Inelastic Collisions of Atoms and Molecules with Metal Surfaces: The Accommodation Coefficient

Abstract

When a gas atom or molecule whose energy corresponds to the temperature T0 strikes the surface of a solid at temperature T1≠T0, an exchange of energy can occur as a result of the interaction of the surface atoms with the incoming atoms (adatoms) or molecules. After remaining on the surface in accordance with their mean adsorption lifetime, the adatoms desorb from the surface with an average energy kT2 (Fig.6.1.a). As a measure of this energy exchange, Knudsen [393, 397] has defined a “thermal accommodation coefficient” α. If the energy exchange between the incident particle with mean energy E0 and the surface with molecular energy E1 is incomplete, so that the particles leave the surface with an energy E2 which lies between E0 and E1, the accommodation coefficient* is defined as $$\alpha = \mathop {\lim }\limits_{E_1 \to E_0 } \frac{{E_2 - E_0 }} {{E_1 - E_0 }}.$$ (6.1-1)