Solvent effects on dynamics of overtone-induced dissociation

We study via classical molecular dynamics simulation the overtone-excitation-induced dissociation of a model for hypochlorous acid, HOCl, in the gas phase and in fluid Ar over a large range of pressures. We find that the lifetime distribution over the first 10 ps after excitation is nonexponential, in both the gas phase and in solution, for an initial excitation of 80 kcal/mol (which, for the model we use, is >30 kcal/mol in excess of what is needed to break the OCl bond). The presence of Ar at moderate pressures (∼100 atm) increases the initial instantaneous rate of dissociation over that in the gas phase. We examine the time-dependent velocity power spectra of the HOCl molecules after excitation, which gives information about the frequencies of the molecular motion as the molecules evolve, and thus about the flow of energy within the molecule. At higher pressures (∼3000 atm), recombination becomes a dominant process and the dissociation probability drops below that at the lower pressures.