Photodissociation of HBr adsorbed on the surface and embedded in large Ar-n clusters

Ultraviolet (UV) photodissociation experiments are carried out for Ar-n(HBr ) clusters in which the HBr is adsorbed on the surface of the Ar-n, and als o on isomers of these systems in which HBr is embedded within the rare-gas cluster. The mean size of the cluster distribution in the experiments is ar ound (n) over bar=130. The kinetic energy distribution (KED) of the hydroge n atoms that left the clusters is measured. Molecular dynamics (MD) simulat ions of the photodissociation of the chemically similar clusters Ar-n(HCl) are used to provide a qualitative interpretation of the experimental result s. The clusters with embedded HBr give a very cold H-atom KED. The clusters with the surface-adsorbed HBr give a KED with two peaks, one corresponding to very low energy H atoms and the other pertaining to high energies, of t he order of 1.35 eV. The theoretical simulations show that already for n=54 , there is a strong cage effect for the "embedded" molecule case, resulting in slow H atoms. The surface-adsorbed case is interpreted as due to two ty pes of possible adsorption sites of HX on Ar-55: for a locally smooth adsor ption site, the cage effect is relatively weak, and hot H atoms emerge. Sit es where the HBr is adsorbed at a vacancy of Ar-n lead to "encapsulation" o f the H atom produced, with a strong cage effect. A weak tail of H atoms wi th energies well above the HBr monomer excess energy is observed for the em bedded case. Simulations support that this is due to a second photon absorp tion by recombined, but still vibrationally hot, HBr. The results throw lig ht on the differences between the cage effect inside bulk structure and at surfaces. (C) 2000 American Institute of Physics. [S0021-9606(00)00925-9].