UV-Vis spectral simulation of polysulfur species using the nuclear ensemble approximation

Polysulfurs (S_X) are thought to be an important part of the Venusian atmospheric sulfur cycle. Among these, S₃, S₄, and S₈ have been proposed to be contributors to the enigmatic near-UV absorption feature observed between 320 and 400 nm in the atmosphere of Venus. Furthermore, the individual UV-Vis spectral signatures of polysulfur isomers remain poorly characterized, hindering the accurate photochemical modeling of these species. In this work, we use the nuclear ensemble approach to simulate the UV-Vis spectra for a comprehensive set of S₂-S₈ structural isomers, generating wavelength-dependent absolute absorption cross sections suitable for use in photochemical models. Benchmark calculations using a set of molecules and radicals were used to validate the chosen spectral simulation approach. The results show that the smaller S₂-S₄ systems have open-chain geometric isomers as their global minima, while cyclic structures are the most stable for the larger S₅-S₈ species. We also identify two low-energy triplet S₄ isomers (cis-³S₄ and trans-³S₄), which may serve as important intermediates. The simulated spectra indicate that the global minimum S₃ structural isomer, linear-S₃, and a high-energy structural isomer of S₄, trigonal-S₄, absorb in the 320-400 nm window, while S₈ shows negligible absorption in the 320-400 nm range. This work advances our understanding of atmospheric sulfur species beyond Earth and supports the interpretation of observations of the atmosphere of Venus and other exoplanets.