The He–Co2 interaction revisited: Approximate rotational energy transfer efficiency indices and computed transport coefficients

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Abstract
Of the several anisotropic interactions suggested over the years for the title system, two of the most recent ones have been tested against experimental total differential cross sections, energy loss angular distributions, and transport coefficients. In the present work, they are examined again in order to compare them in a calculation over a broader range of transport properties, using the approximate infinite order sudden approximation (IOSA) scheme for the dynamics and getting first- and second-order contributions to the Chapman–Cowling expansion in the kinetic theory of dilute gaseous mixtures. The various observables are found to be very close to the experiments for both potential functions, thereby making it still difficult to establish clearcut differences between them from the study of the above properties alone. The novelty of the present study, however, is to show that various recently proposed efficiency indices for the rotational energy transfer (RET) collisions are able to exhibit marked differences between the two surfaces and to unequivocally select one of them as the more anisotropic of the two. Because of the low value of such indices, one is also able to explain why the IOSA approximation should work well for computing the transport coefficients of this system.