Spectroscopic signatures and photoabsorption of HSSCl and H(S)ClS

Chlorine-sulfur coupling is expected to influence Venus's middle-atmosphere photochemistry and may participate, directly or indirectly, in the chemistry linked to the long-standing "unknown" near-ultraviolet (UV) absorber. Here, we present a high-level ab initio study of the HClS2 isomers HSSCl and H(S)ClS, providing benchmark structural, rovibrational, and electronic data required for laboratory identification and photochemical modeling. Equilibrium structures and spectroscopic constants are obtained with CCSD(T)-F12/aug-cc-pVTZ and CCSD(T)/aug-cc-pV(Q+d)Z, while anharmonic fundamentals/overtones and vibrationally averaged rotational constants are computed using a rovibrational configuration interaction treatment. The results confirm HSSCl as the global minimum, with H(S)ClS higher by 0.83 eV, consistent with previous thermochemical surveys. HSSCl exhibits negligible near-UV/visible absorption and a far-UV-dominated cross section near 200 nm, implying photochemical stability in the 200-500 nm window, whereas H(S)ClS shows a strong near-UV band centered near 295 nm. Excited-state scans indicate predominantly direct, repulsive dissociation along the Cl-S and S-S stretching coordinates for HSSCl, favoring radical-forming channels such as SH + ClS and HS2 + Cl over ClS2 + H. These data enable quantitative photolysis-rate calculations and provide spectral fingerprints for experimental and observational searches.