Precursor Mediated Adsorption and Desorption: A Theoretical Analysis.

Existence of a weakly bound precursor to chemisorption has been proposed to explain a variety of kinetic observations in gas-surface reactions. We develop a theoretical approach to adsorption-desorption kinetics which unifies the different kinds of precursor states that have been invoked. Our approach employs a "potential of mean force," or free energy as a function of the reaction coordinate, widely used in liquid state theories. This potential incorporates a rigorous measure of the thermally averaged effects of the ignored coordinates on the reaction rate. As a result it is temperature dependent and it can be quite different from any cut through the potential hypersurface. We define a precursor in terms of the potential of mean force and show that it coincides with the usual qualitative understanding of the term in several model problems. Specifically we consider intrinsic precursors to associative and dissociative chemisorption on a clean surface and an extrinsic precursor model for a partially covered surface. A major conclusion of this study is that the notion of a precursor should not be equated with multiple minima of the gas-surface potential. The one-dimensional potential of mean force picture is sufficient to allow interpretation of equilibrium rates of desorption and general features of thermally averaged sticking probabilities. However, it may be necessary to consider multidimensional aspects of the gas- surface interaction to understand the origin of some features of the reduced potential and to predict detailed dynamical behavior.