H-Atom Product Channel in the Ultraviolet Photodissociation of the Thiomethoxy Radical (CH3S) via the B̃2A2 State.

Abstract

The photodissociation dynamics of jet-cooled thiomethoxy radical (CH3S) via the B̃2A2 ← X̃2E transition was studied in the ultraviolet region of 216-225 nm using the high-n Rydberg H-atom time-of-flight (HRTOF) technique. The H-atom product channel was directly observed from the H-atom TOF spectra (using both dimethyl disulfide and dimethyl sulfide precursors). The H-atom photofragment yield spectrum showed a broad feature in the region of 216-225 nm and three B̃2A2 vibronic peaks at 217.7, 220.3, and 221.5 nm. Several H-atom dissociation pathways were identified. The excited-state CH3S had a repulsive, prompt dissociation pathway to the ground-state H2CS(X̃1A1) + H products, with the product translational energy peaking near the maximum available energy, a predominant C-S stretch vibrational excitation in H2CS(X̃1A1), and an anisotropic angular distribution. The main pathway was the H2CS(X̃1A1) + H product channel via the unimolecular dissociation of internally hot CH3S radical in the ground electronic state after internal conversion from the electronic excited state, with a modest translational energy release (peaking at a low translational energy of ∼11 kcal/mol and extending near the maximum available energy) and a nearly isotropic angular distribution. The H + H2CS(Ã1A2) and H + H2CS(ã3A2) product channels were also observed but were minor channels. The C-H bond dissociation energy of CH3S to the H + H2CS(X̃1A1) products was determined to be 48.8 ± 0.7 kcal/mol.