Surface-plasmon dispersion in simple metals

Abstract
Angle-resolved reflection inelastic electron scattering has been used to measure the dispersion of the surface plasmon for thick films of Na and K. The measured dispersion is negative at small momentum parallel to the surface as predicted by quantum-mechanical calculations of the dynamic response of the electrons at the surface of an interacting electron gas. A simple physical picture is presented to explain the origin of the negative surface-plasmon dispersion in simple metals.