The adsorption of atomic hydrogen on Cu(111) investigated by reflection-absorption infrared spectroscopy, electron energy loss spectroscopy and low energy electron diffraction

363

Surface Science 215 (1989) 363-377 North-Holl~d, Amsterdam

m ADSORPTION OF ATOMIC J3YDROGEN ON Cu(ll1) INVESTIGATED BY REFLECTION-ABSORPTION INFRARED SPECI’ROSCOPY, ELECTRON ENERGY LOSS SPECTROSCOPY AND LOW ENERGY ELECTRON DIFFRACTION Elaine M. McCASH *, Stewart F. PARKER and Michael A. CHESTERS ****

* *, John PRITCHARD

***

School of Chemical Sciences, Universes of Emt Anglia, Norwich, XlZ4 I?.., UK

Received 22 September 1988; accepted for publication 16 January 1989

Hydrogen atoms were adsorbed on Cu(ll1) at 150 K. The vibrational spectrum of the adsorbed hydrogen was measured by both reflection-absorption infrared spectroscopy @AIRS) and electron energy loss spectroscopy (EELS) in the temperature range 90-280 K. The saturated surface displayed a (3 X 3) LEED pattern which was converted to a (2 X 2) pattern on heating to 186 K. The vibrational spectrum observed by EELS was littfe affected by this structural transformation. The major energy loss peak at 1040 cm-l was assigned to the symmetric stretching mode of hydrogen in a two-fold bridge site. The infrared spectrum revealed more detail, including a second sharp absorption band at 1150-1170 cm-’ which was assigned to the first overtone of the deformation mode of hydrogen in a two-fold bridge site. The intensity of the electron energy loss peak, excited by dipole scattering, and of the infrared absorption bands provided an estimate for the effective charge, e*, of the hydrogen adatom of 0.05-0.06e which is similar to values previously estimated for the hydrogen adatom on W(100).

1. Introduction The che~so~tion of hydrogen on transition metal surface has been the subject of continual study, both experimental and theoretical, t~oughout the development of surface science. As the “simplest” possible adsorbate, hydrogen holds a unique position which attracts theoretical modelling. The small * Present address: Cavendish Laboratory, Department of Physics, University of Cambridge, Madingley Road, Cambridge CB3 OHE, UK. ** Present address: Analytical Support and Research Division, BP Research Centre, Chertsey Road, Sunbury on Thames, Middlesex, TW16 7LN, UK. *** Permanent address: Department of Chemistry, Queen Mary College, Mile End Road, London El 4NS, UK. **** To whom correspondence should be addressed.

~39-4028/89/$03.50 0 Elsevier Science Publishers B.V. (forth-Holland Physics Pub~s~ng Division)

