Journal of Physical Chemistry A, Vol.123, No.28, 5849-5858, 2019
H-Atom Product Channel in the Ultraviolet Photodissociation of the Thiomethoxy Radical (CH3S) via the (B)over-tilde(2)A(2) State
The photodissociation dynamics of jet-cooled thiomethoxy radical (CH3S) via the (B) over tilde (2)A(2) <- (X) over tilde E-2 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 (B) over tilde (2)A(2) 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((X) over tilde (1)A(1)) + H products, with the product translational energy peaking near the maximum available energy, a predominant C-S stretch vibrational excitation in H2CS((X) over tilde (1)A(1)), and an anisotropic angular distribution. The main pathway was the H2CS((X) over tilde (1)A(1)) + 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 similar to 11 kcal/mol and extending near the maximum available energy) and a nearly isotropic angular distribution. The H + H2CS((A) over tilde (1)A(2)) and H + H2CS((a) over tilde (3)A(2)) product channels were also observed but were minor channels. The C-H bond dissociation energy of CH3S to the H + H2CS((X) over tilde (1)A(1)) products was determined to be 48.8 +/- 0.7 kcal/mol.