SO Photodissociation at 193 nm Directly Forms S(P) + O(Σ): Implications for the Archean Atmosphere on Earth.

It is well-documented that photodissociation of SO at λ = 193 nm produces O(P) + SO X(Σ). We provide experimental evidence of a new product channel from one-photon absorption producing S(P) + O X(Σ) in 2-4% yield. We probe the reactant and all products with time-resolved photoelectron photoion coincidence spectroscopy. High-level ab initio calculations suggest that the new product channel can only occur on the ground-state potential energy surface through internal conversion from the excited state, followed by isomerization to a transient SOO intermediate. Classical trajectories on the ground-state potential energy surface with random initial conditions qualitatively reproduce the experimental yields. This unexpected photodissociation pathway may help reconcile discrepancies in sulfur mass-independent fractionation mechanisms in Earth's geologic history, which shape our understanding of the Archean atmosphere and the Great Oxygenation Event in Earth's evolution.