Surface Normal Modes

The surface normal modes are a fundamental property of any surface, characterizing the motion of either electrons or atoms. Surely, the best known surface normal mode is the "surface plasmon", having been investigated for over three decades. Ironically, the ordinary properties of the surface plasmon are given by the bulk dielectric response, not by the surface properties. The surface plasmon occurs when varepsilon = -1, which in the Drude model gives an energy omega _{rm sp} = omega _{rm p}/sqrt {2}, where omega_ {rm p} is the bulk plasmon energy. The dispersion of the surface plasmon depends upon the detailed nature of the charge distribution at the surface, and in 1973 Feibelman predicted that the dispersion for simple metals was negative. Theoretical calculations have also indicated that there should be other normal modes in the electronic response at the surface. These new modes, called the multipole modes, have been the subject of considerable theoretical argument even though they have never been observed experimentally, before this study. This study utilized angle-resolved reflection inelastic electron scattering to study the normal modes at the surface of simple metals, like Na, K and Cs. These metals were chosen since they are the simplest of all metals and can be modeled by an interacting electron gas theory. A procedure was developed for growing smooth thick films in an ultra-high vacuum environment. The negative dispersion of the surface plasmon was verified experimentally and a simple physical picture was developed to explain the origin of the dispersion. The existence of the multipole mode was confirmed experimentally and relationship between this mode and the resonance seen in surface photoelectric effect established. The energy and dispersion of both of these modes are in agreement with electron gas calculations. This experimental technique was used to study the dispersion of the surface plasmon on a noble metal, silver and to investigate the development of the surface plasmon in simple metals as a function of film thickness.