Thermal Diffusion in Polyatomic Gases: Nonspherical Interactions

The thermal‐diffusion factor ${\alpha }_T$ , of polyatomic gases is reconsidered and the assumption is dropped that the differential scattering cross section is independent of the entrance and exit channels in inelastic collisions. In addition to the linear relation previously found, linking ${\alpha }_T$ to the partial translational thermal conductivities of the constituents, a new term arises relating ${\alpha }_T$ to the partial internal thermal conductivities. Estimates of this new coupling coefficient were made using the loaded sphere model and an approximate analysis of the thermal separation of ortho‐ and parahydrogen. They both indicate that this coefficient is very small and ordinarily does not contribute much to the total thermal‐diffusion factor. It is shown, however, that there are some situations in which this term becomes dominant. To first order in the eccentricity of isotopic series of diatomic molecules, ${\alpha }_T$ turns out to be proportional to the difference of the moments of inertia, an empirical relation observed by many investigators.