Photochemistry of ClClS and ClSCl isomers

The photochemistry of chlorine-sulfur species is central to Venus's atmospheric chemistry, particularly concerning the unidentified near-UV absorber. This study investigates the electronic structure, spectroscopy, and photochemistry of the ClSCl and ClClS isomers, with implications for their potential role as near-UV absorbers in Venus' atmosphere. High-level CCSD(T) and MRCI+Q calculations indicate that ClSCl is the global minimum, but ClClS is 2.34 eV higher in energy. For ClSCl, the photo-absorption cross-section displays a strong deep-UV peak at 197 nm and a much weaker band centered near 370 nm. In contrast, ClClS shows two comparatively intense near-UV bands at 350 and 271 nm. Potential energy surface analyses show that ClSCl's excited states undergo non-adiabatic coupling upon near-UV irradiation, with deep-UV excitation leading to rapid ClS + Cl formation. Although both isomers absorb in the same near-UV spectral range as the unknown Venus absorber, these isomers are photochemically unstable under the actinic UV radiation and so are unable to play major roles.