The (100) surfaces of chromium and vanadium: Reconsiderations of their structure and reactivity

Surface Science 129 (1983) 79-9 1 North-Holland Publishing Company

THE (100) SURFACES RECONSIDERATIONS

79

OF CHROMIUM AND VANADIUM: OF THEIR STRUCTURE AND REACTIVITY

J.S. FOORD Depurtment of Chemisi~, The Universi~, High~el~ ~vurham~ton SO9 SVH, UK

and A.P.C.

REED

and R.M. LAMBERT

Depnrtment oj Physical Chemistv, University of Cambridge, Cambridge CB2 lEP, UK Received

23 August

1982; accepted

for publication

24 February

1983

The structural properties of clean Cr(lOO) and V(100) surfaces have been examined by LEED, AES, A+ and photoelectron spectroscopy with particular reference to the question of possible reconstruction. When clean, both surfaces exhibit (I x 1) periodicity at 300 K. The c(Z X 2) phase on &r(lOO) is associated with small amounts of adsorbed carbon and oxygen, and the (5 x 1) phase on V( 100) is induced by subsurface oxygen. The nature of the V(iOB)-(5 X 1) surface was examined in detail by studying oxygen and bromine chemisorption on the (1 X I) and (5 XI) surfaces respectively. The surface --) bulk transport of oxygen and the low pressure oxidation of V are characterised; a convenient spectroscopic method for detecting low levels of oxygen in vanadium is described. Electronic and structural aspects of the vanadium-bromine interaction are elucidated.

1. fntroduction

Considerable interest in the structures of the (100) surfaces of V and Cr has been aroused by reports that they respectively exhibit (5 x 1) and c(2 x 2) reconstructions when in an atomically clean state [1,2]. However, Foord and Lambert [3] showed that the Cr c(2 X 2) pattern arises from the presence of adsorbed impurities and Jensen and co-workers have stated that the V (5 X 1) structure is associated with the presence of surface oxygen [4,5]. In the light of this, we have further examined the structural properties of the (5 x 1) and c(2 x 2) surfaces and the conditions under which they form. The interaction between vanadium and oxygen has been the subject of a long standing disagreement in the literature [6-81 and some of the conclusions of a paper on the V(lOO)/Br, adsorption system 191, which was the first to report the phenomenon of low pressure halide growth, are at odds with later investigations of the analogous chemisorption systems, Cr(lOO)/Cl, and Br,, V( 100),X1, [3,10, I I]_ In order to resolve these differences we reinvestigated the adsorption of Oz and Br, and these results are also presented here. 0039~6028/83/0000-0000/$03.00

0 1983 North-Holland

80

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2. Experimental The apparatus has been described previously [3,12], experiments being carried out in two vacuum chambers in which base pressures of 4 X 1O-9 Pa were routinely attained. UPS and XPS were available in one chamber and LEED, AES, TPD and Ar” etching facilities were common to both. The (100) Cr and V samples were prepared from 99.99 + % purity ingots, after orientation to better than OS”, and temperature measurement was by thermocouples spot-welded to the rear of the specimens. Bromine dosing was carried out using a solid state electrolytic source [ 131.

3. Structural chemistry of the Cr(lOO)-c(2 x 2) and V(lOO)-(5 X 1) surfaces 3.1. Cr(lOO) In agreement with our previous work [3], it was found necessary to Ne+ bombard the sample for 400 h (10 PA, 350 eV) while maintaining its temperature at 1000 K, before the specimen could be freed from the bulk N contaminant, which segregates to the surface at elevated temperatures. However, following this treatment, the clean surface (1 x 1) pattern shown in fig. la was observed and thus no reconstruction occurs at T > 300 K. It was found that exposure of the sample to the gases which commonly constitute the background atmosphere in UHV chambers (H,, CO, 02, Nz, CH4, CO*) was ineffective in generating a c(2 X 2) surface. However, extremely intense c(2 X 2) patterns could be observed following annealing of a sample which had been bombarded in an inert gas atmosphere in which the concentration of the above species was above the ppm level. Indeed it was only under the cleanest ion bombardment conditions achievable that (1 x 1) surfaces could be successfully prepared. A number of halogen adsorption experiments were carried out on the c(2 X 2) surfaces, which XPS analysis revealed were contaminated with 0.1 ---)0.4 monolayer of implanted impurity (C, N, 0). An extensive range of compression structures, characteristic of the interaction with the clean surface [3,10], were formed on surfaces for which the contaminant level was less than 0.2 monolayer. although all halogen-induced LEED features were progressively suppressed if the contaminant concentration rose above this value. 3.2. V(100) After insertion of the specimen into the vacuum, NeC bombardment (10 PA, 300 eV) for 12 h with the sample at 1000 K and subsequent annealing, the (5 x 1) LEED pattern observed previously [2,9] was visible (fig. lb). The AES spectrum in fig. 2a was recorded at this stage from which it may be deduced

