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

Abstract

It is well-documented that photodissociation of SO2 at λ = 193 nm produces O­(3P j ) + SO X­(3Σ–). We provide experimental evidence of a new product channel from one-photon absorption producing S­(3P j ) + O2 X­(3Σg –) 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.

Quantum Dynamics of Photodissociation: Recent Advances and Challenges

“…The vibrational excitation can be rationalized by the geometry of this transition state, where the H–H bond lengthens to 1.1 Å, close to the averaged bond length of H 2 ( v = 5). The dissociation mechanism of this central atom elimination channel has been illustrated in several similar systems that the molecular isomerization with one terminal atom moving to the other terminal via a nonadiabatic transition or a roaming process. , In this case, the dissociation process undergoes several internal conversions involving the direct forming of the H–H bond via the transition state and the concurrent breaking of the O–H bonds. This indirect dissociation process leads to a near isotropic product distribution.…”

Roaming in highly excited states: The central atom elimination of triatomic molecule decomposition

“…Rösch et al . ( 39 , 40 ) provided the first evidence for the S( 3 P) + O 2 channel from SO 2 photolysis at 193 nm, and Chang et al . ( 41 ) displayed the experimental observation of the S( 1 D)+O 2 channel in the wavelength range of 120 to 160 nm.…”

Sulfur (3P) Reaction with Conjugated Dienes Gives Cyclization to Thiophenes under Single Collision Conditions

“…It has been proposed as part of a geoengineering approachstratospheric aerosol injectionto increase the Earth’s albedo and mitigate global warming . UV photodissociation of SO 2 has recently been found to give S­( 3 P)+O 2 directly, with significant implications for the early atmosphere of Earth . The astrochemical fate of sulfur has long been a puzzle, with high abundances in diffuse interstellar clouds not maintained as they condense into dark clouds .…”