MODE-SPECIFIC SUBPICOSECOND PHOTODISSOCIATION DYNAMICS OF THE METHYL-IODIDE B-STATE

Vibronically specific, subrotational period lifetimes of the predissociated B state (201-190 nm) of CH3I and CD3I are determined by spontaneous resonance Raman scattering measurements of depolarization ratios and rotationally resolved Raman excitation profiles (REPs). The lifetime of the electronic origin, methyl rocking (nu-6), and methyl umbrella (nu-2) band are the same in CH3I (0.5 ps) and CD3I (1.2 ps). However, the predissociation rate decreases by a factor of approximately 3 when one quantum of nu-3, the methyl-iodine stretch, is excited on the B-state surface. The only vibrational mode which is found to accelerate the B-state predissociation rate is the a1 C-H (C-D) stretch. The lifetime of the nu-1 vibronic level decreases by nearly an order of magnitude for CH3I (60 fs) and by a factor of 2 for CD3I (0.6 ps) relative to the corresponding electronic origin level. In addition, K-resolved REP analysis reveals predissociation rates to be K independent. These mode-specific predissociation rates are ascribed to the shape of the multidimensional reaction coordinate in the region of the curve crossing between the bound (B state) and unbound surfaces.