Cluster catalyzed chemisorption of H2 on Si(111)(1×1): Effects of collision speed and cluster geometry

We have carried out classical dynamical simulations of collisions of Ar12H2 clusters with a rigid Si(111)(1×1) solid surface for a variety of collision speeds and two different cluster geometries. At low cluster temperatures, the most stable cluster geometry is that with the H2 on the outside of the cluster. It is found that dissociative chemisorption of the hydrogen can occur with this cluster geometry at H2 collision energies as low as 0.05 eV; this is markedly lower than the 0.4 eV threshold for ‘‘bare’’ hydrogen on this surface. The reactivity increases with collision energy until a steric limit of roughly 33% reaction is attained. This corresponds to that fraction of collisions in which the H2 can be caged by the Ar cluster on the surface. The other isomer, with the H2 inside the Ar cluster, is less reactive at low energies, since the H2 does not have free access to the surface. However, the reactivity begins to rise steeply away from threshold as the H2 becomes sufficiently energetic to rearrange the Ar ‘‘solvent’’ shell. Studies of the details of the dynamics reveal that the kinetic energy of the Ar cluster can be ‘‘pooled’’ to allow access to relatively high energy channels, such as chemisorption, dissociative trapping, and even collision-induced dissociation.