Adsorption studies by ultraviolet photoelectron spectroscopy

Surface Science 47 (1975) 115-123 0 North-Holland Publishing Company

ADSORPTION STUDlES BY ULTRAVIOLET

PHOTOELECTRON

SPECTROSCOPY

B. FEUERBACHER Surface Physics Group, Astronomy Noordwijk, The Netherlands

Division, European Space Research Organisation,

The application of ultraviolet photoelectron spectroscopy (UPS) for adsorption studies is illustrated using three examples. The observation of difference curves for various gases adsorbed on the same substrate allows those features due to the substrate or the adsorbate to be distinguished. The observation of the coverage dependence of UPS difference spectra allows one to relate the observed features to results from other techniques, as shown for oxygen adsorbed on the (100) face of tungsten. Finally, the measurement of difference spectra of the same species adsorbed on different faces of the same crystal gives information on the substate-adsorbate bonding as illustrated for the case of hydrogen on the (100) and (110) faces of tungsten.

1. Introduction Ultraviolet photoelectron spectroscopy (UPS) has very rapidly become a widespread tool for the investigation of gas adsorption on metal surfaces. The simplicity of such measurements stands in contrast to the serious difficulties that lie in the theoretical interpretation of the results. In this paper three examples of UPS experiments are presented that differ by the experimental parameter varied during the measurements. The various conclusions that may be drawn from the results of such experiments are discussed as well as their inherent limitations. Table 1 gives a summary of these experiments and the information that may be obtained from their results. Most experiments on photoemission from adsorbate covered surfaces have been performed so far by the detection of all electrons emitted into the total half space. On the other hand it has been shown theoretically that angular resolved measurements should provide information on the spatial location of the adsorbate atoms [ 1,2]. The measurements presented here are restricted to electrons emitted normal to the surface under observation. Those electrons have small k-vector components parallel to the surface and therefore sample large-scale properties characteristic of the emitting surface. Measurements on clean tungsten single crystal faces [3] have shown that spectra taken in this way carry information on the one-dimensional density of states near the surface [4]. This view is endorsed by a comparison with

Table 1 Three diffcrcnt I*.xperimental Adsorbate (‘ovcragc Crystal fact

experiments parameter

in UPS spectroscopy ~___ Variation

of parameter

gives information -_

on

Inllucnce of substrate and/or adsorbatc; “fingerprint” Relation to IISD, 7‘11, LIXD, various adsorbate states Influence of surface orientation and substrate bonds

field-emission data [S], where a theoretical interpretation has been given in terms of the density of states near the surface 161, heavily weighted for small parallel components of the k-vector. So photoelectric emission normal to a single crystal surface appears to be a powerful tool to investigate changes in the density of states near the surface induced by adsorbate levels [7].

2. Variation of adsorbate species Measurements of various adsorbates on the same single crystal face will provide information on how much the photoemission spectra reflect properties of the substrate or the adsorbate. UPS spectra are usually presented in the form of difference spectra, where the spectrum of the clean substrate has been subtracted from the spectrum obtained after adsorption. The influence of the substrate is not necessarily cancelled out in such a difference spectrum because, although the bulk properties will not be altered by adsorption of a monolayer, the photoemission following bulk excitation processes might well undergo changes due to the adsorbate. The surface density of states will be different after adsorption. For example, adsorbate resonances may be built up at the expense of depletion of high state densities originally present at the substrate surface. Fig. 1 shows UPS spectra [8] taken normal to a (100) face of a tungsten single crystal for four different adsorbates, namely CO, N,, O,, and Hz. All measurements have been taker1 at 21.2 eV photon energy and saturation coverage. Each panel shows the spectrum of the clean surface as a dashed line. After adsorption, the spectrum changes into that given by the solid line. The corresponding difference spectra are shown by the solid curve in the lower part of the four frames. A number of common features are apparent in the difference curves which must be related to the substrate. Indeed all common features correlate with structure in the spectrum of the clean surface, and an assignment to certain physical processes is possible following an earlier analysis [3]. The clean spectra exhibit two peaks that have been assigned [3] to emission following bulk direct optical transitions, located at - 1.5 and - 6.7 eV. Both these structures appear as dips in all four difference curves, indicating that bulk photoemission is attenuated after adsorption. A much stronger influence on the difference curves is found from structure in the clean spectra

