High-Resolution UV Absorption Cross-Section Measurements of ³²S, ³³S, ³⁴S, and ³⁶S Sulfur Dioxide for the B~1B1−X~1A1 Absorption Band

We report newly measured high-resolution and high-precision ultraviolet absorption cross-sections of 32SO2, 33SO2, 34SO2, and 36SO2 for the B~1B1−X~1A1 band over the wavelength range of 240 to 320 nm at a resolution of 0.4 cm–1. The resolution was improved 20 times compared to that in a previous study. A least absolute deviation linear regression method was applied to calculate cross-sections and derived spectral errors from a set of measurements recorded at a wide range of pressures to ensure the optimal signal-to-noise ratio at all wavelengths. Based on this analysis, error bars on the measured cross-sections ranged between 3 and 10%. The overall features of measured cross-sections, such as peak positions of the isotopologues, are consistent with previous studies. We provide improved spectral data for studying sulfur mass-independent fraction (S-MIF) signatures during SO2 photoexcitation. Our spectral measurements predict that SO2 photoexcitation produces S-MIF enrichment factors 33E = – 0.9 ± 0.2‰ and 36E = – 3.8 ± 0.4‰ (where 33E and 36E are 1000 × [ln(33J/32J) – 0.515 ln(34J/32J)] ‰ and 1000 × [ln(36J/32J) – 1.90 ln(34J/32J] ‰, and 3xJ is the 3xSO2 photoexcitation rate constant). Based on this new result, we found that calculated SO2 photoexcitation isotope effects are smaller than previously thought and generally do not match photoexcitation experimental observations supporting the hypothesis of an intersystem crossing origin of MIF on those experiments.