Two-Dimensional Plasma Resonances in Positive Ions under the Surface of Liquid Helium

Plasma resonances have been observed in a two-dimensional sheet of positive ions held just under the surface of liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. The ion effective mass deduced from the resonant frequencies is found to be weakly temperature dependent, increasing linearly with temperature. Strong nonlinear effects are observed in the wave propagation. The ponderomotive force of the plasma wave induces a static $2k$ distortion of the charge density, which produces large shifts of the resonant frequency with drive amplitude.