Spectroscopic identification of the low-lying electronic states of S₂ molecule

As is well-known, the S₂ molecule is a ubiquitous intermediate in the combustion, atmosphere, and interstellar space. The six low-lying bound states of S₂ have been characterized via photoelectron velocity map imaging and a high-level multi-reference configuration interaction method with the Davidson correction. Spectroscopic constants have been extracted by fitting the potential energy curves extrapolated to the complete basis set limit with a series of Dunning's correlation-consistent basis sets: aug-cc-pV(Q, 5)Z. The calculated spectroscopic parameters well reproduce the experimental results in this work. On the basis of the theoretical calculations, Franck-Condon simulations are performed to assign six adjacent electronic states, especially for three higher overlapping electronic states (c¹Σ_u ^-, A'³Δ_u, and A³Σ_u ⁺). The dissociation energy D_e of the S₂ ^- is evaluated to be 4.111 (4) eV in this work, in agreement with the theoretical prediction (4.056 eV).