Abstract: The application of UPS to the study of chemisorption
Authors: Demuth J. E. , Eastman D. E. 
Published: July 2002

Ultraviolet photoemission spectroscopy (UPS) provides a means of measuring the energy levels of the occupied valence electrons for both the adsorbate and substrate over a wide range of energy. Since chemical bonding directly involves interactions of valence electrons, UPS is a useful tool to study chemisorption. Other techniques, for example, field emission,1 ion neutralization spectroscopy,2 and x-ray photoelectron spectroscopy3,4 (not discussed here) also provide information about occupied valence states. He (21.2 and 40.8 eV) and Ne (16.8 and 28.6 eV) resonance lamps are convenient and the most commonly used radiation sources in UPS chemisorption studies; whereas monochromatized synchrotron radiation is the most desirable source in that it provides stable, continuously tunable, linearly polarized radiation from the visible region up into the x-ray region. In UPS studies, both the orbital ionization energies and line shapes contained in photoemission spectra, as well as their angular and frequency dependence, provide information regarding chemisorption.

The adsorption of atoms or molecules onto a surface induces changes in the UPS spectra which can be interpreted on several levels. The simplest level consists of using the adsorption-induced features as a fingerprint of the chemical state of the adsorbed species. That is, while the nature of the adsorbed species may not be fully understood, distinctions between various chemical phases can be inferred from their respective characteristic ionization features, e.g., the various phases of H or O on W.1,4

A second level of application of UPS consists of identifying the chemical nature of an adsorbed specie (and implicitly the nature of bonding) from its adsorption-induced ionization features. Careful considerations of the differences in the chemisorption spectra from its gas or condensed phase counterpart(s) —including chemical bonding effects as well as initial-state screening and final-state relaxation effects—can result in an identification of the chemical nature (or changes therein) of the adsorbed specie (e.g., the dehydrogenation of C2H4 to C2H2 on Ni5,6). Here the adsorbate (and/or substrate) orbital levels which are shifted and broadened relative to the gaseous or condensed phase (e.g., the π orbitals of chemisorbed C2H2, C2H4, and C6H6 on Ni5,6) can provide immediate information as to which orbitals (including substrate valence bands) are predominantly involved in bonding. Frequency-dependent UPS studies can also aid in the determination of orbital character, especially in cases where ambiguities in relating ionization features to the ’’free’’ adsorbate exist, e.g., CO/Ni.6,7 In some cases, frequency-dependent studies may not be expected to be applicable to adsorbed species whose wave functions are significantly perturbed or admixed with the substrate upon chemisorption. Such interactions can significantly change the frequency dependence of the adsorbed specie from that expected of the free molecule. The most difficult level of interpretation of UPS spectra is to quantitatively determine valence orbital energies, to obtain interaction strengths and bond energies, and to better understand the wave functions of the chemisorption system. In UPS chemisorption studies, when localized electrons are involved (such as in chemisorbed molecules), the many-electron nature of the photoemission process is apparent: all N−1 final-state electrons are strongly perturbed by the removal of an electron of the N electron ground state. Thus, both the initial N electron state and final N−1 electron states are involved and must be considered. Although many-electron effects—including relaxation, shakeup, shakeoff, configuration interaction, etc.8—have been identified and are at least partly understood for the gas and bulk solid phases, their counterparts in the case of chemisorption can be more complex (e.g., due to undetermined chemical changes) and are not yet understood. Thus for chemisorption, a one-electron picture is usually involed and final-state corrections are made. In applying a one-electron picture to interpret UPS spectra, one must consider chemisorption-induced initial-state changes in ionization features due to bonding, charge transfer, and screening of the adsorbate orbitals by the substrate. In addition, final-state effects arise from image-charge screening of the optically excited hole on the adsorbate by the substrate (and nearby adsorbates), which cause additional extra-adsorbate ’’relaxation’’ (as well as possible small modifications in intra-adsorbate ’’relaxation’’10). Initial- and final-state effects are both related to chemical bonding (to varying degrees) and reflect the strengths of the interactions on the various valence orbitals. Thus, it is often possible (except for atomic species or strong bonding involving strong substrate admixing) to directly observe the effects of the chemisorption interaction manifested through changes in ionization features from the free adsorbate (providing the surface specie is not a radical and is stable in the gas phase) and thereby identify the orbitals primarily involved in bonding. Attempts to separate bonding shifts from screening and relaxation shifts in any detail raises immediate questions (which are as yet unanswered in general), since in principle screening and relaxation shifts can be different for different orbitals (and depend on the adsorbate bonding geometry, which is generally not known). In cases where the adsorbate lies flat on the surface, e.g., C2H2 which π bonds,5 such a separation of uniform screening and relaxation shifts from bonding shifts becomes more plausible (due to the geometric dependence of image-charge screening9) than in other cases where the adsorbate stands end-on to the surface, e.g., as believed to be the case for CO chemisorption.6,7 In obtaining a bonding shift from the change in orbital energy position from the free adsorbate, the shift in orbital energies resulting from internal bonding changes caused by the primary chemisorption bond must also be considered. For example, C2H2 and C2H4 chemisorption may result in weakening and expansion of the C–C bond of the adsorbate due to charge redistribution associated with d to π backbonding component in π–d bonding.

