Imaging CH3SH photodissociation at 204 nm: the SH + CH3 channel

Abstract

The SH + CH₃ product channel for the photodissociation of CH₃SH at 204 nm was investigated using the sliced velocity map ion imaging technique with the detection of CH₃ products using state selective (2+1) resonance enhanced multiphoton ionization (REMPI). Images were measured for CH₃ formed in the ground and excited vibrational states (v₂ = 0, 1, and 2) of the umbrella mode from which the correlated SH vibrational state distributions were determined. The vibrational distribution of the SH fragment in the SH + CH₃ channel at 204 nm is clearly inverted and peaks at v = 1. The highly negative anisotropy parameter of the CH₃ (v₂ = 0, 1, and 2) products is indicative of a fast dissociation process for C–S bond cleavage. Two kinds of slower CH₃ products were also observed (one of which was partly vibrationally resolved) that are assigned to a two-step photodissociation processes, in which the first step is the production of the CH₃S (X²E) radical via cleavage of the S–H bond in CH₃SH, followed by probe laser photodissociation of nascent CH₃S radicals yielding CH₃(X²A₁, v₂ = 0–2) + S(³P_j/¹D) products.