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

Abstract

Abstract Laboratory experiments have demonstrated that a mass-independent fractionation (MIF) signature is present in elemental sulfur produced during SO 2 photolysis, but the underlying mechanism remains unknown. I report here the results of chemical kinetics modeling of self-shielding during photodissociation of SO 2 in the C ˜ 1 B 2 − X ˜ 1 A 1 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 32 SO 2 produces MIF signatures in SO and residual SO 2 . Using approximate synthetic spectra for the sulfur isotopologues of SO 2 , I show that SO 2 photolysis yields a sulfur MIF signature that can account for much of the laboratory MIF measured, and is in qualitative agreement with Δ 33 S and Δ 36 S values observed in Archean rocks.