Photodissociation of CH3I: A Full-Dimensional (9D) Quantum Dynamics Study

The photodissociation of methyl iodide in the A band is studied by full-dimensional (9D) wave packet dynamics calculations using the multiconfigurational time-dependent Hartree approach. The potential energy surfaces employed are based on the diabatic potentials of Xie et al. [J. Phys. Chem. A 2000, 104, 1009] and the vertical excitation energy is taken from recent ab initio calculations [Alekseyev et al. J. Chem. Phys. 2007, 126, 234102]. The absorption spectrum calculated for exclusively parallel excitation agrees well with the experimental spectrum of the A band. The electronic population dynamics is found to be strongly dependent on the motion in the torsional coordinate related to the H-3-C-I bend, which presumably is an artifact of the diabatic model employed. The calculated fully product state-selected partial spectra can be interpreted based on the reflection principle and suggests strong coupling between the C-I stretching and the H-3-C-I bending motions during the dissociation process. The computed rotational and vibrational product distributions typically reproduce the trends seen in the experiment In agreement with experiment, a small but significant excitation of the total symmetric stretching and the asymmetric bending modes of the methyl fragment can be seen. In contrast, the umbrella mode of the methyl is found to be too highly excited in the calculated distributions.