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 X¹Σ⁺, A¹Π, B¹Σ⁺, C¹Δ, D¹Σ^-, E¹Δ, a³Π, b³Σ⁺, and c³Δ states of BeS, and even some of the states have no theoretical and experimental reports on the relevant aspects of these transitions. As a consequence, we derived these transition properties in this study. The potential energy curves of X¹Σ⁺, A¹Π, B¹Σ⁺, C¹Δ, D¹Σ^-, E¹Δ, a³Π, b³Σ⁺, c³Δ, d³Σ^-, and e³Σ^- 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 (CASSCF) method, followed by the internally contracted multireference configuration interaction (icMRCI) 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 A¹Π and b³Σ⁺ states, 10^-7 s for the E¹Δ state, 10^-7 – 10^-8 s for the C¹Δ and D¹Σ^- states, and 10^-8 s for the B¹Σ⁺, c³Δ, and e³Σ^- states. the transition properties of the seven Ω states generated by the X¹Σ⁺, A¹Π, B¹Σ⁺, and a³Π electronic states including a number of electric dipole-forbidden transitions were researched. In particular, B¹Σ⁺₀₊ – X¹Σ⁺₀₊, B¹Σ⁺₀₊ – A¹Π₁, and A¹Π₁ – X¹Σ⁺₀₊ systems have strong transitions. In this paper, the radiative lifetimes of A¹Π₁, B¹Σ⁺₀₊, a³Π_0-, a³Π₀₊, and a³Π₁ electronic states were also computed. For the A¹Π₁, B¹Σ⁺₀₊, and a³Π₁ 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.