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117

212cVi/

E BELOW

EF (tb’)

(W+H2)-W EBELOWQ(eV)

Fig. 1. UPS spectra of four gases adsorbed on the (100) face of tungsten, measured at 21.2 eV photon energy. In each frame the dashed line represents the photoemission spectrum of the clean surface, which changes into the upper full fine after adsorption to saturation coverage. The lower full line gives the difference spectrum, same spectrum taken at 10.2 eV photon energy.

with the dash-dotted

line showing

part of the

that arises due to surface emission. The peak in the dashed curves near - 0~4 eV has been assigned (9- 1 l] to surface emission from a surface resonance. This resonance is removed by submonolayer coverages of adsorbates, leading to the very pronounced dip in the difference spectra. The clean-spectrum peak at -4.5 eV is due to surface emission from a high density of states near the surface, which is apparently depleted for all four gases investigated here. The resulting dip in the difference curves gives in some cases rise to a Fano-type line shape (resonance-antiresonance sequence, see difference curves for N2 and 02 in fig. 1) that has been predicted by Penn 271. HOWever, in the present case this agreement is fortuitous, since the dip near - 4.5 eV appears for all adsorbates on the (100) surface of tungsten. Another feature characteristic of the substrate rather than the adsorbate is the peak-dip sequence observed near - 13 eV in all difference curves. This structure is due to a gap in the one-dimensional conduction band density of states, which is reflected in the distribution of ineIastically scattered electrons. Since this structure is derived from the final density of states, it will change its position relative to the Fermi level as the exciting photon energy is altered.

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Clearly the spectra in fig. 1 also carry a lot of information on the adsorbate. The spectra of hydrogen and nitrogen adsorption exhibit well developed resonance levels at - 2.5 and - 6 eV, respectively. The hydrogen resonance is surprisingly sharp with a width of less than 1 eV. A whole series of peaks is observed for oxygen and carbon monoxide adsorption. The latter spectrum much resembles the results of Baker and Eastman [ 131, except for the small dip in the present spectra near 6.5 eV, which has been related to the substrate and appears only in the narrow-angle measurement. No attempt is made here to relate the observed structures to adsorbate orbitals. The difficulty involved in such a procedure is obvious. For the case of CO, only the structure at - 9 eV has been assigned to an intrinsic peak due to CO adsorbed in the a-state [ 131. For the rest of the difference spectrum it is difficult to say whether there is one broad adsorbate induced level, extending from ~ 2 eV to - 8 eV, interrupted by substrate features, or whether there are essentially five peaks of about 1 eV halfwidth. This ambiguity may be partly resolved by measuring at various photon energies or at energies sufficiently high so final-state features will not be strong any more. However, features derived from a change in the substrate surface density of states are of course part of the information one is looking for in such systems. If those features have about the same magnitude as intrinsic adsorbate features, an interpretation in terms of adsorbate orbitals becomes questionable.

3. Variation of the adsorbate coverage Measurements of the photoemission spectra as a function of adsorbate coverage are especially valuable since they link the UPS results to information obtained from other techniques that observe surface properties as a function of coverage, such as thermal desorption, electron stimulated desorption, or work function measurements. An example of such an experiment is presented in fig. 2 for the oxygen-tungsten (100) system [8]. These spectra have been taken at 10.2 eV photon energy and therefore cover only part of the range of the corresponding curves in fig. 1. The lowest curve in fig. 2 shows the energy distribution of electrons emitted normal to the (100) face of tungsten. The upper curves present difference spectra taken at various exposures increasing from top to bottom and displaced vertically such that the exposure may be read from the right-hand scale. Fractional converages have been calculated from the exposure using the data of Madey [ 141 and are indicated on the left hand side of fig. 2. The arrows give the position of the work-function cutoff, decreasing the accessible range of binding energies with increasing coverage. The negative features in the difference curves are essentially the same as those discussed in section 2. A number of positive features are observed that vary strongly with coverage. For very low exposures a narrow peak very close to the Fermi energy is observed. It is conjectured that this is part of a broader resonance level located just above the Fermi energy which is only partly visible due to the Fermi cutoff. This feature could be characteristic of adsorption of single atoms, i.e. the case where ad-

