Mobility and diffusion of protons and deuterons in helium-a runaway effect

The mobility and diffusion of H + and D + ions in He gas are calculated classically, based on an accurate ab initio interaction potential. Comparison with corresponding quantal calculations of the zero-field mobility of H + in He as a function of temperature shows that quantum effects are negligible above 50K, and are only 3% at 10K. Calculations as a function of electric field strength at fixed gas temperature indicate a runaway effect, in which the ions cannot lose enough momentum by collisions to achieve a steady-state average velocity. The drift-tube measurements of Howorka et al. (1979) are consistent with this interpretation.