Long range and temperature-dependent interaction between an alkali atom and a metallic surface; application to surface ionization

For explaining some long range and temperature dependent charge transfer involved in surface processes dealing with particles leaving a surface, we extend the usual chemisorption theory to larger distances. Treating the interaction of an alkali atom (lithium and sodium) on a metal surface (rhenium) in the chemisorption model, we introduce the temperature in the expression for the effective charges of the adsorbed atom. These effective charges are shown to be very sensitive to the temperature for atom-surface distances larger than 5 Bohr radii. The Coulomb repulsion effect between opposite spin electrons on the adsorbed particle allows us to describe the effective charges of both the positive and negative adsorbed ions. We apply our treatment to the positive surface ionization of thermal particles and give a new expression of the degree of ionization which asymptotically tends to the values of the Saha-Langmuir law. We found that the surface ionization process occurs at distances slightly decreasing with increasing temperatures, which are of the order of 13 Bohr radii for lithium on rhenium and of 15 Bohr radii for sodium on rhenium.