Physical Review B
Two previously unappreciated features in photoemission spectra from alkali atoms adsorbed on W(110), namely, the sign of the alkali-induced surface-atom core-level shift of the substrate at low coverage and the very large alkali shallow core-hole lifetime width at all coverages, show that the alkali-substrate interaction is not well described by a transfer of alkali charge. Instead, both features point to the formation of a charge cloud between the alkali adatom and substrate that is derived largely from alkali valence states.
"Nature of the Charge Localized Between Alkali Adatoms and Metal Substrates," G. K. Wertheim, D. M. Riffe, and P. H. Citrin, Phys. Rev. B 49, 4834 (1994).