J.S. Foord et al. / (100) surfaces of chromium and vanadium

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Fig. I. LEED patterns observed from the (100) planes of Cr and V: (a) Cr( lOO)-( 1 X I), 107 V; (b) V(lOO)-(5 x I), 60 V; (c) V(lOO)-(5 x I)-Br, Br, exposure = 2X 10’s molecules m-*, 57 V; (d) V( lOO)-(5 X I)-Br, Br, exposure = 3 X 10’s molecules m -*, 69 V; (e) V(lOO)-(6 x4)-Br, Br, exposure = lOI molecules m-‘, 69 V; (f) V(lOO), ring pattern, Br, exposure = IO*’ molecules m-*, 49 v.

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J.S. Foord et al. / (100) surfaces

of chromium and vanadium

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region.

that no contaminants other than 0 (whose transition overlaps the V peak 509 eV) are present on V(5 x 1). On the other hand Ne+ bombardment of the crystal at 300 K (10 PA, 5 min), followed by annealing at 650 K, resulted in the formation of a (1 x 1) pattern, and the specimen needed to be heated above 750 K before the (5 X 1) pattern appeared. In order to examine the (1 x 1) -j (5 X 1) transition in greater detail, the intensities of the (0, l/5) beams were monitored after annealing of the crystal for 1 min at successively increasing temperatures followed by rapid quenching to 350 K. A graph of the intensities of the (0, l/5) beams versus annealing temperatures is shown in fig. 3, where it can be seen that the (5 X 1) LEED pattern develops in a narrow temperature window around 750 K. XP spectra of the V(2p) and O(ls) region, UP spectra and AE spectra of the V(MW) region recorded before and after the surface phase. transition are shown in fig.

J.S. Foord et al. / (100) surfaces of chromium and oanadium

1

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and surface

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2b. It is apparent that the (1 x 1) + (5 X 1) conversion brings about a considerable increase in the O(ls) XP signal, introduces a prominent feature in UPS at -6 eV which may be thought to arise from O(2p) levels, and reduces the intensity of the V(MW) Auger transition while at the same time introducing a satellite peak 5.5 eV below the main peak. The intensity of the XP 0( 1s) signal was determined as a function of annealing temperature in the same way as the fractional-order LEED beam intensity was monitored, and the results are also presented in fig. 3. It is apparent that the build-up in oxygen concentration in the surface region occurs at precisely the same temperature as that where the (5 x 1) LEED pattern develops. 3.3. Discussion We have here verified our previous suggestion (31 that clean Cr(100) is not reconstructed at 300 K, and since this view has also been expressed recently by the laboratory originally reporting the reconstruction fl4], this issue can no longer be in doubt. It will be of great interest to discover whether the reconstruction takes place at lower temperatures, as is the case with Mo(f00) and W( 100) (15,161. A necessary starting point for such experiments, however, must be the preparation of a (1 X 1) surface at 300 K. An intense c(2 X 2) LEED pattern is clearly visible on surfaces for which the impurity concentration is well below 0.5 monolayer, a feature which led us to suggest that the structure may represent an impurity stabilised displacive phase [IS], rather than a simple adlayer with adatoms in alternate adsorption sites on the (1 X 1) substrate. We now argue against this since the presence of the c(2 x 2)

