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 S(3P)+S(3P) and S(3P)+S(1D) 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 B″3Πu and B3Σu- excited states. The total absorption cross-sections are computed to estimate isotope-fractionation constants, ε, for four most common isotopologues: 32S32S, 32S33S, 32S34S, and 32S36S 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 εZPE 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 36S and 33S with an excitation wavelength-dependent fluctuation.