Adsorption of Oxygen on the (110) Plane of Tungsten

Abstract

The sticking coefficients and desorption kinetics of oxygen on a (110) tungsten crystal have been investigated by a step desorption/reflection technique. Sticking coefficients follow a generalized Kisliuk model, if allowance for reflection on first impact with the surface is made. Unlike CO on (110) W, reflection of O2 is appreciable and appears to have an activation energy of 0.6 kcal, with respect to substrate temperature. Desorption follows first-order kinetics and has an activation energy of ∼ 92 kcal. The process is complex, however, as shown by the variation in pre-exponential with surface treatment. A mechanism involving the formation and subsequent decomposition of two distinct surface oxide phases is postulated and shown to fit the experimental results. On the basis of this mechanism the observed activation energy corresponds to the process oxide→mobile adsorbed O, and it is shown that this implies an upper limit of 126 kcal for the heat of adsorption of O atoms on (110) W.