fact that the technique uses core level photoemission as the probe means that it is possible to investigate the structural environment of the C atoms as well as the 0 atoms in the two molecular adsorbate species, and this can lead to further information. This is illustrated for formate in fig. 3 which shows the C 1s normal emission photoelectron diffraction spectrum compared with the results of a theoretical simulation; in this case the 0 atom sites have already been determined by the 01s PhD, and adjusting the C atom site to optimise the fit to the C 1s data has the effect of modifying the internal structure of the formate. The best fit is obtained by assuming that the O-C-O bond
312
D.P. Woodruff / Structural studies of adsorbed and coadsorbed molecular species
angle in the adsorbed formate is 134” [4], slightly larger than the 124” of the free formate ion. Similar analysis of the C 1s normal emission data from methoxy on Cu(100) leads to the conclusion that this spectrum is mainly sensitive to the angle of the O-C axis relative to the surface normal, and that the best fit is obtained with this axis essentially perpendicular to the surface [5]. We should note that many of the characteristics of the photoelectron diffraction method in the scanned energy mode are similar to those of the SEXAFS technique [6,71 in which the total, rather than angle derivative, photoionisation cross section is measured as a function of photoelectron kinetic energy. The main difference, however, is that the angle derivative photoelectron diffraction method allows the real space direction of substrate scattering atoms to be obtained far more directly, thus offering much more direct site sensitivity; an added experimental advantage is that the angle-resolved photoelectron diffraction method leads to diffraction modulations typically one order of magnitude larger than those found in the angle-averaged SEXAFS method.
3. Normal incidence
standing
X-ray wavefield
standing x-ray wavefield
Bragg
amplitude
intensity
condition reflectivity absorption - on scatterer planes
--I-
- mid-way scatterer
between planes
0;
Fig. 4. Schematic diagram showing the principle of the SXW method. In the upper part of the figure the interfering incident and scattered X-ray wavefronts are shown, together with the amplitude and intensity distributions of the resultant standing X-ray wavefield. In the lower part, reflectivity and absorption profiles are shown around the nominal Bragg condition.
absorption
A quite different structural technique which nevertheless also relies on photoelectric absorption of incident soft X-rays is NISXW [8,9]. The standing X-ray wavefield (SXW) method exploits a particular feature of X-ray diffraction, that when a Bragg scattering condition is met the coherent interference of the incident and diffracted X-ray wavefields is such as to establish an X-ray standing wave in the solid (and beyond the surface where the two waves overlap) which has a periodicity which is equal to, or an integral submultiple of, the subtrate scattering atom periodicity. The nodal (plane) surfaces of this standing wave lie parallel to the scatterer planes of the appropriate Bragg condition. A proper description of this X-ray scattering process requires application of dynamical (multiple scattering) X-ray diffraction theory (without this the scattered intensity could exceed that of the incident intensity) which leads
to the conclusion that the X-rays have a finite penetration (“extinction depth”) into the crystal, and that the total reflectivity condition therefore has a finite range in angle or energy. Within this range the phase of the standing X-ray wave is found to be displaced in a systematic and predictable fashion relative to the location of the scatterer atoms. This means that if we investigate the absorption of this standing X-ray wavefield by an adsorbed atom as a function of the scattering condition within the range of its existence, the location of the adsorbed atom can be found relative to the location of the (extended) substrate scatterer atom planes. This is illustrated schematically in fig. 4. Notice that at one end of the total reflectivity range the nodes of the standing wave lie on the scatterer planes and at the other end they lie midway
D.P. Woodruff / Structural studies of adsorbed and coadsorbed molecular species
between these planes; the X-ray adsorption profiles as a function of scattering condition for atoms located on, or midway between, these (extended) atom plane locations are therefore almost exactly inverted, highlighting the very great sensitivity to a change in layer spacing of hoalfthe bulk layer spacing value, typically l-l.5 A. The purpose of the normal incidence SXW experiment is to extend the range of materials to which the technique can be applied. In general, the total reflectivity range in angle or energy is small; in particular, the angular range may be only a few seconds of arc. This means that the experiment is only viable if the substrate to be studied has a crystalline mosaicity which is better than this small angular range, a condition which is typically met only by extremely perfect crystals of Si or some other semiconductors. However, if one chooses to work at normal