Surface crossings and predissociation dynamics of methyl iodide Rydberg states

We report the results of a calculation of the adiabatic surface crossings between the B and C Rydberg states of CH3I and the dissociative A state. The bound potentials are derived empirically; the dissociative states are based on recent empirical and ab initio results. The surface crossings predissociate the B state at an energy below that of v=1 in the C–I stretching mode, and near the classical turning point for v=2 of the methyl umbrella mode. Our model predicts that at least two dissociative potentials are of importance in the B state predissociation. These results are in agreement with experimental predictions. Classical trajectory results suggest that the predissociation might exhibit mode specificity. The formation of methyl iodide dimers shifts the surface crossings to higher energy, which allows the absorption to v=1 of the C–I stretch and v=2 of the umbrella mode to be observed. This result is also in agreement with experiment.