SO2 Photodissociation at 193 nm Directly Forms S(3P) + O2(3Sg-): Implications for the Archean Atmosphere on Earth

It is well-documented that photodissociation of SO2 at ? = 193 nm produces O(P-3(j)) + SO X(S-3(-)). We provide experimental evidence of a new product channel from one-photon absorption producing S(P-3(j)) + O-2 X(S-3(g)-) 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.

Automated summary

We provide experimental evidence of a new photodissociation pathway of SO2, resulting in the production of S(P-3(j)) + O-2 X(S-3(g)-) with a yield of 2-4%. High-level ab initio calculations suggest that this pathway occurs through internal conversion from the excited state followed by isomerization to a transient SOO intermediate. Classical trajectory simulations qualitatively reproduce the experimental yields, indicating the significance of this unexpected photodissociation pathway in understanding Earth's geologic history and atmospheric evolution.