The Near-UV Absorber OSSO and its Isomers

Disulfur dioxide, OSSO, has been proposed as the enigmatic “near-UV absorber” in the yellowish atmosphere of Venus. However, the fundamentally important spectroscopic properties and photochemistry of OSSO are scarcely documented. By either condensing gaseous SO or 266 laser photolysis of an S₂⋯O₂ complex in Ar or N₂ at 15 K, syn-OSSO, anti-OSSO, and cyclic OS(O)S were identified by IR and UV/Vis spectroscopy for the first time. The observed absorptions (λ_max) for OSSO at 517 and 390 nm coincide with the near-UV absorption (320–400 nm) found in the Venus clouds by photometric measurements with the Pioneer Venus orbiter. Subsequent UV light irradiation (365 nm) depletes syn-OSSO and anti-OSSO and yields a fourth isomer, syn-OSOS, with concomitant dissociation into SO₂ and elemental sulfur.