Low-lying Ω electronic states of BiF: Perturbative versus variational approaches of spin–orbit coupling

The low-lying $\Omega$ 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 ${}^3\Pi$ state of BiF is due to the occupation on the Rydberg 7$s$-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.