MRCI study of the spectroscopic parameters and transition properties of the 36 low-lying electronic states of the B₂...

This paper studied the spectroscopic and transition properties of 36 low-lying states, which came from the first two dissociation limits of B₂ molecule. The potential energy curves were calculated with the complete active space self-consistent field (CASSCF) method, which was followed by the internally contracted multireference configuration interaction (icMRCI) plus Davidson modification (icMRCI+Q) approach. Of these 36 states, the 2⁵Σ_u^-, 1⁵Σ_u⁺, 2⁵Π_u, and 1⁵Δ_u states were repulsive; the B³Δ_u, E³Σ_u⁺, f¹Σ_u^-, g¹Π_g, 2³Π_u, 3³Σ_g^-, 3³Π_u, 1⁵Π_g, and 3³Σ_u⁺ states had double wells; the B³Δ_u, E³Σ_u⁺, G³Σ_u⁺, f¹Σ_u^-, g¹Π_g, 3³Σ_g^-, 2³Π_u, 3³Π_u, 1⁵Π_g, 2⁵Π_g, 2⁵Σ_g^-, and 3³Σ_u⁺ states had one barrier; the 2⁵Σ_g^- state and the second wells of B³Δ_u, E³Σ_u⁺, 1⁵Π_g, f¹Σ_u^-, g¹Π_g, and 2³Π_u states were weakly bound; and the 2⁵Σ_g^- state had no vibrational levels. The avoided crossings existed between the B³Δ_u and 2³Δ_u states, the E³Σ_u⁺ and G³Σ_u⁺ states, the G³Σ_u⁺ and 3³Σ_u⁺ states, the 3³Σ_u⁺ and 4³Σ_u⁺ states, the 2³Π_u and 3³Π_u states, the g¹Π_g and 2¹Π_g states, the 2³Σ_g^- and 3³Σ_g^- states, the 1⁵Π_g and 2⁵Π_g states, the 2⁵Π_g and 3⁵Π_g states, the 2⁵Σ_g^- and 3⁵Σ_g^- states, as well as the F³Π_g and 3³Π_g states. Core-valence correlation and scalar relativistic corrections were taken into account. The extrapolation to the complete basis set limit was done. The spectroscopic parameters and vibrational properties were obtained. The transition dipole moments were calculated. Franck-Condon factors of some transitions were evaluated. The spin-orbit coupling (SOC) effect on the spectroscopic parameters and vibrational properties is tiny and sometimes even can be negligible. The results determined in this paper can provide some powerful guidelines to observe these states, in particular the states which have not been studied in the experiment.