2-DIMENSIONAL ION-PLASMA RESONANCES UNDER THE SURFACE OF LIQUID-HELIUM

The liquid-helium surface is used to confine a two-dimensional plasma of positive helium ions, at a depth under the surface of approximately 300. Plasma-wave resonances are excited and measured over the temperature range from 14 to 400 mK, at ion densities up to 1×108/cm2. The ion effective mass determined from the resonant frequency is measured as a function of temperature, and the limiting T=0 value is found to be m*=301 helium masses. The resonant linewidth measures the ion mobility, which reflects the scattering of the ion from elementary excitations near the helium surface. A change is observed from phonon scattering above 0.2 K to ripplon scattering below that temperature. In the ripplon regime the mobility decreases rapidly as the electric field perpendicular to the surface is increased. The Wigner crystallization transition has been searched for, but no change in the plasma-wave characteristics has been observed over the range 20,000, where is the ratio of Coulomb energy to kinetic energy. © 1990 The American Physical Society.