Femtosecond real-time probing of reactions. XIV. Rydberg states of methyl iodide
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 dependence of the predissociation rates, and coherence effects. The quantal molecular dynamics in two-dimensions 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 CH3I and CD3I 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.
© 1994 NRC Canada. This paper is dedicated to Professor John C. Polanyi on the occasion of his 65th birthday. This article is dedicated to John C. Polanyi, a colleague and friend, whose visionary and insightful work continues to inspire researchers in a number of areas covering science and humanity; it has certainly influenced the contribution reported here. This work was funded by the donors of Petroleum Research Fund, administered by the American Chemical Society, by the National Science Foundation (CHE-9116501) to H.G., and by AFOSR to A.H.Z. Stimulating discussions with Marcos Dantus, Maurice Janssen and Volker Engel are gratefully acknowledged. Arthur Amos Noyes Laboratory of Chemical Physics, Contribution No. 8869.
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