The reactive flux method in the energy diffusion regime. I. Effect of slow vibrational energy relaxation

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Abstract
The Mori–Zwanzig memory function formalism is used to determine the behavior of the solute reactive flux in the energy diffusion regime. We show formally that in the case of slow vibrational energy relaxation the reactive flux can exhibit two plateaus corresponding to two phenomenological rate constants. The second of these plateaus corresponds to the overall solute reaction rate, while the first plateau is a ‘‘false’’ plateau. Such a scenario is equated with a Lindemann-type reaction phenomenology corresponding to two different energy relaxation mechanisms. Time scale arguments are used to show that such a scenario exists in an example of a cubic reaction coordinate coupled to a slow Gaussian friction bath. The geminate recombination of I2 in solution provides experimental confirmation that such behavior also occurs in real chemical systems.