Chapter 5 Photolysis of Long-Lived Predissociative Molecules as a Source of Mass-Independent Isotope Fractionation: The Example of SO₂

Laboratory experiments have demonstrated that a mass-independent fractionation (MIF) signature is present in elemental sulfur produced during SO₂ photolysis, but the underlying mechanism remains unknown. I report here the results of chemical kinetics modeling of self-shielding during photodissociation of SO₂ in the over(C, ̃)¹ B₂ - over(X, ̃)¹ A₁ bands from 190 to 220 nm. This band system is dominated by a bending mode progression that produces shifts in the absorption spectrum upon sulfur isotope substitution. Self-shielding in the rotationally-resolved lines of ³²SO₂ produces MIF signatures in SO and residual SO₂. Using approximate synthetic spectra for the sulfur isotopologues of SO₂, I show that SO₂ photolysis yields a sulfur MIF signature that can account for much of the laboratory MIF measured, and is in qualitative agreement with Δ³³ S and Δ³⁶ S values observed in Archean rocks.