The elementary reaction dynamics of methyl iodide in two Rydberg states leading to an iodine and a methyl radical occur on the femtosecond time scale (M.H. Janssen, M. Dantus, H. Guo, and A.H. Zewail. Chem. Phys. Lett. 214, 281 (1993)). In this article, we consider the dynamics of this elementary process which involves both the Rydberg and valence states. Direct comparisons are made between theory and experiment with special focus on the following observations: large isotope effects, mode

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... ence of the predissociation rates, and coherence effects. The quantal molecular dynamics in twodimensions show that. the initial wave packet motion occurs along a vibrational mode involving the light atoms accompanied by transitions from the Rydberg state to the repulsive state; subsequent dynamics on the dissociative state lead to the C-I bond cleavage. The theoretical calculations also give the decay behavior of the Rydberg states with lifetimes in agreement with those observed in the femtosecond experiments. Moreover, the large isotope effect in observed predissociation rates of CH31 and CD31 has been successfully reproduced by the same model. The two-dimensional dynamics underscore the shortcomings of a one-dimensional picture in which the C-I serves as the sole reaction coordinate. The model presented here offers a viable mechanism for the dynamics of these Rydberg states.