The ripplon-limited mobility of ions trapped below the free surface of superfluid helium

Abstract

New measurements are reported of the damping of two-dimensional plasma resonances in circular pools of negative ions trapped below the free surface of superfluid4He at temperatures down to about 15 mK. At the lowest temperatures the damping is determined by a ripplon-limited ionic mobility. The observed dependence of this mobility on temperature and trapping depth is shown to be similar to that found in earlier experiments on pools of positive ions. A theory of the ripplon-limited mobility is described, based on a calculation of the scattering of capillary waves by a single ion. It is shown that the theory accounts for the observed temperature and depth dependences, but that it yields absolute values that are too large by a factor of about 10 for both species of ion. Possible reasons for this discrepancy are discussed, but no satisfactory explanation has been found.