Theoretical study of low-lying electronic states spectra and transition properties of BeS molecule

There are only limited theoretical results published on the transition properties of the X 1 Σ + , A 1 Π, B 1 Σ + , C 1 Δ, D 1 Σ − , E 1 Δ, a 3 Π, b 3 Σ + , and c 3 Δ states of BeS, and even some of the states have no theoretical and experimental reports on the relevant aspects of these transitions. Therefore, we derived these transition properties in this study. The potential energy curves of X 1 Σ + , A 1 Π, B 1 Σ + , C 1 Δ, D 1 Σ − , E 1 Δ, a 3 Π, b 3 Σ + , c 3 Δ, d 3 Σ − , and e 3 Σ − states of the BeS molecule with their Ω states and the transition dipole moments between them were calculated by the complete active space self-consistent field method, followed by the internally contracted multireference configuration interaction method. For the accurate computation of the transition properties, scalar relativistic corrections and core–valence correlation were considered. The radiative lifetimes were approximately 10 −6 s for the A 1 Π and b 3 Σ + states, 10 −7 s for the E 1 Δ state, 10 −7 –10 −8 s for the C 1 Δ and D 1 Σ − states, and 10 −8 s for the B 1 Σ + , c 3 Δ, and e 3 Σ − states. The transition properties of the seven Ω states generated by the X 1 Σ + , A 1 Π, B 1 Σ + , and a 3 Π electronic states, including several electric dipole-forbidden transitions were researched. In particular, the B 1 Σ + 0+ –X 1 Σ + 0+ , B 1 Σ + 0+ –A 1 Π 1 , and A 1 Π 1 –X 1 Σ + 0+ systems have strong transitions. In this paper, the radiative lifetimes of A 1 Π 1 , B 1 Σ + 0+ , a 3 Π 0− , a 3 Π 0+ , and a 3 Π 1 electronic states were also computed. For the A 1 Π 1 , B 1 Σ + 0+ , and a 3 Π 1 states, the distribution of the radiation lifetime with the rotational angular momentum quantum number J at 0 ≤ υ ≤ 15 and J ≤ 70 was evaluated. It is expected that the outcome of the calculations in this paper can contribute some meaningful guidelines for conducting further theoretical and experimental investigations.