Theoretical study of electronic properties and isotope effects in the UV absorption spectrum of disulfur

The electronic structures of triplet S₂ ground and excited states are studied by ab initio molecular orbital and configuration interaction calculation. Potential energy curves correlated with $\text{S}\left({}^3\text{P}\right)+\text{S}\left({}^3\text{P}\right)$ and $\text{S}\left({}^3\text{P}\right)+\text{S}\left({}^1\text{D}\right)$ at the dissociation limit are evaluated, and electronic terms for a total of 11 states are assigned. Transition dipole moments, as a function of internuclear distance, are determined for two allowed transitions to ${\text{B}}^{″3}{\Pi }_{\text{u}}$ and ${\text{B}}^3{\Sigma }_u^{-}$ excited states. The total absorption cross-sections are computed to estimate isotope-fractionation constants, $\epsilon$, for four most common isotopologues: ${}^{32}{\text{S}}^{32}\text{S}$, ${}^{32}{\text{S}}^{33}\text{S}$, ${}^{32}{\text{S}}^{34}\text{S}$, and ${}^{32}{\text{S}}^{36}\text{S}$ by quantum close-coupling (R-matrix) expansion approach and they are found to lie in a mostly opaque to competing absorbers spectral window. We suggest that the photochemistry and isotopic effects of S₂ are of significant importance and provide data showing high sensitivity of mass-independent fractionation to excitation wavelength. Zero-point energy based constants ${\epsilon }^{\mathrm{ZPE}}$ are estimated as well to compare with the obtained isotope effects and two modes for MIF are present in three-isotope plots; large isotopic effects were observed for both ${}^{36}\text{S}$ and ${}^{33}\text{S}$ with an excitation wavelength-dependent fluctuation.