incidence to the scatterer planes then the usual Bragg equation has a turning point in angle at this condition (2d sin 0 = nh at 0 = 900) and the dependence on the crystal mosaicity is weak [8,9]. This means that normal metal single crystals can be used routinely for the experiment. An example of the application of this method to a molecular adsorbate, methoxy on Al(111) 1101 is shown in fig. 5. We should note that the X-ray energies involved in normal incidence fee (111) Bragg conditions are typically 2.6-3.2 keV, an energy which is large relative to the deepest electronic binding energies of first row elements of the periodic table, so NISXW has limited applicability to these elements due to the low photoabsorption cross section. Nevertheless, in the case of AK1111 at least, it is possible to monitor the absorption of the standing wave at the oxygen atoms by measuring the 0 1s photoemission yield as the photon energy is scanned through the Bragg condition at 2660 eV (the photoelectron kinetic energy being much too high for significant photoelectron diffraction modulation of the signal). This 0 absorption profile is compared with that obtained for the substrate Al atoms by monitoring the yield of AlKLL Auger electrons which provides an absolute energy reference and an experimental curve for which the structural parameters are known and thus from
5r%-emrelative
‘,-0 photon energy
313
I -4 (eV)
0
4
8
Fig. 5. NISXW absorption profiles at 0 and Al atoms for both the (111) and (ill) Bragg conditions from methoxy and from chemisorbed oxygen on Al(111). The dashed lines show theoretical fits to the Al absorption profiles, whilst the lines marked with squares and circles show similar fits to the 0 absorption profiles.
which the non-structural parameters can be obtained. The figure also shows the best fit theoretical curve for the 0 absorption profile in which the O-Al layer spacing perpendicular to the surface has been adjusted to a value of 0.70 + 0.10 A to optimise the fit. Also shown in fig. 5 are similar data collected from chemisorbed atomic oxygen on the same AK1111 surface 1111;this is a well-known precursor phase to surface oxidation on Al which forms no ordered superlattice structure although LEED analysis of this structure has been achieved using an apparent (1 x 1) saturation state. The NISXW data were collected from a surface for which the average coverage was below 1 monolayer, so neither the oxygen nor the methoxy form obvious long range ordered structures. As may be seen from fig. 5, the (111) standing wave data ar,e very similar for the two species, and the 0.70 A top layer spacing is consistent with that deduced from LEED [12,13] and SEXAFS 1141for the atomic oxygen phase. In order to obtain the adsorption site, however, a single layer spacing is insufficient, but this information is obtainable from a second standing wave experiment using scatterer planes at an
314
D.P. Woodruff / Structural studies of adsorbed and coadsorbed molecular species
angle to the crystal surface. In this example NISXW were recorded normal to the (ill) planes at an angle of 70.5” to the surface normal. The combination of the two non-parallel layer spacings then permits a determination of the absolute adsorption site by simple real space triangulation of these two values. A particularly interesting feature of this experiment for the methoxy and atomic oxygen phases on Al(111) is that although the O-Al layer spacings perpendicular to the surface are the same, the layer spacings perpendicular to the (ill) planes are quite different (see fig. 51, indicating the two adsorption sites differ. Simple application of the triangulation establishes that the atomic oxygen occupies fee hollow sites (above an Al atom in the third layer of the substrate) also found in LEED studies of this phase [12,13], but the 0 atom in methoxy occupies the hcp hollow directly above an Al atom in the second layer. This distinction is actually a very interesting one; despite the fact that the two hollow sites may be expected to differ in binding energy by a very small amount, there are rather few cases for which adsorbates adopt both sites, and there appear to be no other examples of any adsorbate actually preferring the hcp site [151. Indeed, the only previous example of uniquely hcp site occupation which has been identified is for ethylidyne on Rh(lll), but in this case the molecule is co-adsorbed with atomic hydrogen and the unusual site has been suggested to be due to the fact that the fee sites are blocked by the adsorbed H atoms [16]. This example clearly serves to illustrate the potential of the NISXW method to determine the local adsorption site of specific elemental species within an adsorbate without the need for long range order. In principle, at least, it is also possible to use this method to probe the site of different elemental species within the same molecular adsorbate but in this particular example the photoabsorption cross section of the other principle atom, carbon, is too low to permit this additional information to be obtained. An alternative example in which this additional information has been obtained by NISXW to clarify the internal structure of the molecular adsorbate is given below in section 6.
