Abstract
An important contribution towards the well-known ionization of alkali atoms chemisorbed at low coverages on transition metal surfaces comes from the upward shift in the alkali valence level due to the image force. The Hartree-Fock approximation for this upward shift, in which the image of the adatom electron cloud is assumed static, may differ by up to a factor of two from that using the result that the self-image energy of an electron at a distance d from a metal surface is given by v=e2/4d. The resolution of this problem depends on the lifetime of an electron in the adatom orbital and the dielectric response of the metal surface. A simple model Hamiltonian is put forward which includes these effects, and which is solved in limiting cases. In particular it is found that the HF approximation should overestimate the upward shift in the case of a substrate such as tungsten. Possible implications for the screening of the Coulomb interaction between electrons in an adatom orbital are discussed.