SO 2 Photodissociation at 193 nm Directly Forms S( 3 P) + O 2 ( 3 Σ g - ): Implications for the Archean Atmosphere on Earth.

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