SURFACE-PLASMON SATELLITES IN THE XPS CORE-LEVEL SPECTRA OF PHYSISORBED SPECIES - FICTION OR REALITY

We analyze the possibility of detecting experimentally the character o f 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) screenin g models have been rather successful in describing the spectral featur es of the XPS core level spectra of the prototype weak chemisorption s ystem CO/Cu, we conclude that this type of system cannot provide a cle ar cut answer as to the character of screening of adsorbate core holes . On the other hand, by selecting systems in which, first, the substra te surface electronic response may exhibit pronouncedly distinct featu res, such as surface plasmons, and second, the adsorbate valence orbit als would remain ''passive'' as regards the charge transfer effects, w e arrive at the conclusion that the XPS core level spectra of Ar atoms physisorbed on Ag or Al surfaces can provide experimental evidence fo r the operation of the image-like or extra-adsorbate screening mechani sm. These conjectures are completed by calculations of the correspondi ng core level spectra and a discussion of the physical conditions for detection of the extra-adsorbate screening effects.