Low-temperature mobility of positive ions in helium vapour

The mobility of positive ions in helium vapour at liquid helium temperatures has been calculated. The radius of the liquid droplet surrounding the ion has been evaluated using classical thermodynamics and the transport cross-section of the droplet has been computed using an absorptive model and a semiclassical WKB method. A comparison with the experimental results of Henson suggests that the liquid droplet is appreciably larger than expected. (author)