The potential model for helium-hydrogen interaction in thermal diffusion

Abstract

The thermal diffusion factors α of small concentrations of all possible hydrogen isotopes were measured in 3He and 4 He between 100°C and 500°C and compared with the values from the Chapman-Enskog theory. It appeared that a Buckingham exp. 6 potential model with parameters from viscosity and second virial coefficients, could fit the separations of the symmetric molecules H 2, D 2 and T 2. It was, however, impossible to fit the values of the asymmetric molecules HD, HT and DT into the Chapman-Enskog theory. An analysis of possible sources of deviations, such as higher order approximations, quantum corrections or inelastic collisions, gave no improvement of the theoretical values. The effect has to be found in the angular dependence of the intermolecular potential, originated by the shift of the centre of mass of the molecule, which is, however, out of the scope of a spherically symmetric theory. The situation is in complete agreement with the mutual thermal diffusion of hydrogen isotopes 2). Both series of experiments could be described by the following approximate formula