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 13Πu and 13Σu- excited states. Furthermore, 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. Zero-point energy based constants εZPE are estimated as well to compare with the obtained isotope effects. Three-isotope plots are shown to express mass-independent fractionation effect; large isotopic effects were found on 36S and 33S in 230-240 nm range, and only on 36S in 240-267 nm range.