Resonance Raman Studies of CH3I Sub-Picosecond Mode-Specific Photodissociation and Radiation-Induced Pure Dephasing Effects
The polarization characteristics of spontaneous resonance Raman (RR) scattering of free rotors are sensitively dependent on the dephasing time of the resonant optical transition.  When this time falls in the time regime between a vibrational and rotational period this frequency domain based polarization technique can determine, with considerable precision, vibronically mode specific lifetimes due to ultrafast dynamical processes in excited electronic states.  The Raman scattering of methyl iodide vapor excited by radiation resonant with the predissociated Rydberg B-state (201 nm-190 nm) provides an excellent example of this method and reveals rather unexpected quantum specific predissociation effects.  Due to the rapid photochemistry all B-state vibronic absorption bands are rotationally diffuse. However, the methyl iodide bond breaking is not so fast as to obscure most vibrational resolution. The goal of these studies is to uncover, on this rapid time scale corresponding to a few vibrational periods, how the photodissociation rate of this isolated molecule depends on excess total vibrational energy above the electronic origin, vibrational symmetry and type of initial nuclear motion excited on the multidimensional electronic surface. It may be anticipated that when statistical descriptions of energy randomization do not apply, unimolecular reaction rates may be enhanced or appear mode selective if the initial excitation can be localized in the apparent reaction coordinate. Such expectations are tested by these studies.