Studies of chemical reactivity in the condensed phase. III. Direct measurement of predissociation and geminate recombination times for iodine in solution

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Abstract
Results are presented which clarify the time scales for predissociation and geminate recombination in the iodine photodissociation/recombination reaction. Direct comparison of absorption features following A state excitation with those following B state excitation suggests that predissociation may be much more rapid (≤1 ps) than previously thought, and that geminate recombination is also very rapid (≤2 ps). Both of these results lead to the conclusion that curve crossing in solution is extremely facile and not the rate-limiting step in recombination. An important consequence of this is that molecular dynamics simulations seem to give reasonable results for recombination dynamics even when curve crossing is treated by oversimplified models. Finally, it is shown that in order to properly interpret the early time bleach kinetics it is necessary to consider dynamics within the ground state population remaining after excitation as well as excited state dynamics.