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...