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size of the hydrogen atom leads to the possibility of several types of adsorbed hydrogen, occupying different surface sites. The vibrational spectra of chemisorbed hydrogen measured by electron energy loss spectroscopy (EELS) have been used as the basis for identification of the type of hydrogen adsorption site, e.g. for the face-centred cubic group VIII metals Ni(ll1) [l], Ni(lOO) [2], Pd(ll1) [3], Pd(lOO) [4] and Pt(ll1) [5,6]. The criteria for relating vibrational frequencies to site geometry in the earlier papers were those originally developed for transition metal cluster hydrides [7] and were based on a simple central force field model neglecting bending force constants. The relevance of this model to adsorbed hydrogen has recently been questioned [8] and the spectrum of hydrogen on Pt(ll1) re-interpreted ]9,10]. Irrespective of the model used for analysis of the vibrational spectra there is agreement that hydrogen adsorbs in three-fold bridging sites on the (111) surfaces and four-fold bridging sites on the (100) surfaces. Apart from a fundamental interest in the nature of hydrogen chemisorption, the studies of Ni, Pd and Pt surfaces can be justified in terms of the importance of these surfaces in catalysis of reactions involving hydrogenation of CO and hydrocarbons. For similar reasons the chemisorption of hydrogen on copper surfaces warrants attention. Dissociative adsorption of hydrogen on copper surfaces is activated [ll] and the heat of adsorption is relatively low. As a result the chemisorbed state may be populated near room temperature by equilibration with a hydrogen gas pressure in the mbar range [12] but is not populated at low temperatures where the rate of adsorption becomes too small. At low temperature it is necessary to pre-dissociate the hydrogen, e.g. on a hot tungsten filament, in order to populate the chemisorbed state [12,13]. It is therefore not surprising that there are relatively few reported experiments on hydrogen chemisorption on copper single crystal surfaces using “surface science”-type techniques in ultra-high vacuum systems. Comsa and David [II] used a novel technique to study the dynamics of associative desorption of hydrogen from Cu(100) and Cu(ll1) surfaces in which the gas was adsorbed at high pressure on one side of the crystal and diffused through to the other face where it desorbed into a UHV system. Greuter and Plummer have recorded UV photoelectron spectra of hydrogen atoms adsorbed on Cu(ll1) and report some similarities to spectra of hydrogen on Ni and Pt surfaces [13]. Pritchard and coworkers have studied hydrogen che~so~tion on UHV evaporated copper films and single crystals. Adsorption was monitored by surface potential measurements both for atomic hydrogen adsorption at low temperature and reversible adsorption of molecular hydrogen at near room temperature. Measurement of adsorption isotherms allowed the determination of isosteric heats of adsorption, mostly on polycrystalline films [14]. In this paper we report the vibrational spectra of hydrogen atoms adsorbed on Cu(111) measured by both the electron energy loss spectroscopic technique, EELS, and the reflection-absorption’infrared spectroscopic technique, RAIRS.

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2. Experimental The electron energy loss spectra were recorded with a Leybold Heraeus ELS22 spectrometer in an ultra-high vacuum system equipped with a retarding field analyser for low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The resection-abso~tion infrared spectra were recorded in a similarly equipped, separate UHV system which has been described elsewhere [15]. The Mattson Cygnus 25 spectrometer was operated at 8 cm-’ resolution and, unless otherwise stated, 4000 scans were accumulated for each spectrum (23 minutes 37 seconds total scanning time). A “narrow band” mercury cadmium telluride (MCT) detector was used which limited the spectral range to 4000-800 cm-‘. The copper crystal was prepared by mechanical polishing followed by a chemical etch in a 1: 1: 1 mixture of glacial acetic acid, concentrated nitric acid and orthophospho~c acid and was cleaned by several cycles of argon ion bomb~~ent followed by annealing at 670 K. Atomic hydrogen and deuterium were generated by thermal dissociation on a tungsten filament placed 15 mm from the front face of the crystal in the EEL spectrometer and 25 mm from the crystal face in the RAIRS chamber. The procedure involved establishing a dynamic hydrogen pressure in the chamber, typically 1 x 1O-7 Torr in EELS, and 1 x 10F6 Torr in RAIRS, and heating the tungsten filament for measured time intervals. The exposures quoted are not the exposures to hydrogen atoms but refer to the molecular hydrogen pressure and the time for which the filament was operated.

3. Results 3. I. Low energy electron diffraction The clean Cu(lll)surface exhibited a sharp (1 X 1) LEED pattern. Adsorption of hydrogen atoms at a substrate temperature of 98 K initially resulted in a (2 x 2) pattern with split half-integral spots, fig. la, at a nominal exposure of 30 L. Further exposure up to 60 L resulted in a sharp (3 X 3) pattern, fig. lb. The sequence of LEER patterns could be reversed by heating the crystal in vacuum. After heating to 180 K briefly (- 120 s) and re-cooling to 98 K, the two LEED patterns were found to coexist implying the presence of islands of the two different structures. Heating to above 186 K produced the (2 x 2) pattern alone while the sharp (1 x 1) pattern was recovered after heating to 263-278 K. The adsorbate was found to be very susceptible to disruption by the electron beam. Under the influence of an electron beam of energy 200 eV the (3 X 3) pattern could be transformed to a mixture of (3 X 3) and (2 X 2) after

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Fig. 1. (a) LEED pattern of (2

et al. / Ahorption

X

of atomic hydrogen on Cu(lI1)

2) structure primary energy = 195 eV. (b) LEED pattern of (3 X 3) stmcture primary energy = 248 eV.

