Nonadiabatic perturbations and fine structure splittings in the 1,2 3Πg states of B2: An analysis based on adiabatic and rigorous diabatic states

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Abstract
Nonadiabatic effects and fine structure splittings in the 1,2 3Πg states of B2 were studied. Adiabatic electronic wave functions Ψa(J 3Πg) were computed at the multireference configuration interaction level. The interstate derivative couplings 〈Ψa(1 3Πg)‖(d/dR)Ψa(2 3Πg)〉 were computed and used to construct ‘‘rigorous’’ diabatic electronic states. The spin–orbit interactions responsible for the fine structure splitting in the 1,2 3Πg states were determined, as were the 1,2 3Πg∼1 1Πg spin–orbit interactions. These electronic structure data were used to determine the vibrational structure of the 1,2 3Πg states in both coupled adiabatic and coupled diabatic states bases. As a result of an avoided crossing of the 1,2 3Πg states the fine structure splitting constants exhibit a marked geometry dependence. The fine structure splitting constants for the (1 3Πg, v=0–4) levels were found to be −4.2(−4.4), −3.7, −2.8, −1.8, and −0.83 cm−1, respectively. The (1 3Πg, v=0) value is in good agreement with a recent experimental determination given parenthetically. While the diabatic basis provides illuminating qualitative insights into the electronic structure of the states in question the adiabatic basis is preferred computationally.