Classical Theory of Small Energy Accommodation Coefficients. Application to 3He and 4He on W

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Abstract
Some recent experimental data on the energy accommodation coefficients of the systems 3He–W and 4He–W are compared with the predictions of the classical theories of gas–solid interactions. For reasons which are explained, these data should provide a particularly good test of theory. The classical lattice theory of energy accommodation coefficients is generalized, using results from other theories, to take account of the three‐dimensional nature of the gas; this generalization involves, for example, elimination of the assumption of normal, head‐on encounters of gas atoms with surface atoms and consideration of the hopping of gas atoms at the surface. The gas–solid interaction potential is the recently proposed modified exponential repulsion, the use of which allows considerable simplification of the analysis. The assumption that accommodation coefficients are independent of solid temperature, allowing use of a lattice model at zero temperature, is retained and discussed at some length. The application to He–W allows the calculations to be simplified by means of a perturbation procedure which is valid only for small accommodation coefficients. Although there are some disappointing features, in view of the approximations still existing in the theory the comparisons made are satisfactory. For example, there are no grounds for concluding, on the basis of these comparisons, that a classical model is inadequate for dealing with the accommodation of He–W.