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ofatomic estrogen on

Cufl 11)

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just 8-10 s exposure. A typical exposure time used to obtain the photographs was 5-7 s, which had no noticeable effect at low beam energies, - 50 eV, but did begin to change the pattern at high beam energies, - 200 eV. 3.2. Electron energy 10s~ spectra A nominal exposure to hydrogen atoms of 60 L at 150 K followed by cooling to 98 K resulted in the EEL spectrum shown in fig. 2a and the (3 x 3) LEED pattern. Higher exposure to hydrogen atoms produced no further changes. Adsorption of hydrogen atoms at lower temperatures led to contamination of the surface with water, which desorbed at 150 K. Heating the hydrogen saturated surface to 186 K produced the (2 x 2) LEED pattern and the EELS spectrum in fig. 2b. In both cases the main loss peak appeared at 1040 + 15 cm-’ and was accompanied by a weak feature at 1400 cm- ‘. The peak at 2080 cm-’ is due to a low level of carbon monoxide cont~nation, A broad shoulder centred at approximately 800 cm-’ is possibly due to residual water contamination. It should be noted that all EEL spectra were recorded with the crystal temperature at 98 K. The EEL spectrum resulting from exposure of the clean surface to 1 L of water at 98 K is shown in fig. 3. The pattern of loss features is in agreement with reported EEL spectra of adsorbed water 1161and shows an intense, broad feature near 800 cm-‘. The spectrum resulting from adsorption of deuterium atoms to saturation is shown in fig. 4. The shift in the position of the main peak confirms that it results from excitation of a hydrogen& vibrational mode.

I

x 3300

0

1037

MO0

AE/,,_l

2000

Fig. 2. (a) Electron energy loss spectrum after 60 L exposure at 150 K and cooling to 98 K. Corresponds to (3 X 3) LEED structure. (b) Electron energy loss spectrum after heating hydrogensaturated surface to 186 IS and cooling to 98 K. Corresponds to (2 X 2) LEED structure. Primary energy was 5 eV and elastic peak width 62 cm- ’ for all EEL spectra.

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et al. / Adsorption

of atomic hydrogen on Cu(lll)

AE /cm-’ Fig. 3. Electron

energy loss spectrum

after exposure

of clean surface

to 1 L of water at 98 K.

The peak positions in the spectrum corresponding to the (2 x 2) LEED structure are not significantly different to those in the spectrum of the saturated surface displaying the (3 x 3) LEED pattern. Superficially the main loss peak intensity in the (2 x 2) surface spectrum appeared to be lower than for the saturated surface but when normalised to the elastic peak which also varied in intensity there was no significant difference. 3.3. Reflection-absorption

infrared spectra

The RAIR spectrum of the saturated surface resulting from exposure to hydrogen atoms at 150 K, fig. 5a, shows a broad peak at 1039 cm-’ (half-width 42 cm-‘) and a sharp peak at 1151 cm-‘. The half-width of the sharp peak was spectrometer-resolution-limited but was found to be 4 cm-’ in spectra recorded at 2 cm-’ spectrometer resolution. (Note that the infrared spectrum could be recorded at any temperature since the crystal heater current does not disturb the measurements whereas EEL spectra could only be recorded after cooling with the heater current switched off.) The effect of

750

I

62cm-’

.