superstructure has little detectable effect on halogen adsorption, at low impurity levels. Instead, it seems to be the case that c(2 x 2) impurity islands form, which give rise to an extremely intense fractional order beam, yet which leave large areas of the surface with the (1 X 1) atomic configuration characteristic of clean Cr( 100). Turning to V(lOO)-(5 X l), the XPS results clearly confirm previous AES work [4,5] which indicated that a substantial concentration of oxygen is present on the (5 X 1) substrate. Furthermore, since the (1 x 1) ---f(5 x 1) structural transition occurs in precisely that narrow temperature regime for which the oxygen concentration builds up to its limiting value as a result of bulk -+ surface diffusion, during heating of the specimen, it may be concluded that the presence of 0 is not merely coincidental but a necessary requirement for formation of the (5 x 1) structure. Given this, it is of interest to establish the likely structure since kinematical scattering calculations f2] gaye good agreement with a model in which the first interlayer spacing is 1.0 A greater than that in the bulk. The important observation, which is presented in more detail in section 4.1.2, is that bromine forms the LEED pattern in fig. lc for B < 0.5. It should be noted that this pattern would not be generated from coexisting (5 x 1) and V(lOO)-c(2 x 2)-Br domains. The appearance of (2n + 4, 2m + 4) beams with associatedfifth order maxima reveals that Br forms a c(2 x 2) array un top of the (5 X 1) substrate. This in turn implies that the outermost layer of the (5 X 1) substrate consists of a normal four-fold symmetric plane of (100) atoms. Since A+ measurements show that oxygen rapidly diffuses below the surface for T > 300 K, it seems reasonable to suppose that the oxygen in the V structure lies underneath this outermost layer. The most likely structure for the (5 x 1) phase is therefore very similar to that proposed for the W( lOO)-(5 X 1)-C surface [17] in that it consists of an outermost layer of metal atoms of (100) symmetry and a sub-surface layer of adsorbate atoms with (5 x 1) translational symmetry. This adsorbate array presumably increases the first intra-metal layer spacing and leads to a systematic fit with the kinematical scattering calculations in which this feature is incorporated. As regards future work on V involving AES, we point out that although detection of oxygen via the normal O(KLL) transition is difficult because of its proximity to V transitions, adsorbed 0 does strongly attenuate the V(M,,W) emission and introduces a prominent satellite feature 5.5 eV below the main peak. Thus, even with low resolution AES, monitoring these features should permit the detection of low concentrations of surface 0. Photoemission revealed no energy shifts in either the vanadium core levels or the metal valence band at the oxygen concentrations necessary to induce this remarkably pronounced feature. However, since UPS indicated that the O(2p) levels lie 5-6 eV below the peak in the V d-band emission, it seems likely that an efficient cross-transition is responsible for the appearance of the satellite peak, involving valence levels on both V and 0.

J.S. Foord et al. / (IOO) surfaces of chromium and oanadium

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4. Adsorption of Br, and 0, on V(100) 4.1.

Results

1, Oxygen adsorption Oxygen adsorption on the V( lOO)-( 1 X 1) surface was studied by XPS, UPS, AES and A$ determination techniques. XP spectra of the V(2p)-O(ls) region are shown in fig. 4a. Oxygen exposures of less than 4 L at 300 K produced a smooth increase in the O(ls) signal and attenuation of the signals from the V(2p) levels. After higher exposures at 300 K, broad chemically shifted V(2p) signals emerged at a binding energy some 3.6 eV greater than that of the parent peaks. If the adsorption process was instead carried out at 400 K, the only apparent change was an enhancement of the rate of oxidation. UP spectra monitored during oxidation are shown in fig. 4b. Adsorption of 30 L 0, at 300 K produced broad intense O(2p) emission features at - 5.5 eV and the V(3d) emission intensity was reduced to a broad peak at - 1.0 eV. Little change was observed in the spectrum following further exposure of the 4. It

