Electronic excitations in thin alkali-metal layers adsorbed on metal surfaces

Abstract

The dynamical linear-response properties of realistic Na and K layers adsorbed on a semi-infinite jellium substrate corresponding to the electron density of Al are studied with the aim of elucidating the nature of the adlayer electronic excitations and their variation with coverage. The ground-state properties are described by a first-principles method and the dynamical response in the long-wavelength limit is treated within the time-dependent density-functional approach. At coverages near the work-function minimum, the adsorbate-induced excitations are dominated by intra-atomic excitations between adatom resonant states. Nevertheless, these atomiclike transitions do not lead to any spectral features in the electron-energy-loss function because of the strong hybridization between adatom and substrate states. Instead, as a result of surface screening processes and matrix element effects, a broad loss peak appears near the threshold for emission. This mechanism explains the frequently observed correlation between the coverage dependence of the work function and that of the energy loss induced by the alkali-metal adlayer. As the coverage is increased to one monolayer, the threshold mechanism is replaced by collective excitations consisting of heavily broadened volume plasmons and multipole surface plasmons of the alkali-metal adsorbate. At double-layer coverage, these two modes become very sharp and can clearly be resolved. These collective excitations show only small influence due to the lattice structure of the alkali-metal adlayer.