IP!I!z 0

1?60

1000

AE /cm-’ Fig. 4. Electron

energy

loss spectrum

of surface saturated cooled to 98 K.

with deuterium

atoms

at 150 K and

E. M. McCash et al. / Ahorption

of atomic hydrogen on Cu(1 I I)

369

b)

1148

1151 I 1200

I 1100

I 1000 _ V/Clll_'

I 900

Fig. 5. (a) Reflection-absorption infrared spectrum of surface saturated with hydrogen atoms at 150 K. (b) Reflection-absorption infrared spectrum of (a) after cooling the crystal to 91 K.

cooling to 91 K was to shift the broad peak to 1046 cm-’ and reduce the half-width to 33 cm-‘, fig. 5b. There was an apparent decrease in the intensity of the sharp feature but spectrometric accuracy for this peak is poor as its inherent width is smaller than the instrumental resolution (8 cm-‘) and similar to the data point separation (4 cm-l). Heating to 180 K, to give the “mixed” (2 x 2)/(3 X 3) LEED pattern, resulted in considerable broadening of the main peak to a half-width of 55 cm-’ and the appearance of a second “sharp” feature at 1170 cm-‘, fig. 6a. Cooling this surface produced a much sharper band at 1045 cm-‘, half width 24 cm-‘, and a single sharp band again at 1151 cm-‘, fig. 6b. Heating to 225 K, to produce the (2 x 2) LEED pattern, resulted in further broadening of both peaks in the spectrum, fig. 7a. The main band at 1035 cm-’ of half-width 60 cm-’ is accompanied by a single, sharper band at 1166 cm-‘. Cooling to 150 K produced a rather ill-defined spectrum, fig. 7b. This was partly due to the problems of producing a flat background in the ratio spectrum, particularly when the sample temperature is varied, but it does appear that the

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McCash et al. / Adsorption

of atomic hydrogen on Cu(I 11)

91 K

Fig. 6. (a) Reflection-absorption infrared spectrum of hydrogen-saturated surface after heating to 180 K and measured at that temperature. (b) Reflection-absorption infrared spectrum of (a) after cooling to 91 K.

xca)225K

I

1200

I

1100

I

I

1000

900

ij/cm-'

Fig. 7. (a) Reflection-absorption infrared spectrum of hydrogen-saturated surface after heating to 225 K and recorded at that temperature. (b) Reflection-absorption infrared spectrum of (a) after cooling to 91 K.

E. M. ~UcCu.sh et al. / Adsorptionof atomichydrogen on Cufl 11)

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spectrum has become weaker. Under the same conditions the EEL spectrum shows the main loss feature substantially intact. This remains as an unexplained discrepancy between the results of the EELS and RAIRS experiments. The appearance of two peaks in the infrared spectrum of the adsorbed hydrogen atoms in contrast to the one peak in the electron energy loss spectrum may be explained by the inability of the low resolution EELS technique to resolve the two peaks.

4. Discussion Under ideal circumstances the combination of vibrational spectra with corresponding LEED patterns can lead to an un~biguous identification of the bonding sites of simple adsorbates on a single crystal surface. The small size of the hydrogen atom, and consequent uncertainty concerning the number of adsorbate atoms per unit mesh, makes this a difficult task particularly without precise coverage measurements. A variety of LEED structures have been reported for hydrogen layers on other (111) surfaces. The (2 X 2) pattern on Ni(ll1) was analysed in some detail by Christmann et al. [17] who concluded that hydrogen was located at three-fold sites to a coverage B = 0.5. The weak ad~tional diffraction features (1%2% of the intensity of the integral order beams) were believed to arise from a hydrogen scattering rather than from a reconstruction of the metal surface. A (0 X fi)R30 o pattern observed for hydrogen on Pd(ll1) has been found to exist for two surface coverages, 0 = l/3 and 2/3. In the latter case the pattern may be regarded as a (6 x &)R30” array of empty sites [18]. On Pt(ll1) a (1 x 1) LEED pattern has been associated with adsorption at fee three-fold sites to form a complete monolayer, 8 = 1.0 [19]. The real-space unit mesh corresponding to the (3 x 3) LEED pattern reported here is rather large and must contain many hydrogen atoms. Thermal desorption spectra of hydrogen atoms adsorbed on Cu(ll1) reported by Greuter and Plummer [12] show a main desorption peak at 300 K with a smaller shoulder at 220 K. Consideration of our results indicates that the main desorption peak is associated with the (2 x 2) hydrogen layer which Greuter and Phunmer suggested corresponds to a surface coverage @ = OS. The saturated surface coverage then corresponded to 8 = 0.7, given the measured ratio of peak intensities for the two TDS peaks. A surface coverage of 8 = 0.7 for the (3 X 3) surface structure corresponds to six hydrogen atoms per unit mesh (6 = 0.67). In the case of the (2 X 2) layer, there would be two hydrogen atoms per unit mesh. An alternative interpretation of the LEED patterns is to consider (3 x 1) and (2 x 1) surface structures each existing in domains of three orientations related by a 60’ rotation. The adsorbed hydrogen atoms could then fill alI the two-fold sites in parallel rows with every third row

