Surface collective excitations in reduced dimensionality

The collective excitations in simple metal surfaces have been extensively studied and well understood now. However, the effects of lattice potential and the complications caused by reduced dimensionality on the surface collective excitations are still challenging subjects in both theory and experiment. Here we present the experimental studies of these problems and comparison between experiment and theory. The surface plasmon dispersion and damping for thick Hg films have been measured using momentum resolved inelastic electron spectroscopy. Hg has been investigated to study the effects of the crystal potential on the surface plasmon dispersion, and to correlate the measurements on a surface with those for collective excitations in clusters. The crystal potential effects are shown to lead to an overall lowering of the plasma frequency, to a flattening of the momentum dispersion, and to a sizable broadening of the plasmon line width relative to the behavior obtained for the simple metals. Even though the band structure effects in Hg are significant, there is a quantitative correlation between the normalized energy and width of the surface plasmon as a function of [special characters omitted] and the Mie resonance in Hg clusters as a function the inverse of the radius R. The surface collective modes of thin K films on Al(111) have been investigated using photoyield measurements and inelastic electron scattering. Theoretical calculations have predicted a richer set of features in the thin films than for the semi-infinite metal surfaces because there are the interference between two interfaces (substrate-film and film-vacuum) and heavy damping on the substrate. The use of an optical probe and electron scattering has allowed us to draw a more complete picture of the dynamic screening in thin films. The number, dispersion, damping and optical activity of the collective modes of the thin films have been measured as a function of K film thickness. At [special characters omitted] = 0, the bulk-like overlayer plasmon and the multipole mode were observed. At finite [special characters omitted], these modes undergo a transition towards the K multipole and monopole surface plasmons. With increasing coverage, the overlayer excitations turn into the collective modes of K. The experimental results confirm the theoretical predictions