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

The photodissociation dynamics of jet-cooled thiomethoxy radical (CHS) via the B̃A ← X̃E transition was studied in the ultraviolet region of 216-225 nm using the high- 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 B̃A vibronic peaks at 217.7, 220.3, and 221.5 nm. Several H-atom dissociation pathways were identified. The excited-state CHS had a repulsive, prompt dissociation pathway to the ground-state HCS(X̃A) + H products, with the product translational energy peaking near the maximum available energy, a predominant C-S stretch vibrational excitation in HCS(X̃A), and an anisotropic angular distribution. The main pathway was the HCS(X̃A) + H product channel via the unimolecular dissociation of internally hot CHS 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 + HCS(ÃA) and H + HCS(ãA) product channels were also observed but were minor channels. The C-H bond dissociation energy of CHS to the H + HCS(X̃A) products was determined to be 48.8 ± 0.7 kcal/mol.