Theory of Surface Collective Modes in Semi-Infinite Plasmas

Abstract

The test-charge problem in a semi-infinite plasma is formulated by taking appropriate account of the electrodynamic boundary conditions in the interface between a plasma and a dielectric medium. The general expression for the dispersion relation of the surface collective mode is thereby obtained. The properties of the surface collective modes associated with the electron plasma oscillations and the ion-acoustic waves are investigated; the criterion for the instability of the ion-acoustic surface mode is analyzed and compared with that of the corresponding volume mode. It is remarked that the field fluctuations observed outside the plasma domain should clearly convey the collective character of the surface mode, while the volume mode loses such a feature because of the screening action of the surface.