Angle-resolved UPS measurements provide more information than is obtainable with angle-integrated UPS, but with the added complexity that such measurements are often more difficult to interpret. Angular UPS measurements can be used to detect or enhance various chemisorption or surface features, e.g., CO on Ni(111)6 or H on W(100),4 and Si surface states,11 since various orbitals will have hν-dependent emission maxima in different directions. A detailed understanding of angular photoemission distributions should ultimately provide information regarding the orientation of the adsorbate and its orbitals in the ground state relative to the surface,12,13 although matrix-element effects and final-state effects must be understood and ’’separated’’ to do so. (Frequency-dependent UPS measurements will likely be necessary to sort out final-state interference effects.) A noteworthy limitation of UPS for chemisorption studies is that it provides only ’’half’’ the available spectroscopic information in the sense that it probes only the occupied orbitals. In many cases it would be desirable to have additional information concerning the low-lying unoccupied orbital energies, e.g., optical transition-state energies. As an example, for chemisorption of many molecules (e.g. C2H2 or CO) on transition metals, the interaction between the first unoccupied level and the occupied substrate d electrons contributes the largest component to the heat of adsorption.5,6 Since it is difficult and many times ambiguous to interpret substrate d-band changes induced by this interaction, it would be useful to examine the chemical shifts of these unoccupied adsorbate levels. Thus, in chemisorption studies, surface reflectance spectroscopy (SRS),14 electron-energy-loss spectroscopy (ELS),15 and yield spectroscopy16 applied to chemisorption provide information complimentary to conventional UPS.

Theoretical models which provide the ground-state orbital energies of chemisorbed species on a jellium surface,17 for a cluster of atoms,18–22 or a single-crystal surface23 have been used to examine chemisorption bonding. However, the comparison of many model calculations to experimental UPS spectra can be limited both by the sensitivity of orbital energies to bonding interactions and by the difficulties in taking account of many-electron screening and relaxation effects (for adsorbates with localized wave functions). For example, SCF Xα calculations24 can take at least partial account of the N−1 final state, but (to date) contain limiting approximations such as the use of spherical potentials and small clusters. Nonetheless, useful descriptions of UPS spectra have been made for two different calculations of CO chemisorbed in Ni.22,24 Phenomenological models have also been used to interpret UPS spectra25–27 and, although only semiquantitative, can provide insight into the observed ionization spectra and the nature of the bonding for chemisorbed species. Caution must be used in applying such simple models as Mulliken’s acceptor–donor model25 to determine heats of adsorption from orbital energy shifts observed in UPS spectra (e.g., CO chemisorbed on various substrates), since UPS cannot provide information regarding unoccupied orbital interaction parameters. Also, as previously mentioned, the difficulties in separating primary bonding shifts from final-state relaxation shifts and initial-state ’’rehybridization’’ shifts limit the application of such models. Several recent UPS chemisorption studies which provide some new information (and raise interesting questions) regarding chemisorption on stepped surfaces, semiconductor surfaces, and noble-metal surfaces are now briefly mentioned. We have examined hydrocarbon chemisorption on a Ni-stepped surface ([n (111) × (100)], where we believe n to be ∠6 to 4, i.e., a [755] to [544] surface). For ethylene chemisorption at 100 K, we find a new dehydrogenated phase coexisting with chemisorbed ethylene. This dehydrogenated phase is characterized by a new level at 4.4 eV below EF. When the sample is warmed to T∠400 K, ethylene desorbs and leaves only this feature; the C2H4→C2H2+H2↑ (g) dehydrogenation reaction observed on Ni(111) is not observed. We speculate that this new feature may likely be chemisorbed carbon which results from the decomposition of C2H4 due to a stronger interaction at the steps than on the (111) surface.