4. Coadsorption structures and chemical site distinctions As surface science experiments on well-characterised surfaces move closer to those of relevance to surface chemical reactions (including those in commercial catalysts), the ability to investigate coadsorption systems becomes crucial. Any local element-specific structural probe has the potential to be applied to problems of this kind, and a recent photoelectron diffraction study [17] serves to illustrate this potential; the results of this study also highlight the limited understanding which we have of such problems due to the relative dearth of this type of structural investigation. The system investigated was NO adsorbed on Ni(lll), both with, and without, coadsorbed atomic oxygen. This adsorption system is of considerable interest because vibrational spectroscopy [18-201 (particularly high resolution vibrational spectroscopy in the form of infrared reflection absorption spectroscopy [19,20]) indicates that there are three distinct states of NO involved which have been attributed to different local adsorption structures. Vibrational spectroscopy has undoubtedly been one of the most successful methods of studying surfaces having two or more coexistent adsorbed species (especially different molecular fragments) due to its high spectral resolution relative to its spectral range. Its power to provide “fingerprinting” characterisation of such problems is therefore not in doubt, but its use as a quantitative structural method is less strongly based. In the NO/Ni(lll) case, three quite distinct N-O vibrational stretch frequencies are seen, the lowest being characteristic of low coverages of NO on bare Ni(lll1, the intermediate value being the only frequency seen at high NO coverages, and the highest frequency being the state seen when coadsorbed with atomic oxygen. These three distinct vibrational states have been proposed to be characteristic of three different adsorption geometries, tilted bridge, bridge, and atop, respectively 119,201. In applying photoelectron diffraction to this problem, we note that the NO bonds to the surface through the N atom, so N 1s photoelectron diffraction spectra should allow quantitative
D.P. Woodruff / Structural studies of adsorbed and coadsorbed molecular species
I
1
*
I
*
1
300 100 200 photoelectron energy
*
1
I
400
(eV)
Fig. 6. Nls normal emission experimental photoelectron diffraction spectra from NO adsorbed on Ni(ll1) in three different states (characterised by different N-O vibrational stretch frequencies), and from NH 3 on Ni(ll1).
determination of the NO adsorption sites (as do the 0 Is spectra for the different oxygen-containing species on Cu(100) - fig. 2), whilst the presence of the coadsorbed atomic oxygen will not influence this measurement. Of course, 0 1s photoelectron diffraction spectra obtained from these systems will be more complex, because in the case of the oxygen coadsorption phase there are two distinct 0 sites associated with the NO and the atomic 0, and the spectra will provide an incoherent sum of the photoelectron diffraction from these two sites). Fig. 6 shows normal emission N Is photoelectron diffraction spectra [17] recorded from the three different adsorption phases which have been found to be characteristic of the three different N-O vibrational frequencies. The three spectra are extremely similar, differing only in the relative intensities of the fine structure, and contrast strongly with the spectra of fig. 2 for which structural changes are known to occur. For comparison, fig. 6 also includes a similar N 1s normal emission photoelectron diffraction spectrum from NH, on Ni(ll1) [21] which is quite different (in this case the molecule occupies an atop site). The qualitative message of fig. 6 is clear; NO occupies essentially the same
315
