Surface Ion Mobility Measurements in Liquid Helium.

A system for the study of positive and negative charge carriers in helium at the liquid vapor interface has been developed for use at temperatures below one Kelvin. Low frequency impedance measurements of the two-dimensional charge layer have been performed using a bridge circuit. From these measurements the mobility for both species of ions has been calculated. The mobilities have been found to essentially agree with those measured in the bulk liquid. The presence of the interface was found to have little effect on the mobility. In these experiments, the ions were about 300 (ANGSTROM) under the liquid surface, and this relatively large distance may be the reason for the lack of coupling. For temperatures above 0.1 K and charge densities on the order of 10('7)/cm('2), no evidence for localization or crystallization was found. A model for current flow in a cylindrical charge sheet is presented for use in the mobility calculation.