Imaging the photodissociation dynamics of vibrationless and vibrationally excited CH3S radicals

Abstract

The photodissociation dynamics of CH3S radicals was studied at different excitation energies within the first absorption band. The CH3S radicals were produced with a broad vibrational energy distribution from the photodissociation of CH3SH at 210 nm, which allowed us to study the effect of the vibrational excitation on the photodissociation dynamics. The photofragments, CH3(ν), S(3PJ), and S(1D) were detected by resonance enhanced multiphoton ionization schemes in slice-imaging experiments, and the corresponding translational energy and angular distributions were obtained for each fragment. Vibrationless CH3S radicals are excited to the Ã2A1 state that predissociates via three possible dissociative states (ã4A2, B̃2A2, and 4E), leading to the formation of CH3(ν = 0), CH3(ν2) with inverted population and S(3PJ) with J = 0, 1, and 2. Instead, vibrationally excited CH3S radicals are excited in the Franck–Condon region of the B̃2A2 dissociative state, where they dissociate directly to produce fast and anisotropic S(3P0) fragments, according to the adiabatic correlation established by the new calculated potential energy curves reported in this work. The B̃2A2 state is crossed by the dissociative 4E state, and this crossing leads to the formation of slower and less anisotropic S(3P1) and S(3P2) fragments that apparently correlate with CH3 populated with one quantum in the C–H stretch (ν1), suggesting that this vibrational mode is involved in the non-adiabatic dynamics associated to the B̃2A2/4E crossing. Finally, S(1D) fragments show a Boltzmann-like kinetic energy distribution with an isotropic angular distribution, associated with slow fragments produced by statistical dissociation from the locally excited Ã2A1 state of CH3S.