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119

0.01

-6

-4

-2

E BELOW

EF(eV)

0

Fig. 2. Photoelectron difference spectra for oxygen adsorbed on the (100) face of tungsten for various exposures, measured at 10.2 eV photon energy. The bottom curve gives the spectrum of the clean surface. The exposure increases from top to bottom for the difference curves (righthand scale). A calculated fractional coverage is given on the left side for each curve. The arrows mark the work-function cutoff.

sorbate-substrate interaction outweighs adsorbate-adsorbate interaction. For higher coverages, up to 0 = 0.5 (half monolayer), two peaks 1 and 2 are observed. As the coverage is increased to 0.75, peak 2 suddenly disappears, while peak 1 is found at a constant position up to saturation coverage. The results of measurements performed by other techniques on the same system [02 on W (loo)] are shown in fig. 3. The curve marked ESD presents the O+ ion I

0

0.2

04

0.6 COVERAGE

0.8

1.0

MONOLAYERS

Fig. 3. Electron-stimulated desorption (ESD) and thermal desorption (TD) yield for oxygen adsorbed on the (100) face of tungsten as a function of adsorbate coverage. ESD gives the O+ yield following ref. 14, TD shows the atomic oxygen (0) and oxide (WO,) yield as reported in ref. 15.

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yield on electron stimulated desorption as observed by Madey [ 131. The yield increases monotonically up to about 0.5 monolayers, than decreases to a minimum which is followed by a steep increase. The two curves marked TD show the results of thermal desorption measurements of Ptushinskii and Chuikov [ 15 J The yield for oxygen (upper curve) saturates at about half a monolayer coverage. At the same coverage oxides (WO,, WO,) are observed in the desorption products. All three experiments (UPS, TD and ESD) demonstrate that a profound change in the adsorbate layer takes place at about a half monolayer coverage. Some conclusions may be drawn on the structure observed in the spectra of fig. 2. Peak 1 may be associated with atomic oxygen. According to fig. 3, oxygen desorbs thermally with increasing yield up to about 0 = 0.5 and with constant yield for higher coverages. This is approximately the behaviour observed for peak 1 (fig. 2). This structure, which is also observed for adsorption of carbon monoxide on the same substrate [8], is therefore assigned to atomic oxygen. On the other hand, the disappearance of peak 2 at half monolayer coverage may tentatively be associated with the formation of surface oxides.

4. Variation of crystal face Measurements of the difference spectra due to the same adsorbate on different faces of the same crystal may shed light on the substrate-adsorbate bonding and the role of the various substrate orbitals. An example of such a measurement is presented in fig. 4, which shows difference curves for hydrogen on the (100) and the ( 110) face of tungsten for various exposures [ 161. The figures are drawn in the same way as fig. 3. Coverages corresponding to the individual difference curves are indicated on the left hand side of each trace as calculated using the data of Tamm and Schmidt [ 171, The correlation of these spectra to measurements made by other techniques has been elaborated elsewhere [ 161, so the present discussion will focus on the differences introduced by the two crystal faces. The difference curves for hydrogen on the (100) face of tungsten show two low-coverage states I and 2. A broad hump centered at about - 3 eV developes for increasing coverages. This hump is strongest as hydrogen is saturated in the & state, which is characterized by a c (2 X 2) superstructure due to an ordered half monolayer of atomic hydrogen. As the coverage is increased further, the broad hump develops into a sharp structure at - 2.5 eV which is only about 0.5 eV wide. This is the same peak as seen in the hydrogen spectra taken at 2 1.2 eV (fig. 1). The difference spectra of hydrogen on the (110) face show a broad low-coverage state marked 1 in the right part of fig. 4. This feature is strongest for a saturated half monolayer of atomic hydrogen, as characteristic for the pZ state. As the coverage is further increased, a new peak 2 emerges near -4 eV. This structure coexists with peak 1 at high coverages up to saturation.

B. FeuerbacherlAdsorption

-6

-4 E BELOW

-2 EF(&‘)

0

121

studies by UPS

-6

4 E BELOW

-2 EF (eV)

0

Fig. 4. Adsorption of hydrogen on two different single crystal faces of tungsten observed by UPS. The bottom curves represent the photoemission spectra taken normal to the (100) and (110) face. The upper curves are difference spectra for increasing coverage from top to bottom (right-hand scale). Calculated coverages are indicated on the left side of each figure.