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b Fig. 8. Real-space surface structures suggested to account for the observed LEED patterns. (a) (3x11 structure. (b) (2 X 1) structure. Large open circles are copper atoms, small solid circles denote hydrogen atom positions.

missing for the (3 x 1) structure and every second row missing for the (2 x 1) structure, fig. 8 {the choice of a two-fold adsorption site is justified below). This is an attractively simple interpretation of the LEED results and accounts for the half order pattern very well. In the case of the third order pattern there are very clear spots at the (l/3, l/3) and relation positions which would not be produced by three domains of a (3 X I> structure. It is possible that the third order pattern corresponds to a mixture of (3 x 1) and (0 x JS^)R30” patterns each arising from a hydrogen atom coverage of 8 = 0.67 but it could equally well correspond to a true (3 x 3) pattern. We are not able to distinguish between these possibilities. The extra diffraction features are relatively weak and therefore could arise from hydrogen scattering. The small splitting of the extra spots in the half order pattern may be explained in terms of interference between antiphase domains of (2 x 1) structure. This is illustrated for (2 X 1) domains oriented along one azimuth in fig. 9. Turning to the vibrational spectra, the observation of a metal-hydrogen stretching band, resulting from dipole scattering, at 1050 cm-’ in the EEL spectrum would, by analogy with vibrational spectra of transition metal cluster hydrides [7], be assigned to the symmetric metal-hydrogen stretch of a hydrogen atom bridging two metal atoms. According to a simple model neglecting bending force constants one can determine the frequency of the corresponding antisymmetric stretch pas from the relationship Y,,/Y, = tan (Y

E.M. McCash et al. / Adsorption

0

0

i

:

of atomic hydrogen on Cu(lI1)

0

373

:

I \\

1 “,c2”

; \\ I :

\/

Q

‘?

ooJ :

oO,l

I[+ I

’

0

t

0

:

9.0 *

:

0

:

0

:

0

Fig. 9. (a) Real space mesh, aI, a2 mesh vectors of substrate; b,, b2 mesh vectors of overlayer; c,, c2 mesh vectors linking antiphase domains. (b) Reciprocal space, C,*, C,* reciprocal mesh vectors defining mesh of split spots. Note: the observed splitting corresponds to a domain size about twice that illustrated in the diagram.

where (Yis the semi-vertical angle of the hydrogen bridge [7]. Using a covalent radius of hydrogen of 0.37 A a value of 1300 cm-’ for vaSis predicted. We do observe a very weak feature at 1400 cm-‘, in rather poor agreement with this model bearing in mind that neglecting the angle bending force constant would lead to an overestimate in vaS.We also find on the basis of both specular and off-specular spectra that the band at 1400 cm-’ appears to arise from dipole scattering whereas symmetrical bonding at a two-fold bridging site would lead to an antisymmetric stretch strictly forbidden by the surface selection rule. If we assume that the hydrogen atom is bonding at a two-fold bridging site then assignment of the weak feature at 1400 cm-’ to v,, is far from satisfactory. An alternative interpretation of the observed band frequencies is suggested by the theoretical work of Hamann et al. [8,9] who have estimated hydrogen

