Surface plasmon satellites in the XPS core level spectra of physisorbed species: fiction or reality?

Abstract

We analyze the possibility of detecting experimentally the character of the mechanism of screening of core holes in photoionized adsorbates weakly bound to the substrate surface. On noticing that both the image-like (extra-adsorbate) and charge transfer (intra-adsorbate) screening models have been rather successful in describing the spectral features of the XPS core level spectra of the prototype weak chemisorption system CO/Cu, we conclude that this type of system cannot provide a clear cut answer as to the character of screening of adsorbate core holes. On the other hand, by selecting systems in which, first, the substrate surface electronic response may exhibit pronouncedly distinct features, such as surface plasmons, and second, the adsorbate valence orbitals would remain “passive” as regards the charge transfer effects, we arrive at the conclusion that the XPS core level spectra of Ar atoms physisorbed on Ag or Al surfaces can provide experimental evidence for the operation of the image-like or extra-adsorbate screening mechanism. These conjectures are completed by calculations of the corresponding core level spectra and a discussion of the physical conditions for detection of the extra-adsorbate screening effects.