MRCI study of the spectroscopic parameters and transition properties of the 36 low-lying electronic states of the B 2 molecule.

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 25 Σu - , 15 Σu + , 25 Πu , and 15 Δu states were repulsive; the B3 Δu , E3 Σu + , f1 Σu - , g1 Πg , 23 Πu , 33 Σg - , 33 Πu , 15 Πg , and 33 Σu + states had double wells; the B3 Δu , E3 Σu + , G3 Σu + , f1 Σu - , g1 Πg , 33 Σg - , 23 Πu , 33 Πu , 15 Πg , 25 Πg , 25 Σg - , and 33 Σu + states had one barrier; the 25 Σg - state and the second wells of B3 Δu , E3 Σu + , 15 Πg , f1 Σu - , g1 Πg , and 23 Πu states were weakly bound; and the 25 Σg - state had no vibrational levels. The avoided crossings existed between the B3 Δu and 23 Δu states, the E3 Σu + and G3 Σu + states, the G3 Σu + and 33 Σu + states, the 33 Σu + and 43 Σu + states, the 23 Πu and 33 Πu states, the g1 Πg and 21 Πg states, the 23 Σg - and 33 Σg - states, the 15 Πg and 25 Πg states, the 25 Πg and 35 Πg states, the 25 Σg - and 35 Σg - states, as well as the F3 Πg and 33 Πg 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.