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vibration frequencies using a thin slab model for the metal surface and performing total energy calculations for (1 x 1) hydrogen layers on Pt(lll), Ru(OOOl), Cu(ll1) and Cu(lll)/Ru(OOO1) model surfaces. The main thrust of their conclusions is that the central force field model seriously underestimates the symmetric stretching frequency because the bonding involving the delocalised electrons of the substrate is largely non-directions. They conclude that hydrogen atoms will bond to three-fold fee sites on these (111) surfaces, including Cu(lll), and calculate v, for H on Cu(ll1) at 1300 cm-’ with a value of vas at 30% lower i.e. - 1000 cm-‘. We cannot assign our strongly dipole excited band at 1050 cm-l to the antisymmetric stretch which would be dipole forbidden. It therefore seems that our results are not compatible with these calculations. The infrared spectrum of the hydrogen-saturated surface provides additional useful evidence for the nature of the adso~tion site. The observation of the main band at 1045 cm-’ is in good agreement with the EELS result and confirms that the energy loss process must have a significant dipole scattering contribution. The appearance of a second, sharper band at 1150 cm-’ is rather reminiscent of the infrared spectrum of hydrogen adsorbed on W(100) reported by Chabal[20]. There is general agreement that hydrogen bonds at a two-fold bridge site on W(100). Chabal assigned a broad band at 1070 cm--’ to the s~et~c metal-hydrogen stretch and a second, sharper band at 1270 “wag” in which the hydrogen cm-’ to an overtone of the metal-hydrogen moves parallel to the surface. The fundamental frequency of this wagging mode had been measured by EELS at 645 cm-‘. If hydrogen sits at a two-fold site on Cu(ll1) we can similarly assign the band at 1150 cm-* to the overtone of the wagging mode. As all overtones are totally symmetric they are allowed by the metal surface selection rule, but would generally be infrared forbidden for a strictly simple harmonic model. Resonant anharmonic coupling between an overtone and a nearby totally symmetric fund~ental commonly results in a significant transfer of intensity into the overtone band (Fermi resonance). Since hydrogen bonded at a three-fold bridging site will not possess a low frequency wagging mode, but rather a symmetric stretch and a two-fold degenerate a symmetric stretch, the assignment of the band at 1150 cm-’ to an overtone may only be made on the basis of hydrogen adsorbed at a two-fold bridging site. The intensity of the Mets-hy~ogen stretching band in our EEL spectra is rather higher than generally observed for hydrogen on (111) fee surfaces, e.g. Ni(lll), Pt(ll1) and, quite apart from characteristic frequencies, this suggests that the hydrogen atom sits in a different site on the copper surface. The effective charge, e *, associated with the metal-hydrogen stretching band may be estimated from the intensity of the loss peak relative to the elastic peak using the equation given by Newns [21]. If we assume a coverage 8 = 0.7 [13] we estimate a value of e* = 0.05e. Similarly the total integrated intensity of

E.M. McCash et al. / Adsorption of atomic hydrogen on Cu(1 I I)

