Symmetry Effects in Gas Kinetics II: Ortho- and Para-hydrogen

The Manchester University Mk I computer has been used to calculate accurately the viscosity cross section for hydrogen molecules, and its dependence on the symmetry of the wave function for binary collisions. The calculations assume a central intermolecular potential of (exp, 6, 8) type, and cover reduced temperatures in the range 0.05 ⩽ T * ⩽ 2.0. Comparison is made with estimates of the relative difference in viscosity cross sections for para-para and para-ortho collisions derived from Becker and Stehl's measurements. Assuming elastic collisions only, fairly good agreement is found at 15°K, but at higher temperatures the theoretical results become increasingly too small. A central interaction is seemingly inadequate to explain the relatively large ortho-ortho cross sections observed; in this connection the effect of an increased range of interaction is briefly discussed and shown to be unimportant.