Low-lying [formula omitted] electronic states of BiF: Perturbative versus variational approaches of spin–orbit coupling

Abstract

The low-lying Ω states of bismuth monofluoride (BiF) below 50000 cm−1 have been theoretically studied using the multi-reference configuration interaction and the equation-of-motion coupled-cluster methods, where the spin–orbit coupling (SOC) effects are respectively considered perturbatively or variationally. Since the perturbative treatment of SOC is not a good approximation for the heavy atom Bi, the latter method exhibits higher accuracy and better agreements with the experimental spectroscopic constants. Our results show that the second 3Π state of BiF is due to the occupation on the Rydberg 7s-shell of Bi, which gives rise to the so-called “triplet C” state and its sub-states reported in the early literatures. With the help of our theoretical results, some experimentally assigned spectral bands with considerable confusion have also been clarified and reassigned.