Full nine-dimensional ab initio potential energy surfaces and trajectory studies of A-band photodissociation dynamics: CH3I*→CH3+I, CH3+I*, and CD3I*→CD3+I, CD3+I*

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Abstract
The full nine‐dimensional potential energy surfaces (PESs) of the 3 Q 0 and 1 Q 1 states of CH3I have been calculated with the ab initio contracted spin–orbit configuration interaction method. The results are fitted to three diabatic potential terms and their couplings as functions of all the internal degrees of freedom. The transition dipole at the Franck–Condon region has also been calculated. Surface hopping quasiclassical trajectory calculations on these potential energy surfaces have been performed to examine the photodissociationdynamics of both CH3I and CD3I in the A‐continuum. The results are in general good agreement with the recent experimental findings. The reasonable I*/(I*+I) branching ratio can be obtained with these PESs when the contribution of direct transition to the 1 Q 1 state is considered. The rotational distribution of the CH3 and CD3 fragments and its I*/(I*+I)‐channel selectivity are determined by the shape of the PESs with respect to the bending angle outside the conical intersection region. The vibrational distribution of umbrella mode is closely related to the shape of PESs for the umbrella angle; the sudden switch of reaction coordinate from 3 Q 0 to 1 Q 1 at the conical intersection is the origin of vibrational excitation in the I* channel. The larger umbrella excitation of the CD3 fragment in both I and I* channels, in comparison with the CH3 fragment, is related to the larger separation of the reaction coordinate from the Franck–Condon geometry. The symmetric stretching energy increases during the dissociation, which is related to the shape of PESs with respect to this coordinate, and the excitation of symmetric stretching mode seems to be possible.

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