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surface to oxygen at 300 and 400 K. The V(MVV) Auger transition was monitored during oxidation under conditions where the electron dose received by the crystal was minimised (typically 2 PA, 2.5 keV, 30 s> to reduce possible electron beam damage and indeed no ESD effects were observed on the accessible time scale. The resulting spectra are presented in fig. 4c. For oxygen exposures of up to 5 L at 300 K, the only change apparent in the spectra was an attenuation of the MW signal and the appearance of a satellite peak shifted by - 5.5 eV. At this stage the spectrum was qualitatively similar to that of the (5 x 1) surface. After further exposures to oxygen, a considerable broadening of the valence band spectrum was visible, and if the oxygen exposure was carried out at 400 K, this broadening resolved into a clear chemical shift of -4.0 eV. Work function measurements showed that exposures of the surface to oxygen at 300 K caused a smooth increase in A+ of up to +2.0 eV for oxygen exposures of up to 5 L, after which there was little change. Oxygen adsorption studies were also carried out on the (5 x 1) and ion bombarded surface: no significant change apart from rather small kinetic effects could be detected. 4.1.2. Bromine adsorption Some specific experiments involving bromine adsorption on the (5 X 1) surface were carried out in order to clarify certain aspects of the earlier work reported from this laboratory 191. Bromine formed no ordered LEED structures if the crystal temperature was maintained at 300 KI the existing (5 x 1) pattern merging into the background after high Br, exposures (IO*’ molecules me*). However, a series of patterns could be seen if the specimen was briefly annealed at 750 K, subsequent to the Br, exposure. For Br, doses between lo’* and 2 X 1018 molecules rn-’ the pattern shown in fig. lc was visible. This pattern exhibits (2n + 5, 2m + 4) spots, each of which shows prominent fifth order satellites. If the bromine exposure increases to 3 x 10” molecules tn-‘, the fifth order maxima completely disappear from the LEED pattern, and a simple c(2 x 2) pattern emerges (fig. Id). From bromine exposures in the range 4 X IOr to 3 X lOI molecules m-*, the previously reported (6 X 4) pattern shown in fig. le was seen. Finally, if the bromine exposure was increased to 10” molecules m-*, the pattern shown in fig. If appeared. This consists of twelve symmetrically spaced spots arranged in a ring, each spot being elongated along the ring circumference. At higher beam voltages a second ring was visible with a radius 1.74 times that of the first. Such patterns arise from the presence of two domains of hexagonal symmetry oriented at right angles to each other on the surface, with some degree of rotational disorder about the surface normal. For gas exposures in the range 5 X lOI to 102’ molecules mm2, domains of the hexagonal LEED pattern were found to coexist with the (6 X 4) structure and calibration of the spot positions arising from the two structures enables the hexagonal lattice vector to be calculated as 3.8 + 0.1 A.

J.S. Foord et al. / (100) surfaces

of chromium

87

and vanadium

Work function measurements made during the adsorption of 10” molecules rnem2 of Br, at 300 K followed by crystal heating at progressively higher temperatures are shown in fig. 5a. Although the adsorption process produced the expected large increase in A$ (+ 1.9 eV) the behaviour found on warming the specimen is very interesting. The A+ value steadily drops to exhibit a shallow minimum at the temperature at which the surface layers convert from the hexagonal structure to the (6 x 4) structure, increases as the C(2 x 2) structure forms, and then falls off to zero as all of the remaining bromine thermally desorbs. Auger spectra monitored during this procedure confirmed the earlier view [9] that the Br Auger signal of the (6 X 4) structure is approximately 1.5 times that of the c(2 X 2) adlayer. Changes observed in the V(M,,,W) and Br(M*,N,,,N,,,) Auger transition regions of the secondary electron spectrum during the course of these experiments are shown in fig. 5b. As the Br, exposure increases from 3.0 x lOI

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88

J.S. Foord et al. / (100) surfaces of chromium and oanudtum

molecules rn-’ to 102’ molecules me2, the bromine Auger peak exhibits a broadening and shifts by 0.9 eV to lower kinetic energies. Correspondingly, the original V(MVV) transition is completely suppressed, and a new peak appears displaced 4.0 eV to lower kinetic energies. If the sample was heated to 550 K, shifts to lower kinetic energies by 0.9 eV occur in both these transitions. heating the specimen to higher temperatures to generate the (6 x 4) and c(2 x 2) structures causes the Br transition to shift back to its original (low coverage) position, removes the new peak from the spectrum, and restores intensity to the V(MW) transition. Finally, it was noted that when surfaces dosed at 300 K with the minimum amount of Br, necessary to form the (6 X 4) structure were annealed to form such a structure, the V(MVV) transition decreased in intensity by some 40%. A decrease in work function of 0.5 eV accompanied this process. XP spectra of the Br(3p) region and the V(2p) region were recorded after bromine adsorption, and the latter are illustrated in fig. 5c. If the surface was dosed with 102’ molecules m- 2 of Br,, poorly resolved shoulders corresponding to a chemical shift of 0.9 eV appeared in the V(2p) spectra, and these underwent a further shift of + 1.0 eV when the specimen was annealed at 550 K. After further heating of the specimen to restore the (6 x 4) and c(2 x 2) LEED structures, the shoulders to the main 2p transitions are seen to disappear. Emission from the Br(3p) levels at high resolution was generally too weak to be chemically useful, but again it was noticeable that annealing the specimen at 550 K after high bromine doses caused a shift of + 1.0 eV in the binding energy of the 3p levels. He I excited valence band spectra are shown in fig. 5d. After high bromine exposures the vanadium 3d emission is strongly attenuated, a broad Br(3p) envelope appears in the - 3.0 eV to - 6.5 eV region, and a prominent feature becomes visible at - 1.0 eV. Annealing the specimen at 550 K causes a binding energy shift of these additional features by + 1.0 eV. When the crystal is heated to generate the (6 X 4) and c(2 X 2) patterns, the feature at - 1.0 eV (- 2.0 eV on the annealed surface) disappears, and certain changes become visible within the Br(3p) emission band. 4.2. Discussion 4.2.1. Low pressure oxidation During oxygen exposure of up to 4 L at room temperature, the increase in work function by 2.0 eV, and the straightforward attenuation of the V(2p) photoemission on V(MW) Auger peaks with increased oxygen exposure are clearly consistent with the formation of an electronegative oxygen overlayer. However, the chemical shift by 3.6 eV to higher binding energy of the V(2p) XPS signal during further oxidation at 300 K clearly shows that a surface [7]. Increasing the oxide begins to form, in which V4+ species predominate

