Size Effects on Photodissociation and Caging of Hydrogen Bromide Inside or on the Surface of Large Inert Clusters:  From One to Three Icosahedral Argon Layers

Abstract

Molecular dynamics simulations have been employed for the study of photolysis of hydrogen bromide placed inside or on the surface of Ar 12, Ar 54, Ar 97, and Ar 146 clusters, representing one to three icosahedral solvation shells. A large set of classical Wigner trajectories, which take into account the initial quantum rotational or librational delocalization of HBr, is generated and analyzed in terms of transient hydrogen populations inside the cluster and final kinetic energy distributions. The key result is that for fully solvated HBr the size effect on caging is dramatic in the studied range of cluster sizes, while it is only moderate for surface isomers. Simulations also demonstrate that caging can be efficiently turned off by a librational preexcitation of HBr on argon clusters. Calculations are compared to results of cluster experiments which have measured the kinetic energy distribution of hydrogens originating from HBr photolysis at 243 nm in or on argon clusters with an average size of 115 atoms, and a near-quantitative agreement is found.