Photoelectron spectroscopy of adsorption layers

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Abstract
The ionization potentials of a system can be derived from measurements of the kinetic energies of electrons liberated from that system as a consequence of the absorption of a light quantum, i.e., from photoelectron spectroscopy (PES). Such spectra give access to the electronic binding energies of the system. Therefore, soft X-ray-induced photoelectron spectroscopy (XPS, often called ESCA)1,2 and ultraviolet photoelectron spectroscopy (UPS)2,3 have long been used to study the electronic structure of atoms, molecules. and solids by direct study of the photoionization energies of valence electrons in both cases or by study of the intensities and shifts of core electron energies in the former. The application to surface studies under well-defined conditions, however, was delayed by the late realization of the inherent surface sensitivity of these methods and by the nonavailability of ultrahigh vacuum (UHV) spectrometers; only in the first years of this decade did a number of groups working in this field become aware of the potential to be utilized here. Following the early UPS4 and XPS5 investigations of adsorption on well-defined surfaces, an increasing number of papers have continued to appear in this area. PES has gained a certain prominence among the many methods used to characterize surfaces and adsorbed species. A critical assessment of this work shows that many interesting new insights into surface problems have been provided by PES. On the other hand, it should be noted that a number of important problems pertaining to the interpretation of such spectra have yet to be solved; therefore, the full information content of such spectra cannot (yet) be utilized.