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

Abstract

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 25Σu−, 15Σu+, 25Πu, and 15Δu states were repulsive; the B3Δu, E3Σu+, f1Σu−, g1Πg, 23Πu, 33Σg−, 33Πu, 15Πg, and 33Σu+ states had double wells; the B3Δu, E3Σu+, G3Σu+, f1Σu−, g1Πg, 33Σg−, 23Πu, 33Πu, 15Πg, 25Πg, 25Σg−, and 33Σu+ states had one barrier; the 25Σg− state and the second wells of B3Δu, E3Σu+, 15Πg, f1Σu−, g1Πg, and 23Πu states were weakly bound; and the 25Σg− state had no vibrational levels. The avoided crossings existed between the B3Δu and 23Δu states, the E3Σu+ and G3Σu+ states, the G3Σu+ and 33Σu+ states, the 33Σu+ and 43Σu+ states, the 23Πu and 33Πu states, the g1Πg and 21Πg states, the 23Σg− and 33Σg− states, the 15Πg and 25Πg states, the 25Πg and 35Πg states, the 25Σg− and 35Σg− states, as well as the F3Πg and 33Πg 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.