adsorption site in all three vibrationally distinct phases. Quantitative structural analysis using model scattering calculations reveals that the common local adsorption site in these three phases is actually the hollow 1171, and not the bridge or atop sites, a result confirmed for the high coverage Ni(lll)/NO phase by an independent SEXAFS investigation [22]. The cause of the large vibrational frequency shifts appears to be due to vibrational coupling (in the high coverage NOonly phase) and electronic charge transfer (in the coadsorption phase) rather than site change. It is appropriate to remark that NO, with its singly occupied 25~ antibonding state, is probably particularly susceptible to internal vibrational frequency shifts due to local electronic interactions, but this result also highlights the danger of using internal vibrational frequencies as a signature of specific adsorption sites without proper supporting evidence. It also illustrates the potential of these element specific local structural probes to provide proper quantitative structural analysis of coadsorption phases independent of their long range order. Despite this proven capability, however, element specificity is inadequate to investigate many coadsorption problems of interest because the same elements may be involved in both adsorbates (indeed this is the case for NO/O, but here the N atom is unique to one species, and is also the atom bonding to the surface and so likely to give the clearest site determination). An obvious case of interest is that involving hydrocarbon fragmentation in which the only accessible atom (H does not have any core electrons!) in all the fragments is carbon. One route to studying the structure of these fragments independently is to exploit the different phoemission core level binding energies associated with these different chemical states. Photoelectron diffraction is clearly ideally suited to exploiting these “chemical shifts” in the measured binding energies because it is intrinsically a core level photoemission technique, but the NISXW method could also be used in the photoemission absorption detection mode as used in the AK1111 example given above to provide similar site specificity. Clearly the key to the
316
D.P. Woodruff / Structural studies of adsorbed and coadsorbed molecular species
viability of structure determination of chemically distinct atoms through their photoemission chemical shifts lies in the comparative sizes of the chemical shifts to be exploited and the limits of detectability set by the instrumental resolution and by the intrinsic photoemission linewidths. However, the fact that X-ray photoelectron spectroscopy (XPS) has proved so succesful as a laboratory method for studying surface chemistry bears witness to the potential of this route for investigating many chemically important coadsorption problems if instrumental resolution comparable to that obtained in laboratory XPS instruments can be obtained. This instrumental target is certainly achievable, and we have already made measurements on two relevant model systems. In one of these we have tackled a slightly different aspect of the problem of determining the adsorption site of atoms of the same element but different chemical environments which is the case of such atoms within a single (larger) adsorbed molecular species. Specifically we have determined the adsorption site of the two different C atoms within surface acetate (CH,COO-) and trifluoroacetate (CF,COO-) species on Cu(ll0). A substantial (m 3 eV> chemical shift is seen between the C 1s photoemission peak from the methyl carbon and the carboxyl carbon in the acetate, whilst in the trifluoroacetate a chemical shift of a similar magnitude but opposite sign is seen in the CF, carbon relative to that in the carboxyl group. We find that the photoelectron diffraction spectra of the carboxyl and methyl (or trifluoromethyl) groups are quite different and can be reconciled, with the aid of model calculations, with their different sites above the surface [231. A second rather different example concerns the adsorption of PF, on Ni(ll1). This species is extremely sensitive to electron or ultraviolet photon irradiation, and is known to dissociate, apparently into PF,, PF and P fragments. As has been shown in similar core level photoemission experiments of fragmented PF, on Ru(OOOl), there are substantial chemical shifts in the P2p photoemission peak between these different components [24], so that chemical shift photoelectron diffraction offers the possibility of determining the local
adsorption site of each fragment whilst they are coexisting on the surface. We have collected data for this system and are currently in the process of analysing them [25]. Although the chemical shifts between these fragments are quite large (almost 2 eV) this demonstration experiment is important in establishing what should be possible with many more coadsorption systems following the appropriate instrumental development.
