Experiments on ions trapped below the surface of superfluid4He

Abstract

Preliminary results are reported of experiments on a two-dimensional pool of negative ions trapped below the surface of superfluid 4He. The observation of plasma resonances in such a negative-ion-pool is reported for the first time. Studies of the resonances have been made in the temperature range from 50 to 130 mK, and ionic masses and mobilities have been obtained from the resonant frequencies and linewidths. Nonlinear effects have been observed at large plasma-wave amplitude and it is possible that vortex nucleation has been observed at ionic velocities exceeding 51 m s-1. Pools of ions have been cooled below the temperature predicted for transition to a Wigner lattice, but no corresponding change in behaviour of the pool has been observed so far. Measurements have also been made with a capacitance-conductance bridge, yielding values of the ionic mobility in the temperature range from 0.1 to 1K. The work complements existing work by J. Poitrenaud and F.I.B. Williams (1972) and by G.A. Williams and co-workers.