Charge transfer processes and sputtering processes of self-sustaining alkali metal coatings on metal surfaces

Abstract

It is well known that adsorption of electronegative or electropositive atoms on a metal surface results in charge transfer between the metal conduction band and the adsorbate atom. This charge transfer results in the formation of a coverage-dependent image dipole and a consequent change in the work function and adsorbate binding energy. Additionally, the secondary electron yield and the charge state of surface atoms desorbed by thermal, electron impact or ion impact desorption processes are closely related to the effective adsorbate charge. We discuss here the effect of Gibbsian segregation as a means of producing similar effects for electropositive (alkali metal) layers on a number of metal substrates. Classical chemisorption theory is used as a means of estimating the adsorbate effective charge and the results are compared with models used to predict the secondary ion fraction of sputtered atoms. The resulting concept of self-sustaining sputter-resistant coatings is analyzed in the context of applications to the fusion environment as well as to, e.g., materials for high current electrical contacts.