5. Vibrational adsorbed
anisotropies species
and amplitudes
of
So far the role of these structural techniques in determining the static local structure of adsorbed (and coadsorbed) atomic and molecular species has been stressed. These methods are also capable, however, of providing information on the vibrational amplitudes of the adsorbed species in a directionally sensitive fashion. In diffraction (interference) phenomena generally, atomic vibrations enter into the physics because such vibrations produce an uncertainty in the atomic positions and hence in the interference path lengths; this is manifest by a Debye-Waller factor which attenuates the amplitude of a giving scattering interference term. This effect, of course, appears in LEED and the vibrational amplitude of the scattering atoms (including the adsorbates) appear as a fitting parameter in the theory. In LEED, however, the vibrations are generally assumed to be isotropic and any anisotropy would be difficult to extract, even if inserted into the theory, because multiple scattering introduces several different scattering vectors, and thus vibrational amplitudes in different directions enter into each scattered beam intensity in a complex beam and energy dependent fashion. In the case of photoelectron diffraction the fact that different collection geometries can be chosen to optimise the role of different individual subtrate backscatterer atoms means that it is significantly easier to obtain vibrational amplitudes, in specific directions. The first evidence of this effect was in a photoelectron diffraction study of NH, on Ni(ll1) [21,26] at a fixed temperature (N 110 K) in which it was found that the ampli-
D.P. Woodruff / Structural studies of adsorbed and coadsorbed molecular species
tude of the photoelectron diffraction modulations were strongly suppressed in off-normal emission geometries relative to normal emission. Theoretical simulation reveals that the ammonia occupies atop sites so that this effect is in part due to the strong nearest-neighbour backscattering obtained in normal emission, but the effect is too pronounced to be explained by this alone, and it appears to be due to a much larger vibrational amplitude parallel to the surface than perpendicular to the surface. This is entirely reasonable; for an atop adsorbed species there is a soft wagging vibration which is expected to have a large amplitude. A very similar situation appears to occur in the case of PF, adsorption which is also found to adsorb in atop sites on the basis of both NISXW measurements (supported by SEXAFS) [27] and photoelectron diffraction [251. In the case of NISXW the information on vibrational amplitudes [28] derives from one of the structural fitting parameters, the so-called coherent fraction. If the X-ray absorbing atoms are partially disordered, either statically or dynamically, the SXW absorption profile is modified in that it becomes a sum of the ideal profile for atoms at the mean layer spacing, attenuated by a DebyeWaller term, and a matching incoherent term which simply has the form of the reflectivity curve. The coherent fraction therefore provides a measure of the mean square displacement of the absorber atoms in the direction perpendicular to the scatterer planes. In the case of (111) and (ill) NISXW from PF, on Ni(lll), this means that the (111) profile provides a measure of the mean square displacements perpendicular to the surface, whilst the (ill) profile provides a value for the displacement almost parallel to (actually 20” from) the surface. The fits to the P absorption profiles for the (111) and (ill) NISXW, shown in fig. 7 do, indeed show a substantially reduced coherent fraction for the (ill> NISXW which we interpret as being due to a similar large vibration wagging vibration. Very recently we have also investigated this same system by photoelectron diffraction at different temperatures [25] and find clear evidence that the origin of the enhanced mean square
317
1.4 1.2 g 1.0 ‘0 $j 0.8 2 1.2 E
1.0
relative photon energy WI
Fig. 7. NISXW absorption profiles at both P and F atoms (as well as the substrate Ni atoms) recorded at the (111) and (ill) nominal Bragg conditions for PF, adsorbed on Ni(ll1). The dashed lines are fits to the substrate (Ni) absorption profiles, whilst the lines marked with squares and circles are the fits to the adsorbate atom absorption profiles.
displacements parallel to the surface is indeed thermal in origin; i.e. they are due to dynamic rather than static displacements. Notice that this adsorption system, in which we are able to measure both the P and F NISXW absorption profiles, illustrates the potential of the NISXW technique to determine, independently, the location of of atoms of different elemental species within an adsorbed molecule. The results provide us with clear information that the molecule is adsorbed in a highly symmetric configuration, with its C,, axis perpendicular to the surface, and the internal bondlengths and bond angles show no sign of distortion due to the metal substrate interaction.
6. Conclusions In summary, this short paper has provided experimental demonstrations of the utility of the photoelectron diffraction and NISXW methods to determine the local adsorption structure of adsorbed atoms and molecules in the absence of long range order, and in some cases in the presence of coadsorbed species. The potential to extend these methods through the use of chemical shifts in photoemission binding energies has also been described. Finally, examples have also been
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given of how these techniques can provide information on the magnitude and anisotropy of the vibrational states of these adsorbates. These achievements and potentials take surface structural methods further into the range of complexity of most relevance to understanding problems in surface reactions and catalysis.
Acknowledgements
As has already been mentioned, the work described here is the results of a series of major collaborations as indicated in the references cited, and without the efforts of these collaborators none of this work would have been possible. Much of this work has stemmed from longstanding and fruitful interactions with the groups of Professor Alex Bradshaw of the Fritz Haber Institute using the BESSY storage ring, and Dr. Rob Jones of Nottingham University using the Daresbury SRS. The financial assistance of the Science and Engineering Research Council, and of the European Commission under the SCIENCE and Large Scale Facilities programmes, is also gratefully acknowledged.
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