The interaction of atoms and molecules with solid surfaces III—The condensation and evaporation of atoms and molecules

Abstract

In the first two papers of this series (to be referred to as papers I and II) calculations were made of the probability that an atom adsorbed on a solid surface would be excited to states of higher vibrational energy and to states in the continuum, equivalent to evaporation. In this paper we carry the theory of evaporation a stage further and also develop a theory of condensation and show the relation of the theory to the method of statistical mechanics. Langmuir showed by a simple dynamical argument that under certain assumptions a solid surface in contact with a gas would be partially covered with adsorbed atoms and that the fraction of the surface covered could be expressed in terms of the pressure of the gas by the simple relation θ = ap /(1 + ap ). The parameter a is proportional to the product of a quantity τ, which is the average time spent by an atom in the adsorbed phase, and a quantity c , which is the probability that an atom striking the surface shall be adsorbed. Recently bowlers has shown that Langmuir's formula is essentially a thermodynamic one and can be obtained without involving a precise mechanism either of adsorption or evaporation. By adjusting the parameter a , the formula can be made to fit many of the experimental results and so the value of the product c τ can be inferred. This is probably as far as the statistical method can go. It cannot determine either c or τ uniquely, and if estimates are to be made of then information about c must be obtained from other sources. It is often assumed that c is of the order of unity, but this cannot be true in all cases, or even in the most interesting ones, for, as Roberts has shown, the accommodation coefficient (which is a measure of the probability that an atom will gain or lose energy on striking a solid surface) is sometimes very small. It is as low as 0·05 for helium striking tungsten and only 0·07 for neon striking tungsten at room temperature.