Photoabsorption cross‐section measurements of32S, 33S, 34S, and 36S sulfur dioxide for the B1B1‐X1A1 absorption band

Abstract

We report measurements of the ultraviolet absorption cross‐sections of32SO2, 33SO2, 34SO2 and 36SO2 recorded from 250 to 320 nm at 293 K with a resolution of 8 cm−1. This is the first reported measurement of the 36SO2cross‐section. This work improves earlier measurements of the32SO2, 33SO2 and 34SO2cross‐sections and is in good agreement concerning fine structure and peak widths, with localized differences at the peak maxima when isotope effects are taken into account. SO2 samples were produced in an identical process via combustion of isotopically enriched S0, eliminating effects due to variation in oxygen isotopic composition. Peak positions for the rare isotopologues are red shifted relative to the 32SO2 isotopologue. Starting at the origin the shift increases linearly through the band. A linear shift model based on the spectrum of 32SO2was used to estimate the cross‐sections of33,34,36SO2; the average of the wavelength resolved absolute difference between the modeled and experimental spectra is 77.4, 107 and 139 ‰ respectively. While the peak‐to‐valley amplitude of36SO2 tends to be smaller than the other isotopologues throughout the spectrum, integrated band intensities for all isotopologues are conserved to within 4% relative to 32SO2. The cross‐sections were used in a photochemical model to obtain fractionation constants to compare with photochemical chamber experiments. We conclude that planetary atmospheres will exhibit isotopic fractionation from both photoexcitation and photodissociation, and that experiments in the literature have isotopic imprints arising from both the B1B1‐X1A1 and the C1B1‐X1A1 bands.