Application of Transition-State Theory to Gas-Surface Reactions: Barrierless Adsorption on Clean Surfaces

Abstract

Transition-state theory (TST) has long been applied to gas-surface reactions for the purpose of characterizing and predicting desorption rate coefficients. However, current perceptions of its use exclude the use of TST from meaningful description of the sticking coefficient, either for clean or for partially covered surfaces, principally because of the limitations of the theory in describing adsorbate-adsorbent energy transfer. TST has thus previously been neglected as an interpretative and predictive tool for the temperature and coverage dependence of the sticking coefficient, particularly for processes without a barrier to adsorption. It has previously been generally believed that only variational minimum dividing surfaces at infinite adsorbate-surface separation could be possible for barrierless adsorption processes, implying a TST sticking coefficient of unity. Reassessment of this point suggests that this is not necessarily the case even for clean surfaces; a form for the desorption rate coefficient that insists all trajectories have sufficient energy to desorb is derived, and a model calculation of the temperature dependence of the sticking coefficient on a clean surface is presented that demonstrates that other variational minimum dividing surfaces are indeed possible.