Plasma Modes of Low-Dimensional Electron Systems over a Helium Film

The dispersion laws for collective modes in low-dimensional electron systems over a superfluid helium film covering a metal substrate are evaluated within the many-body multisubband formalism. In contrast to the electron system over bulk helium, the longitudinal mode is acoustic-like. The transversal mode is optical-like with a gap given by the one-electron spectroscopic intersubband frequency. We found that the effect of depolarization shift is supressed in the quasi-twodimensional system.