Short-time photodissociation dynamics of A-band and B-band bromoiodomethane in solution: An examination of bond selective electronic excitation

We have obtained resonance, Raman spectra and absolute Raman cross section measurements at eight excitation wavelengths in the A-band and B-band absorptions of bromoiodomethane in cyclohexane solution. The resonance Raman intensities and absorption spectra were simulated using a simple model and time-dependent wave packet calculations. Normal mode vibrational descriptions were used with.the results of the calculations to find the short-time photodissociation dynamics in terms of internal coordinates. The A-band short-time photodissociation dynamics indicate that the C-I bond becomes much longer, the C-Br bond becomes smaller, the I-C-Br angle becomes smaller, the H-C-Br angles become larger, the H-C-I angles become smaller, and the H-C-H angle becomes a bit smaller. The B-band short-time photodissociation dynamics indicate the C-Br bond becomes much longer, the C-I bond becomes slightly longer, the I-C-Br angle becomes smaller, the H-C-I angles become larger, the H-C-Br angles become smaller, and the H-C-H angle becomes slightly smaller. Both the A-band and B-band short-time photodissociation dynamics appear to be most consistent with an impulsive "semi-rigid" radical model qualitative description of the photodissociation with the CH2Br radical changing to a more planar structure in the A-band and the CH2I radical changing to a more planar structure in the B band. We have carried out a Gaussian deconvolution of the A-band and B-band absorption spectra of bromoiodomethane, as well as iodomethane and bromomethane. The absorption spectra, resonance Raman intensities, and short-time photodissociation dynamics sueeest a moderate amount of coupling of the C-I and C-Br chromophores.