Photodissociation and caging of HBr and HI molecules on the surface of large rare gas clusters

Abstract

Photodissociation experiments were carried out at a wavelength of 243 nm for single HBr and HI molecules adsorbed on the surface of large Ne_n, Ar_n, Kr_n and Xe_n clusters. The average size is about 〈n〉 = 130; the size ranges 〈n〉 = 62–139 for the system HBr–Ar_n and 〈n〉 = 110–830 for HI–Xe_n were covered. In this way the dependence of the photodissociation dynamics on both the size and the rare gas host cluster was investigated. The main observable is the kinetic energy distribution of the outgoing H atoms. The key results are that we do not find any size dependence for either system but that we observe a strong dependence on the rare gas clusters. All systems exhibit H atoms with no energy loss that indicate direct cage exit and those with nearly zero energy that are an indication of complete caging. The intensity ratio of caged to uncaged H atoms is largest for Ne_n, decreases with increasing mass of the cage atoms, and is weakest for Xe_n. On the basis of accompanying calculations this behaviour is attributed to the large amplitude motion of the light H atom. This leads to direct cage exit and penetration of the atom through the cluster with different energy transfer per collision depending on the rare gas atoms. The differences between HBr and HI molecules are attributed to different surface states, a flat and an encapsulated site.