Diatomic sulfur: Low lying bound molecular electronic states of S ₂

We present here the results of self-consistent field (SCF) and configuration interaction (CI) type calculations on thirteen low-lying electronic states of diatomic sulfur. The basis set was one of double zeta quality augmented with polarization functions. The CI space for each electronic state consisted of all configurations constructed from single and double excitations of electrons from the valence orbitals of the Hartree-Fock configuration. There are several significant findings of this study. First, we report the discovery of a previously unobserved and bound ¹Π _u state which lies approximately 37000 cm^-1 above the ground state. This state dissociates to two ground state sulfur atoms. Second, we provide new predictions of excitation energies and properties for the three states e ¹Π_g, c ¹Σ_u ^-, and B″³Π_u. These states were suspected or known to be bound, but experimentally determined properties were uncertain. Finally, we find that systematic application of a formula of Davidson, which estimates the contribution of unlinked energy terms in a singles and doubles CI calculation, leads to improved predictions of excitation energies.