Electronic Raman scattering at disordered noble- and alkali-metal surfaces

Abstract

Electronic Raman spectra with continuous Stokes shifts between 250 cm⁻¹ and 3500 cm⁻¹ are observed from potassium and noble-metal films of thickness 150 nm grown on a smooth substrate at 40 K in ultrahigh vacuum. We investigate the irreversible transformation of the spectra during thermal annealing of the surface structure. In the case of potassium, the Raman yield shows a continuous decline upon annealing from 40 K to 300 K, roughly following the photoelectron yield. In the case of silver we observe a decrease in halfwidth by a factor of 2 and an increase of the peak intensity by a factor of 10 upon annealing to 250 K. This behaviour of silver is explained by intraband excitations of the electronic surface state at (111) facets. The observations are, in principal, in agreement with an evaluation of the transition phase space.