Theory of the Thermal Accommodation Coefficients of a Diatomic Gas

A quantum-mechanical theory of the thermal accommodation coefficient is discussed for a diatomic gas whose molecules can possess internal energy of rotation as well as translational energy. Expressions are obtained, in terms of transition probabilities, for αt, the accommodation coefficient associated with the change in translational energy of a beam of gas molecules reflected from a solid surface, and αi, the accommodation coefficient associated with the change in internal energy of the gas molecules. The gas molecules are considered as rigid plane rotators; and, on striking the surface, a gas molecule experiences a change in rotational and translational energy, while an atom of the solid loses or gains vibrational energy. Some general conclusions are drawn concerning αt and αi and their relation to each other, and a special case is considered.