Vibrational excitation and caging following the photodissociation of small HBr clusters in and on large Ar clusters

Abstract

The photodissociation of a small (HBr)_x cluster adsorbed on or embedded in larger Ar_n clusters has been studied at 243 nm. We measured the kinetic energy distribution of the H atom products in a time-of-flight mass spectrometer using (2 + 1)-resonance-enhanced multiphoton ionization (REMPI) spectroscopy. This observable provides information both on the amount of caged H atoms with zero velocity and on the vibrational excitation of HBr molecules in the cluster which have undergone a collision with unperturbed fast, already dissociated H atoms. In the surface case, generated by the pick-up technique, (HBr)_x clusters from x = 1 to 4 were investigated on Ar_n clusters with 〈n〉 = 139. While the ratio of caged to uncaged atoms R_c is about 2, similar to the result for surface adsorbed monomers, the vibrational excitation is accompanied by rotational excitation. It is mainly attributed to the constrained geometry of surface adsorbed (HBr)₂ dimers. In the embedded case, the (HBr)_x clusters of 〈x〉 = 8 were investigated in Ar_n clusters of variable size from 〈n〉 = 100 to 12. Here, the amount of caged to uncaged atoms varies drastically, by a factor of 50, as a function of the argon cluster size. In the inelastic collisions, well resolved peaks were observed for the vibrational excitation into = 1 and = 2.