Near-threshold enhancement in photoemission from alkali metal overlayers

The total surface photoabsorption and the emitted part of it are calculated for thin Cs layers adsorbed on a semi-infinite metallic substrate. The jellium model is used to evaluate the electronic properties of this system and the dynamical response is treated within the time-dependent density functional approach. It is shown that the self-consistently screened electronic excitations lead to an enhancement of the p-polarized emission current in the vicinity of the threshold. Photoemission measurements for Cs overlayers on Ag are carried out at various coverages. These spectra indeed show a striking increase of the p-polarized intensity within about 1 eV above threshold.