J. S. Foord ef al. / (100) surfaces

of chromium ond oanadium

89

oxidation temperature to 450 K merely results in an increased rate of oxidation, but does not affect the oxidation state of the vanadium species formed. It is also notable that during this oxidation process, the UPS spectrum rapidly attains a profile characteristic of VO, [ 181, the two peaks arising from the emission from the V(3d) and O(2p) bands. Clearly, the low pressure interaction between oxygen and V( 100) first forms an oxygen overlayer and then generates an apparently uniform surface oxide phase in which V4+ is the predominant cation. Vary similar conclusions were reached by Brundle previously in the case of evaporated vanadium films [7] and indeed we obtain similar results on the annealed and ion bombarded (disordered) V( 100) surfaces, suggesting that the observations are not very sensitive to the surface topography. In particular this work would appear to discount previous conclusions [6,7] that the oxidation of V( 100) results in the formation of thick oxide phases in the range VO, (0 < x < 1.5). Finally, we point out that the determination of the peak profile of the V(M,,,W) Auger transition may also be used to monitor the oxidation process on vanadium. The peak intensity is simply attenuated during formation of the oxygen overlayer, but a clear chemical shift of -4.0 eV takes place when the vanadium oxide phase forms. The magnitude and direction of the shift (to lower kinetic energies) presumably is the net result of (a) a positive chemical shift in the binding energy of the M,,, core levels which is opposed, and more than outweighed, by (b) an increased binding energy of the valence levels as a result of charge transfer from the weakly bound d-band in the V metal to the more strongly bound O(2p) band. 4.2.2. Bromine adsorption Studies of the interaction of chlorine and bromine with Cr( 100) and chlorine with V(100) [3,10,1 I] show these systems follow a set pattern. Atomic overlayers form at low coverages, evolving Xc,,, MX(,,, or MX,(,, during subsequent thermal desorption. At high exposures island growth of the layer-structured metal dihalide occurs, with a common epitaxial relationship being adopted whereby the close-packed halogen planes lie parallel to the metal surface. Earlier work on the V(lOO)-Br system [9] suggested instead that layer-by-layer growth of the dihalide takes place so that the surface is covered by a uniform monolayer of halide (associated with a (6 X 4) LEED pattern) before 3D growth sets in and, furthermore, it was thought that subsequent layers may also be associated with the (6 X 4) structure thus implying completely different epitaxial relationships. The present work seeks to resolve these apparent differences between the V( lOO)-Br system on the one hand and the Cr( lOO)-Cl, Cr( lOO)-Br and V( lOO)-Cl systems on the other. It was shown that annealing bromine covered surfaces at constant coverage to form the (6 x 4) superstructure from the disordered phase initially present induces a drop in AC+ of 0.9 eV and reduces the intensity of the V(M,,,W) transition. In addition, the work function of the (6 X 4) is lower than that of

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J.S. Foord et al. / (100) surfaces of chromium and vanadium

