Three-dimensional photodissociation dynamics of methyl iodide

Abstract

The photodissociation dynamics of methyl iodide is investigated on a three-dimensional grid using a time-dependent quantum-mechanical method. Two electronic states which correlate with I and I* fragments are explicitly included in the calculation. The potential-energy functions and the nonadiabatic coupling are adapted from a recent ab initio calculation by Morokuma and co-workers. The dynamically active degrees of freedom include the dissociation coordinate, the umbrella bend of the methyl group which is treated as a stretch between a hypothetic atom X (X=H3) and the carbon atom, and the H3–C–I bend. The discrete variable representation is used to describe the dynamics in the bending coordinate θ while the other two degrees of freedom are treated by a fast Fourier transform (FFT) based approach. The time propagation of the wave packet is carried out using the Chebychev expansion of the time propagator and the grid in the translational coordinate space is shifted during the propagation to avoid reflection at the end of the grid. Absorption spectra calculated from these ab initio surfaces are found to be blue shifted by a few thousand wave numbers and have broader widths when compared with experimental results. These discrepancies can be attributed to the topology of the ab initio potential-energy surfaces in the Franck–Condon region. Vibrational and rotational distributions for both the CH3 and CD3 fragments are calculated at several photon wavelengths. The agreement with experimental results is excellent. The vibration of the methyl fragment has a weak dependence on the photon wavelength and is found to be largely separable from its rotation. The rotational distribution of the methyl fragment is cold in the I* channel, but relatively hot for the lower I channel. The I* yield is strongly influenced by the initial excitation scheme. When both excited states are considered to be optically active, the calculated I* yield gives a better agreement with experimental observations. Our calculations are also compared with several previous theoretical works.