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

Abstract

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.