Theoretical study of low-lying electronic states of BiN, BiP, BiAs, and BiSb

Abstract

For the pnictogen atoms, the first two levels ${}^4{S}^{\circ }$ and ${}^2{D}^{\circ }$ may couple together through the spin–orbit coupling (SOC) effects with the aid of the third level ${}^2{P}^{\circ }$, leading to a complicated computational procedure to take SOC into account. In this work, we theoretically study the low-lying valence states of BiPn (Pn = N, P, As, and Sb), where 102 $\Lambda$-S states from nine dissociation asymptotes split into 242 $\Omega$ states. In view of too many $\Lambda$-S and $\Omega$ states, some special treatments have been considered. In the $\Lambda$-S state calculation, which is the most computationally intensive part, we employ the recently developed multi-reference configuration interaction (MRCI) method in the efficient static-dynamic-static (SDS) framework. The more reliable two-component MRCI method has also been performed to compute some spectroscopic parameters for validation. The calculated spectroscopic constants of low-lying $\Omega$ states are generally in good agreement with the available experimental ones, but the experimental dissociation energies of BiP and BiSb seem seriously underestimated and need to be remeasured. Some new spectral transitions have also been predicted, which provide useful references for future experiments.