The Mobility of Helium Molecular Ions in Helium

A theoretical study of the low field mobility of He₂⁺ in its parent gas is described. Forces between He₂⁺ and He are calculated by means of perturbation theory. Determination of the scattering phase shifts then yields the momentum transfer cross section as a function of collision energy. Use of the Chapman-Enskog theory of diffusion permits the calculation of the mobility as a function of temperature.The temperature variation of the mobility of He₂⁺ observed by Tyndall and Pearce is analyzed by means of classical theories, yielding an effective potential for the He₂⁺-He interaction. The discrepancy between theoretical and experimental mobility is ascribed to certain defects in the calculated potential. When account is taken of ionic clustering, the observed temperature dependence is explained on the basis of a simple potential model.