Polarized resonance Raman spectrum as a probe of nonadlabatic transitions in photodissociation: A theoretical treatment

The polarized resonance Raman spectrum of a two-state system (methyl iodide) is studied by use of a time-dependent formalism. The dissociation coordinate, i.e., I-CH₃ stretch, is the only variable taken into consideration in our model. Potential energy curves for the relevant states are adapted from ab initio data, and the nonadiabatic coupling between them is modeled to give the correct I/I* photodissociation branching ratio. The dissociation dynamics is characterized by using one-dimensional quantum wavepackets in a diabatic electronic basis set. The anisotropy of the resonance Raman cross section, β, is calculated from these time-dependent wave functions, and it is shown that β provides a direct measure of the relative populations of the two diabatic surfaces during dissociation. Comparison with experiment shows a discrepancy which suggests that the ab initio potential energy curves may need modification.