Vibration mediated photodissociation dynamics of CH3SH: manipulation of the dynamic energy disposal into products.

The S-H bond dissociation dynamics of CH3SH have been investigated for the S1-S0 transition mediated by either the S-H stretching (2608 cm-1) or CH3 symmetric stretching (2951 cm-1) mode excitation in the S0 state. The S-H and C-S bond extensions are strongly coupled in the S1 state through the S1/S2 same-symmetry conical intersection, giving the C-S stretching mode excitation of the CH3S˙ fragment during the prompt S-H bond rupture on S1. In the IR + UV transition mediated by the S-H stretching mode, the vertical transition seems to access the Franck-Condon region where the S-H bond is shortened while the coupling to the C-S bond stretching becomes stronger compared to the case of one-photon UV transition, indicating that the intramolecular vibrational redistribution (IVR) is little activated in S0. When the IR + UV excitation is mediated by the CH3 symmetric stretching mode, on the other hand, the Franck-Condon region in S1 encompasses the enlarged molecular structures with respect to both S-H and C-S bond extensions, presumably due to the rapid IVR in S0 prior to the vertical transition. This leads to the inverted vibrational state population of the C-S bond stretching mode of the CH3S˙ fragment. This work demonstrates that the reaction dynamics upon the IR + UV excitation of CH3SH is highly mode dependent and the energy disposal dynamics could be controlled by the manipulation of the Franck-Condon region through the particular vibrational-state mediation in the ground state, shedding new light on the structure-dynamics relationship.