Photodissociation of H 2 S: A New Pathway for the Production of Vibrationally Excited Molecular Hydrogen in the Interstellar Medium.

Hydrogen sulfide (H 2 S) is the most abundant S-bearing molecule in the solar nebula. Although its photochemistry has been studied for decades, the H 2 fragment channel is still not well-understood. Herein, we describe the photodissociation dynamics of H 2 S + h v → S( 1 S) + H 2 (X 1 Σ g + ) with the excitation wavelength of 122 nm ≤ λ ≤ 136 nm. The results reveal that the H 2 (X) fragments formed are significantly vibrationally excited, with the quantum yields of ∼87% of H 2 (X) fragments populated in vibrational levels v ″ = 3, 4, 5, and 6. Theoretical analysis suggest that these H 2 products are formed on the H 2 S 4 1 A ' state surface following a nonadiabatic transition via an avoided crossing from the 3 1 A ' to 4 1 A ' state. The estimated quantum yield of the S( 1 S) + H 2 channel is ∼0.05, implying this channel should be incorporated into the appropriate interstellar chemistry models.