Electron thermalization in gases. I. Helium

Abstract

A theory is presented for the thermalization of subexcitation electrons in a monoatomic gas and calculations performed for helium. Evolutions of both the velocity and the position distribution functions are considered in an approximate manner. Of particular interest are the time scale for thermalization and width of the spatial distribution at thermalization. Satisfactory agreement has been obtained with experiments and earlier calculations, where comparison is possible. Additionally, time-dependent relaxation rates, diffusion coefficient and distributional widths have been introduced and evaluated. At a given gas temperature thermalization parameters are not sensitive to initial energy within the subexcitation range. Variation of gas temperature, on the other hand, produces significant changes in these quantities. Momentum transfer cross sections for lower energies have been taken from swarm drift data and, for higher energies, constructed from phase shifts measured by Andrick and Bitsch. Calculated thermal electron mobilities agree well with experiment at different temperatures.