Work Function, Optical Absorption and Second Harmonic Generation from Alkali Atoms Adsorbed on Metal Surfaces.

We consider the electronic properties of a low coverage of alkali atoms adsorbed on a metal surface. We calculate the alkali induced changes in the work function, optical absorption and second harmonic generation within a simple but realistic model. The alkali-induced peaks typically observed at a few eV loss energy in inelastic electron scattering is explained as resulting from electronic transitions from the filled part of the ns- resonance to the empty part of the same resonance and to the np- resonance. We predict a strong alkali-induced increase in the low frequency lossy response of an alkali covered metal surface. This theoretical prediction is confirmed by new inelastic electron scattering data for the K/Cu(100) system. A strong increase in second harmonic generation (SHG) is predicted at low alkali concentration (laser photon energy h omega ~ 1.16 eV). The dependence of the SHG signal on the coverage (at low coverage) and on the incident photon energy is in rough agreement with experimental data. Finally, we discuss the temperature dependence of the work function and of the local fluctuating electric field which exists in the adsorbate system. Experimental results for the temperature dependence of the workfunction for the K/Cu(100) system are presented and are found to be in good agreement with the theoretical predictions.