375

the infrared bands in fig. 5b gives a value of e* = 0.06e using the equation given by Chesters and Canning [22] which is a simplified version of the equation given by Ibach and Mills [23]. The agreement between the two methods of measurement is satisfactory and the results compare with e* = 0.06e calculated for hydrogen on W(100) [24]. It is reasonable to expect that hydrogen in a two-fold bridging site, which sits further above the top plane of metal atoms than does hydrogen in a three-fold bridging site, will be less shielded by metal valence electrons and hence will retain a higher effective charge. The effective charge associated with hydrogen adsorbed at three-fold sites has also been determined in the calculations of Hamann and Feibelmann to be - 0.05e [lo]. This seems a small value, as pointed out by these authors, particularly when compared with an effective charge associated with the C-O stretch of adsorbed carbon monoxide of l.Oe. However it must be remembered that the strength of dipole scattering in EELS and of infrared absorption is determined by the dynamic dipole moment which is the product of the effective charge and the amplitude of motion. Our values of effective charge for hydrogen on Cu(ll1) correspond to a dynamic dipole moment of - 0.03 D compared with values of - 0.2 D for chemisorbed carbon monoxide [21]. The result reported here, and for hydrogen on the W(100) surface [24], demonstrate that an effective charge of - 0.05e for adsorbed hydrogen atoms is sufficient to result in an EEL spectrum dominated by dipole scattering and a measurable infrared spectrum. We conclude that the effective charge associated with hydrogen adsorbed in three-fold sites such as on Pt(ll1) must be significantly smaller than 0.05e since the dipole scattering contribution to the EEL spectrum is found to be very weak. In analysing our LEED and vibrational spectroscopic results we have assumed that only one form of chemisorbed hydrogen exists on the surface. There is some evidence that other forms might exist, for instance the weak peak at 1400 cm-’ in the EEL spectrum, excited by dipole scattering, might be assigned to hydrogen in a triangular adsorption site [9,10]. The apparent lack of correlation between the intensity of the main electron energy loss peak and coverage on converting from the (3 x 3) to (2 x 2) structures may also arise from the existence of other hydrogen surface species which do not contribute strongly to the infrared or electron energy loss spectra. Finally we return to the detailed interpretation of the infrared spectrum of hydrogen atoms on Cu(ll1) and particularly to the origin of the two peaks in the spectrum. We assign the broad band near 1050 cm-’ to the symmetric metal-hydrogen stretch and the sharp peak near 1150 cm-’ to the overtone of the deformation mode of hydrogen adsorbed at a two-fold site and we explain its significant intensity in terms of Fermi resonance. The analysis of the spectrum of hydrogen on W(100) [20] and more recently of hydrogen on W(lOO), Mo(100) and Cr(100) [24] by Chabal and co-workers cites a different mechanism for transfer of intensity into the overtone in which the existence of

a particular surface state associated only with the (1 x 1) hydrogen saturated surfaces is crucial. The surface state has the same symmetry as the fundamental of the deformation and the excited state associated with the “extra” band in the infrared spectrum is believed to involve coupled vibrational and electronic excitation. Coupling of a discrete vibrational excitation with a quasi-~ont~uous excitation of electron-hole pairs leads to the so-called “Fano” lineshape in the overtone band. We do not observe a highly asy~etric lineshape in the band at 1150 cm-‘, although some distortion is apparent, and it therefore appears that this coupling mechanism is not so important on the Cu(ll1) surface, presumably because of the absence of suitable electronic surface states. The band at 1150 cm-’ remains relatively sharp at all temperatures while the width of the band assigned to the symmetric stretch is very temperature dependent. The observation of some reversible and some irreversible changes in the width of this band on varying the substrate temperature indicates that both i~onlogeneous and homogeneous broadening processes are responsible. The reversible changes in peak width could be due to a dephasing process involving coupling between the copper-hydrogen symmetric stretch and metal phonons, in which case the deformation mode appears to be rather less susceptible to such processes. A rather curious result observed for hydrogen on W(lOO), Mo(100) and Cr(100) is that the overtone band is seen in the infrared spectrum but not in the electron energy loss spectrum, even though it is sufficiently separated from the fundamental to be resolved [24]. Our explanation for the appearance of the overtone excitation in the spectra of hydrogen on Cu(ll1) would apply to both electron energy loss and infrared spectroscopy but the separation of the two bands does not allow them to be resolved by EELS. The only indication of the existence of a higher frequency band in the EELS spectrum is an asymmetry in the peak at 1040 cm-‘, e.g. fig. 4b, which is consistent with the width of the EELS loss peaks generally (- 100 cm-‘) and the lower intensity of the RAIRS band at 1150 cm-’ (less than half the intensity of the 1040 cm-’ band).

