Measurement of Ionic Mobilities in Liquid ³He by a Space Charge Method

Abstract

The behavior to be expected of a charged impurity moving in a Fermi fluid was first discussed in detail by Abe and Aizu^l and independently by Clark.² They concluded that, although mobility should be inversely proportional to viscosity at high temperatures (classical limit), a T ⁻² law should be obeyed in the low-temperature (degenerate) limit. Davis and Dagonnier have discussed the situation at intermediate temperatures in terms of quantum mechanical Brownian motion.³ These predictions were not borne out by experiment.^4,5 In a more recent theoretical investigation Josephson and Lekner⁶ show that, although the T ⁻² law may still be expected at sufficiently low temperatures, some form of weaker dependence should be observed at higher temperatures. Since there seems to be little consensus as to the form of this weaker dependence or as to the characteristic temperature at which one regime might give way to the other, it is important that accurate experimental mobility data should be obtained. For negative ions such data are now available down to 17 m°K.⁵ However, for positive ions data below 1°K,⁴ measured by a time-of-flight method, suffer from severe inconsistencies and hysteresis effects, apparently experimentally based. In this paper we describe mobility measurements down to 0.25°K by a completely different technique: space-charge-limited emission of ions from a sharp metal point. This method appears to be more accurate than earlier space charge techniques⁷ and avoids the complication of thermal gradients due to heating at a radioactive source.

Work supported by the Science Research Council under contract BSR9251.