Dynamics of precursor-mediated chemisorption

Abstract

We have developed theoreticalmodels of precursor‐mediated (nondissociative) molecular chemisorption and used the stochastic classical trajectory method to simulate experiments capable of determining whether a precursor is present. The simulations employ empirical many‐dimensional potentials and include the full effects of surface vibrations, coupling among molecular degrees of freedom, and coupling between the molecule and surface. We find that coupling between molecular rotational and translational modes strongly affects the experimentally observable quantities. As a result, reasoning based on the usual one‐dimensional picture of a precursor is unreliable. The most notable effect of a precursor is the strong rotational polarization it induces in desorbing molecules. We discuss the origins of rotational polarization and conclude that with no precursor, polarization will be weak and opposite in sign to that of the precursor case. The sticking probability as a function of incident energy and surface temperature can also distinguish whether precursor mediation is important but, again, inferences from one‐dimensional models can be misleading. We also find that equilibrium diffusion rates for precursor molecules are faster than for chemisorbed molecules. However, the equilibrium process is relatively slow and until it is complete, chemisorbed molecules may well move farther across the surface than precursors.