Physical mechanisms responsible for core-level shifts of alkali metals adsorbed on Si(111)

The origin of the core-level shift of the Si 2p level of surface atoms and that of the (n - 1)s level of adsorbate metal atoms has been investigated by means of all-electron ab initio Hartree-Fock cluster model wave-functions. The calculated initial-state core-level shifts for both levels is much smaller than the one expected from an ionic bond where the interacting systems are charged, and seem to indicate that these results are not compatible with the formation of an ionic bond. Nevertheless, the main trends are in agreement with the experimental findings. However, a careful analysis carried out by means of the CSOV method permits an understanding of the origin of these small calculated (or experimental) core-level shifts and allows us to identify the physical mechanisms responsible for this behavior.