Effects of solvation by a single atom on photodissociation: Classical and quantum/classical studies of HCl photolysis in Ar⋅⋅⋅HCl

Abstract

The photolysis of HCl in the cluster Ar⋅⋅⋅HCl is studied theoretically with the objective of elaborating on the effect of a single ‘‘solvent’’ atom on the dynamics of chemical bond breaking. The focus is on observable properties, such as the velocity distribution and the angular distribution of the H atom product, on how these properties reflect the ‘‘solvent’’ effect, and on the physical mechanisms involved. The main results obtained are the following. (1) There is a high probability for at least a single ‘‘hard’’ collision between the H photofragment and the Ar atom before the H atom leaves the cluster. Multiple collisions between the H and the heavy atoms also occur with significant probability. (2) The final kinetic-energy distribution of the H atom shows a long pronounced tail due to energy transfer in the collisions with the heavy atoms. (3) There are pronounced peaks in the angular distribution of the H atom due to the single and multiple collision events. (4) Comparison of photolysis of Ar⋅⋅⋅HCl with that of Ar⋅⋅⋅DCl shows a large isotope effect, again due to collisions within the cluster during its fragmentation. The results were mostly obtained from classical trajectory calculations, but also in part from calculations using a hybrid quantum/classical method in which the H atom is treated by quantum wave packets while the heavy atoms are described classically. The quantitative results show some quantum effects, but for most purposes the classical description is sustained. Implications of the results for experimental studies of molecular photodissociation in clusters are discussed.