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

Abstract

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.