Close-coupling calculations of transport and relaxation cross sections for H2 in Ar

Abstract

We present benchmark close‐coupling calculations of relaxation, transport, and Raman line shape cross sections for H₂ colliding with Ar, using the BC₃ (6,8) potential energy surface of Le Roy and Carley. The experimentally observable cross sections calculated include those for shear viscosity, binary diffusion, depolarized Rayleigh light scattering, and flow birefringence. The agreement with available experimental data is fair, except for the pressure shifting of the pure rotational Raman lines of H₂ and the cross section for depolarized Rayleigh light scattering. It is shown that the Raman line shift cross sections are very sensitive to the dependence of the intermolecular potential on the H₂ stretching coordinate, and the line shift data should be useful in determining this feature of the potential more accurately. Conversely, line shift data should not be used to determine intermolecular potentials when the vibrational dependence of the surface is neglected. The role of orbiting resonances in transport and relaxation cross sections is also investigated. Their effects are found to be small for transport cross sections, but they can contribute up to 30% to relaxation and linewidth cross sections at very low temperatures (<100 K). The resonance effects decrease quickly as the temperature is increased and are negligible at room temperature and above. Nevertheless, the existence of resonant effects must be remembered when performing thermal averaging of cross sections, since one energy point accidentally on resonance can greatly distort calculated results.