Surface energy-loss function for the inelastic scattering of electrons from a metal substrate with an overlayer of adsorbed alkali-metal atoms

Abstract

We investigate the spectrum of elementary excitations of the conduction electrons associated with an overlayer of sodium atoms adsorbed on a metal surface. We compute the surface energy-loss function for the small-angle backscattering of electrons as function of coverage (up to two full monolayers of sodium atoms are considered). The ground-state problem is dealt with by using Lang’s uniform-background model of alkali-metal-atom–metal-surface chemisorption. The dynamical response of the electron gas is treated in the random-phase approximation. Previous theoretical work has, for the most part, assumed that an overlayer of atomic thickness responds as bulk matter. Our microscopic calculation reveals that for one-monolayer coverage the overlayer’s response in fact deviates significantly from bulklike behavior. For two-monolayer coverage the response is bulklike, with a prominent collective (plasmon) mode. The quantum-mechanical nature of the overlayer response is displayed in detail. A case is made for the need that an angle-resolved electron-energy-loss experiment be performed, using an s-p-bonded metal, such as aluminum, as the substrate (instead of a transition metal).