Photodissociation, electronic relaxation and recombination of HCl in Ar_n(HCl) clusters Non-adiabatic molecular dynamics simulations

Abstract

The photodissociation of HCl embedded in argon clusters is studied by semiclassical molecular dynamics, based on a surface-hopping approach for the non-adiabatic transitions. The diatomics-in-molecules (DIM) method is used to construct the 12 electronic potential energy surfaces that are involved, and the non-adiabatic couplings. Caging effects, including recombination and electronic relaxation are investigated for Ar₁₂(HCl) and Ar₅₄(HCl), corresponding respectively to one and two complete solvation layers. The effects of the process on the cluster, e.g. fragmentation and structural deformation, are also studied. The main findings are: (1) non-adiabatic transitions play a major role in the dynamics for both clusters; (2) some recombination occurs in Ar₁₂(HCl), and it is much greater, about 7%, in Ar₅₄(HCl); (3) all 12 electronic states are visited, at least to some extent, in the process, but the distributions remain non-statistical throughout in both systems; (4) rates of spin-forbidden transitions are roughly of similar magnitudes to those of spin-allowed transitions between electronic states; (5) the energy gap law of radiationless relaxation theory does not work well for these systems. Symmetry and shape of the electronic states greatly affect the relaxation rates; (6) the clusters undergo melting and extensive evaporation in the processes.

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