Ions trapped below the surface of superfluid helium. I. The observation of plasma resonances, and the measurement of effective masses and ionic mobilities

Abstract

The results are reported of experiments on two-dimensional pools of positive and negative ions trapped below the surface of superfluid 4 He. The results of detailed computations on the static characteristics of an ion pool are presented, from which it is possible to deduce the size of the pool from the known potentials applied to the surrounding electrodes and the measured total charge in the pool. At the higher tem peratures, between 1 K and about 300 mK, the ionic mobility can be obtained from measurements of the non-resonant response of the ions in the pool with a capacitance-conductance bridge connected to a pair of the surrounding electrodes; the theory underlying this technique is described in detail, and experimental results are presented. At lower tem peratures sharp plasm a resonances can be excited, and detailed studies of these resonances have been made in the temperature range from 25 mK to 130 mK for negative ions and from 15 mK to 450 mK for positive ions. Ionic effective masses have been deduced from the observed resonant frequencies, but the resulting values are in some respects unreliable for reasons that are explained. Ionic mobilities have been deduced from the observed damping of the plasma resonances; they are compared with the results of earlier experiments at higher temperatures, but the comparison with theory will be presented in a separate paper. The ion pools have been cooled on many occasions below the temperature at which there is a transition to a Wigner crystal, but no corresponding change has been observed in the behaviour of the mobility or of the plasma resonances.