Vibronic origin of sulfur mass-independent isotope effect in photoexcitation of SO₂ and the implications to the early earth’s atmosphere

Signatures of mass-independent isotope fractionation (MIF) are found in the oxygen (¹⁶O, ¹⁷O, ¹⁸O) and sulfur (³²S, ³³S, ³⁴S, ³⁶S) isotope systems and serve as important tracers of past and present atmospheric processes. These unique isotope signatures signify the breakdown of the traditional theory of isotope fractionation, but the physical chemistry of these isotope effects remains poorly understood. We report the production of large sulfur isotope MIF, with Δ ³³S up to 78‰ and Δ ³⁶S up to 110‰, from the broadband excitation of SO ₂ in the 250–350-nm absorption region. Acetylene is used to selectively trap the triplet-state SO ₂ ([Formula] ³B ₁), which results from intersystem crossing from the excited singlet ([Formula] ¹A ₂/[Formula] ¹B ₁) states. The observed MIF signature differs considerably from that predicted by isotopologue-specific absorption cross-sections of SO ₂ and is insensitive to the wavelength region of excitation (above or below 300 nm), suggesting that the MIF originates not from the initial excitation of SO ₂ to the singlet states but from an isotope selective spin–orbit interaction between the singlet ([Formula] ¹A ₂/[Formula] ¹B ₁) and triplet ([Formula] ³B ₁) manifolds. Calculations based on high-level potential energy surfaces of the multiple excited states show a considerable lifetime anomaly for ³³SO ₂ and ³⁶SO ₂ for the low vibrational levels of the [Formula] ¹A ₂ state. These results demonstrate that the isotope selectivity of accidental near-resonance interactions between states is of critical importance in understanding the origin of MIF in photochemical systems.