Reaction mechanism in chemisorption kinetics: nitrogen on the {100} plane of tungsten

Abstract

Kinetic models are developed for the formation of a chemisorbed overlayer on a homotattic surface with fourfold symmetry, taking into account the existence of short range order in the overlayer. It is shown that where the pairwise lateral interaction energy between nearest neighbour adatoms is large (repulsive or attractive), and the overlayer is consequently highly ordered, dissociative adsorption follows a pseudo first-order rate mechanism. Accurate sticking probabilities as a function of surface stoichiometry are reported for the interaction of nitrogen with the {100} plane of tungsten, obtained by a molecular beam technique, over a wide range of both surface and gas temperatures. The results are shown to be in quantitative agreement with the kinetic model, allowing for initial trapping into a mobile physisorbed state, and consistency is established with structural (low-energy electron diffraction) studies.