5. Conclusions The electron energy loss and infrared spectra of hydrogen atoms adsorbed on Cu(ll1) indicate that the adsorbate sits at two-fold bridging sites on this surface, unlike on other fee (111) surfaces where hydrogen adsorbs at three-fold sites. The assignment of the adsorption site was based on the intensity of the spectra and the two-peaked structure in the infrared spectrum, which led to analogies with H/W(lOO), rather than soley on characteristic hydride stretching frequencies over which there appears to be some confusion at present.

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The LEED patterns observed have been correlated with (3 X 1) and (2 X 1) overlayers at @= 0.67 and 0.5 respectively. However it is recognised that hydrogen in other adsorption sites may be present. A weak loss peak in the EEL spectrum at 1300 cm-’ could be associated with hydrogen in three-fold sites [lo].

Equipment grants from the SERC and research assistantships Cash, SERC, and SF. Parker, MOD) are gratefully acknowledged.

(E.M. Mc-

References [l] W. Ho, N.J. Dinardo and E.W. Phnnmer, J. Vacuum Sci. Technol. 17 (1980) 134. [2] P.-A. Karlsson, A.-S. K&rtensson, S. Andersson and P. Nordlander, Surface Sci. 175 (1986) L759. [3] H. Conrad, M.E. Kordesch, R. Scala and W. Stenzel, J. Electron Spectrosc. Related Phenomena 38 (1986) 289. (41 C. Nyberg and C.G. Tengstal, Surface Sci. 126 (1983) 163. [S] A.M. Baro, H. Ibach and H.D. Bruchrnan, Surface Sci. 88 (1979) 384. [6] A.M. Baro and W. Erley, Surface Sci. 112 (1981) L751. [7] M.W. Howard, U.A. Jayasooriya, S.F.A. Kettle, D.B. Powell and N. Sheppard, J. Chem. Sot. Chem. Connnun. (1979) 18. [8] R. Biswas and D.R. Hamann, Phys. Rev. Letters 56 (1986) 2291. [9] P.J. Feibelmann and DR. Hamann, J. Vacuum Sci. Technol. A 5 (1987) 424. [lo] P.J. Feibelmann and D.R. Hamann, Surface. Sci. 182 (1987) 411. [ll] G. Comsa and R. David, Surface Sci. 117 (1982) 77. [12] C.S. Alexander and J. Pritchard, J. Chem. See. Faraday Trans. I, 68 (1972) 202. [13] F. Greuter and E.W. Plurntner, Solid State Comrnun. 48 (1983) 37. 1141 J. Pritchard, T. Catterick and RK Gupta, Surface Sci. 53 (1983) 1. [15] M.A. Chesters, J. Electron, Spectrosc. Related Phenomena 38 (1986) 123. [16] B.A. Sexton, J. Vacuum Sci. Technol. 16 (1979) 1033. [17] K. Christman, R.J. Behm, G. Ertl, M.A. Van Hove and W.H. Weinberg, J. Chem. Phys. 70 (1979) 4168. [18] T.E. Felter, S.M. Foiles, M.S. Daw and R.H. Stulen, Surface Sci. 171 (1986) L379. [19] Jihwa Lee, J.P. Cowin and L. Wharton, Surface Sci. 130 (1983) 1. [20] Y.J. Chabal, Phys. Rev. Letters 55 (1985) 845. [21] D.M. Newns, in: Vibrational Spectroscopy of Adsorbates, Ed. R.F. Wilhs, Vol. 15 of Springer Series in Chemical Physics (Springer, Berlin, 1980) p. 7. [22] N.D.S. Canning and MA. Chesters, J. Electron Spectrosc. Related Phenomena 29 (1983) 69. 1231 H. Ibach and D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations (Academic Press, New York, 1982). [24] J.E. Reutt, Y.J. Chabal and S.B. Christman, J. Electron Spectrosc. Related Phenomena 44 (1987) 325.