Photodissociation of HCI adsorbed on the surface of an Ar₁₂ cluster: Nonadiabatic molecular dynamics simulations

The photodissociation of HCl adsorbed on the surface of an Ar₁₂ cluster is studied by semiclassical molecular dynamics simulations, using a surface-hopping approach for the nonadiabatic transitions. The DIM method is used to construct the 12 potential energy surfaces that are involved, and the nonadiabatic couplings. The results are compared with previous studies on HCl embedded inside Ar clusters and on the triatomic Ar-HCl cluster. The main findings are the following: (1) There is a yield of about 1 % for recombination onto the ground electronic state of HCl, roughly the same as for HCl embedded inside Ar₁₂. (2) Photodissociation lifetimes much longer than for Ar-HCl are found. (3) The kinetic energy distribution of the H atom shows large energy transfer to the cluster, greater than in the case of HCl in the embedded geometry in (Ar)₁₂HCl. (4) An interesting mechanism leads to the formation of some fraction of very "hot" Cl atoms. (5) About 10% of the Cl is left trapped in (Ar)_mCl clusters. (6) The branching ratio P_1/2:P_3/2 for the Cl atoms that leave the cluster shows electronic cooling compared to the isolated HCl molecule case. The results throw light on the role of local geometry in photodissociation/recombination processes, and in particular on the mechanisms pertinent in the case of surface-adsorbed species. The nature of the results, showing strong cage effects at the surface geometries is to a large extent a consequence of the encapsulation of the H atom, obtained for the structure of the (Ar)₁₂HCl cluster.