APPLICATION OF TRANSITION-STATE THEORY TO GAS-SURFACE REACTIONS - BARRIERLESS ADSORPTION ON CLEAN SURFACES

Transition-state theory (TST) has long been applied to gas-surface rea ctions for the purpose of characterizing and predicting desorption rat e coefficients. However, current perceptions of its use exclude the us e of TST from meaningful description of the sticking coefficient, eith er for clean or for partially covered surfaces, principally because of the limitations of the theory in describing adsorbate-adsorbent energ y transfer. TST has thus previously been neglected as an interpretativ e and predictive tool fdr the temperature and coverage dependence of t he sticking coefficient, particularly for processes without a barrier to adsorption. It has previously been generally believed that only var iational. minimum dividing surfaces at infinite adsorbate-surface sepa ration could be possible for barrierless adsorption processes, implyin g a TST sticking coefficient of unity. Reassessment of this point sugg ests 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 s urface is presented that demonstrates that other variational minimum d ividing surfaces are indeed possible.