The adsorption kinetics of hydrogen on the principal faces of tungsten have been studied thoroughly by Tamm and Schmidt [ 17- 191. The results indicate that hydrogen adsorption on the (100) and the (110) faces exhibits some similarities. On both surfaces hydrogen is adsorbed in atomic form [20]. Two binding states fil and f12 with different binding energies may be distinguished on both faces. The values for the binding energies are about the same for the (100) and the (110) face. The fi2 state, which has the higher binding energy, is populated first and saturates at a coverage that corresponds to half an atomic monolayer. It is characterized by a superstructure observable in LEED [21] or RHEED [22] for both surfaces. At saturation hydrogen forms a mono-atomic layer on both crystal faces in the p1 state. The two measurements shown in fig. 4 also exhibit some similarities. Both spectra show a relatively broad resonance level characteristic for the f12 state with its high binding energy. The lower binding energy of the p1 state is reflected in a narrower adsorbate level for both crystal faces. However, there is a fundamental difference between the two spectra. For the (100) face the low-coverage state merges into the high-coverage state for increasing coverage and only one state is observed at saturation (see also fig. 1). On the (110) face, the two adsorbate-induced resonances develop sequentially and coexist at high coverages. It is not easy to understand this result in a simple model. Theoretical investigations have been performed on the interaction of adatoms on surfaces [23]. Calculations for hydrogen adsorbed on the (100) surface of a metal [24,25] predict the formation of a superstructure at half monolayer coverage due to an oscillatory behaviour in the interaction potential. However, theory is not yet in a state to account for differences in the interaction potential on a (100) and ( 110) surface. Qualitative

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differences on the two surfaces, however, are evident. On the (100) crystal face the hydrogen is bonded in a site of four-fold symmetry [26] in the f12 state. This is either the position above a surface atom or a centered bridge position. A nearest neighbour repulsive interaction therefore gives rise to a c (2 X 2) superstructure. At full monolayer coverage all adatoms are bound in the same way, resulting in a (1 X 1) LEED pattern. The (110) surface, on the other hand, offers only adsorption sites of twofold symmetry. It may be expected that a similar interaction potential is responsible for the (2 X 1) superstructure observed for half-monolayer coverage on this crystal face. This potential may now be different for the (100) and (11 1) azimuth, giving rise to two unequivalent adsorption sites at full coverage that might be responsible for the observed coexisting adsorbate levels.

5. Conclusions The use of ultraviolet photoelectron spectroscopy has been discussed using three examples of experiments that allow different conclusions on chemisorption systems. The observation of photoelectron difference spectra of various gases on the same crystal surface allows of a distinction between substrate and adsorbate features. This has been shown using the examples of oxygen, hydrogen, nitrogen and carbon monoxide on a tungsten (100) face. Some gases (Hz, Nz) exhibit strong resonance-like peaks that allow easy “fingerprint” identification, while for other gases like 0, and CO the observed structures in the resonance spectra are broad and relatively featureless, so an interpretation in terms of resonance levels becomes doubtful. Measurements of difference spectra as a function of coverage allows UPS data to be related to results of other techniques such as thermal desorption, electron-stimulated desorption, work function measurements or LEED observations. An example is chosen of oxygen on the ( 100) face of tungsten. The UPS spectra reveal the sudden disappearance of a peak at a coverage of 0.5 monolayer. Comparison with thermal desorption and ESD results shows that other techniques also show a distinct change in surface properties at this coverage. The comparison allowed the assignment of one of the peaks in the difference spectra to atomic oxygen, while the disappearance of the other peak was related to the formation of surface oxides. Interesting information on substrate-adsorbate bonding is obtained by comparing the UPS spectra of different crystal faces adsorbing the same species. Considerable differences are observed in hydrogen adsorption on the (100) and (1 IO) face of tungsten, indicating directionality in the bonds responsible for chemisorption. Both faces show two adsorption states filling consecutively with coverage. On the (100) face the low-coverage state merges into the high-coverage state, so only one adsorbate level is observed at saturation, while two states appear to coexist at high coverages on the (I 10) face.

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