Theoretical study of low-lying electronic states of BiCl, BiBr, and BiI

Ab initio calculations on the low-lying Omega states of BiX (X = Cl, Br, and I) below 45 000 cm-1 have been performed using the multi-reference configuration interaction and the equation-of-motion coupled-cluster methods with the spin-orbit coupling effects being included perturbatively and variationally, respectively. Totally ten Omega states are found to be bound ones for each molecule, including the ground state X10+, the first excited state X21, five higher-lying valence excited states (a2, A ' 0-, A0+, A '' 1, and B0+), and three Rydberg states (C10-, C20+, and C31) originating from the Bi 7s occupation. The derived spectroscopic constants as well as the estimated radiative lifetimes are overall consistent with the available experimental values. Our results may enhance the understanding on the distribution of electronic states of BiX and provide theoretical references for future spectroscopic observations.