Experiments on the exchange of energy between gas, solid, and adsorbed layer in vacuo. —I. A method of detecting variations in the thermal efficiency of molecular collisions

Abstract
1. Introduction . —Following Knudsen and Smoluchowski, the thermal efficiency of encounters between the molecules of a rarefied gas and of a solid surface may be characterised by the “accommodation coefficient,” α = T 1 – T 2 /T 1 – T' 2 , where T 1 is the temperature corresponding to the energy of a gas molecule approaching the solid surface, T 2 the temperature on leaving the surface, and T 2 ' the temperature of the molecules which constitute the surface layer. The value of α has been measured in certain cases, and is generally considered to depend on the physical condition of any given surface; for instance, Langmuir has given reasons for expecting α at a clean metal wire not to be the same as when the metal carries adsorbed vapour, while Hughes and Bevan have shown that α for H 2 on nickel can be increased from 0.25 to 0.48 by oxidation. Since Langmuir has shown that a connection exists between the time taken for gas molecules to reach thermal equilibrium with a surface and the probability of their condensing thereon, variations in α , for different surface conditions of any given solid in any given gas, are of importance to the physics of adsorption. Since differences in thermal exchange at a bare lattice and at an adsorbed layer will share largely in determining whether the latter is monomolecular or multimolecular in structure, these differences are among the data preliminary to a physics of heterogeneous chemical reaction, in so far as this depends on the structure of the interfacial layer.

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