the c(2 X 2) adlayer, even though the former contains 50% more bromine. These results suggest that a Br overlayer formed at 300 K undergoes conversion to a halide corrosion layer during sample annealing, the drop in intensity of the M2,3W signal being associated with V --$ Br charge transfer. Good support for the previously presented interpretation of the (6 x 4) structure therefore exists. Turning to the chemistry exhibited at much higher coverages, the detection of the ring pattern (fig lf) shows that halide growth does not proceed via formation of successive (6 X 4) layers. The hexagonal ring pattern fits exactly the basal plane of VBr, (a = 3.77 A) and is analogous to the similar pattern seen in the Cr/Cl, Br and V/Cl chemisorption systems. Furthermore, the coexistence of (6 X 4) and hexagonal domains at intermediate coverages shows that island growth once again occurs. The interaction of Br with V(100) is therefore very similar to that of Cl with V(100) and Cl and Br with Cr(lOO): an atomic bromine overlayer forms during exposures of - lOI molecules rnw2; after which 3D island growth of the layer-structured VBr, occurs, the close packed Br planes lying parallel to the V surface. The only apparent difference is the fact that Br overlayers on V(100) convert entirely to halide corrosion phases at a temperature below that at which the constituent MX, units become volatile, whereas this is not the case in the other halogen adsorption systems we have investigated. All of the electron spectroscopic techniques confirm the view that growth of VBr, occurs during high gas exposures (- 10” molecules M-‘). Thus the chemical shift of 1.9 eV to greater binding energy of the V(2p) peak in XPS is characteristic [3] of the formation of a halide phase in which +2 is the predominant oxidation state. As regards UPS, apart from the changes in the Br(3p) region of the spectrum ( - 4.5 + - 7.0 eV), another prominent change is the decay of emission near E, and the concomitant growth of a feature at - 2.0 eV. This can be straightforwardly interpreted [19] as arising from the from the d-band of octahedrally coordinated V2+ 3T3p +- 4Aig PES transition species in the VBr, lattice. The complete suppression of the V’(M,,yV) Auger transition and the appearance of a peak displaced 5.0 eV to lower kinetic energies is also indicative of the growth of a halide phase of substantial thickness on the surface. The new peak can then be interpreted in terms of an M2,3W transition in the halide. It was particularly interesting to note that the entire electron emission spectrum (XPS, UPS, AES) from the halide layer shifts by - 0.9 eV to lower kinetic energies when films formed at 300 K are annealed at 500 K. This must arise from changes in reference level caused by the development of an electric field across the halide. Since it was also noted that A+ drops during the annealing procedure it appears that films formed at 300 K possess an excess Br concentration in the outermost layers, giving rise to a high work function and a negative potential drop across the halide. Subsequent annealing then results in the interdiffusion of V-and Br species causing the

J.S. Foord et al. / (100) surfaces of chromium and vanadium

disappearance of this potential drop, a reduction reference level of atoms in the surface region.

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Acknowledgements J.S.F. thanks Selwyn College, Cambridge, for financial support during the course of this work. A.P.C.R. thanks BP Ltd. and Newnham College, Cambridge, for financial support. We are grateful to Johnson Matthey Ltd. for the loan of precious metals.

References [I] [2] [3] [4] [5] [6] (71 [8] [9] [lo]

[ 1I] [12] (131 [14] [15] [16] [ 171 [ 181 [19]

G. Gewinner, J.C. Peruchetti, A. Jaegle and R. Riedinger, Phys. Rev. Letters 43 (1979) 935. P.W. Davies and R.M. Lambert, Surface Sci. 107 (1981) 391. J.S. Foord and R.M. Lambert, Surface Sci. 115 (1982) 141. V. Jensen, J.N. Anderson, H.B. Nielsen and D.L. Adams, Surface Sci. 112 (1981) L785. V. Jensen, J.N. Anderson, H.B. Nielsen and D.L. Adams, Surface Sci. 116 (1982) 66. F.J. Szalkowski and G.A. Somorjai, J. Chem. Phys. 56 (1972) 6097. C.R. Brundle, Surface Sci. 52 (1975) 426. F.J. Szalkowski and G.A. Somorjai, Surface Sci. 52 (1975) 431. P.W. Davies and R.M. Lambert, Surface Sci. 95 (1980) 571. J.S. Foord, A.P.C. Reed and R.M. Lambert, in preparation. A.P.C. Reed and R.M. Lambert, in preparation. J.S. Foord, P.J. Goddard and R.M. Lambert, Surface Sci. 94 (1980) 339. P.J. Goddard and R.M. Lambert, Surface Sci. 67 (1977) 180. G. Gewinner, J.C. Peruchetti and A. Jaegle, Surface Sci. 122 (1982) 383. M.K. Debe and D.A. King, Surface Sci. 81 (1979) 193. T.E. Felter, R.A. barker and P.J. Estrup, Phys. Rev. Letters 38 (1977) 1138. J.B. Benzinger, E.I. Ko and R.J. Madix, J. Catalysis 54 (1978) 414. H. Becher, E. Dietz, U. Gerhardt and H. Angermuller, Phys. Rev. B12 (1975) 2084. Photoemission in Solids II, Eds. L. Ley and M. Cardona, Topics in Applied Physics, Vol. 27 (Springer, 1979).