The presence of steps appears to disrupt the reaction path for the ethylene-to-acetylene dehydrogenation reaction observed on Ni(111). Thus, our results on stepped nickel concur with the general conclusions of Somorjai28 that stepped surfaces are more ’’reactive’’ and form some type of nearly completely dehydrogenated ’’carbonaceous deposit,’’ which is believed to play an important role in catalytic reactions. We also observe that electron irradiation (∠1 μA, 150 eV for 2 min) of C2H2 or C2H4 on Ni(111) produces this same new phase; also, a 133-Pa s room temperature exposure of propylene to Ni(111) yields this same phase together with chemisorbed acetylene.

UPS studies show that formic acid exposure, as well as coadsorption of CO+H on the nonpolar ZnO(11̄00) surface, produces chemisorbed CO (plus atomic O for formic acid decomposition), whereas CO does not significantly adsorb on the clean surface.29 In contrast with CO adsorption on transition metals, the bonding shifts for chemisorbed CO appear to suggest that both 1π and 5σ molecular orbitals interact with the directed orbitals of the partially covalent semiconductor. The chemisorption of other simple adsorbates as well as larger organic molecules on these surfaces has also been studied.29

CO and hydrocarbon chemisorption on the noble metals Cu, Ag, and Au is also in progress to better understand the role of s and d electrons in chemisorption.30 CO chemisorbs readily at T∠78 K on polycrystalline Cu and Au for monolayer exposures, while it does not chemisorb on polycrystalline Ag (T∠78 K) even for pressures up to 6.7×10−3 Pa. The UPS spectra for CO on Cu and Au are similiar—a clearly resolvable four-peak structure in addition to strong preferential attenuation near the top of the substrate d bands. Since a very different three-peak structure is observed for CO on NI,6,7 Pd,7 and Pt,31 the bonding of CO to Cu or Au appears to be different. Such bonding differences are consistent with the known reduction in the heat of adsorption of CO on these noble metals from that on transition metals.32 Striking coverage-dependent changes in the UPS spectra for CO chemisorbed on Cu(100) below half-monolayer coverage are also observed which are not observed on polycrystalline Cu.30 Such changes might be associated with adsorbate–adsorbate interactions and, in addition to frequency dependence studies, prove useful in determining the orbital character of chemisorbed CO on Cu. We can also relate this phase change to an anomalous coverage dependence of the isoteric heat of adsorption for CO on Cu(100) observed by Tracy,33 which has not been observed for CO on Ni34 or Pd.35 Any bonding model proposed for CO on Cu must not only account for the differences in the UPS spectra for CO on Cu from that of CO on Ni, Pd, or Pt, but should also account for such coverage-dependent phase changes. A model of bonding to explain these unusual features of CO bonding to Cu (and Au) is discussed elsewhere.30

Thus, in summary, UPS can provide considerable insight into chemisorption even though there is much to learn about the many-electron effects which complicate detailed interpretations of UPS spectra. Continued development of angle-resolved and frequency-dependent UPS chemisorption studies, as well as correlations with SRS, ELS, or partial yield measurements, will certainly add to our knowledge of chemisorption bonding.