Studies of Chemical Reactivity in the Condensed Phase. IV. Density Dependent Molecular Dynamics Simulations of Vibrational Relaxation in Simple Liquids.

Molecular dynamics simulations of the photodissociation/recombination process for iodine in liquid xenon at several densities are reported in this paper. These simulations were performed to aide in the understanding and interpretation of recent picosecond experimental investigations on model chemical reaction systems. From these calculations, it was found that geminate recombination occurs primarily within a few picoseconds at all densities considered. This is in agreement with previous molecular dynamics simulations with significantly smaller systems, and with the current interpretation of experimental results. Simulated iodine ground electronic state vibrational relaxation times range from about one nanosecond at the lowest density to approximately 250 picoseconds at the highest density reported here. In addition, the functional form of the decay of the average iodine vibrational energy was observed to be nearly independent of density. This result is discussed in terms of simple gas phase Isolated Binary Collision models. Various force correlation functions projected on to the iodine vibrational coordinate were also examined, and indicate that the iodine molecule significantly perturbs the local solvent environment. These force correlation functions may be helpful when assessing the usefulness of liquid phase theories of vibrational relaxation of highly excited molecules. Finally, the simulation results on iodine vibrational relaxation are compared with the available experimental data. These comparisons indicate that the molecular dynamics calculations overestimate the rate of vibrational relaxation over the lower third of the iodine ground electronic state potential surface, and that the efficiency of V-TR transfer, relative to V-V transfer, may have been underestimated. The sensitivity of these results to several system parameters are analyzed.