Ab initio relativistic configuration interaction calculations of the spectrum of bismuth oxide: Potential curves and transition probabilities

A series of configuration interaction calculations employing relativistic effective core potentials including the spin–orbit interaction is reported for the X1 2Π1/2 ground and numerous low-lying excited states of the bismuth oxide molecule up to 30 000 cm−1. Special difficulties connected with the treatment of open-shell systems and double-group irreducible representations are discussed and a feasible computation scheme is developed for dealing with them. The spin–orbit interaction is found to cause a high level of mixing between a variety of low-lying λ–s states, producing a number of avoided crossings which play a key role in determining the character of the BiO spectrum. A comparison with existing experimental data for both the energy locations and intensities of a large number of band systems indicates that the present calculations are capable of predicting Te values to an accuracy of 0.1–0.2 eV. Corresponding radiative lifetime results generally agree within a factor of 2, with the best experience occurring for relatively strong transitions. The state which was originally assigned as A 2Π1/2 actually turns out to be dominated by the 4Π λ–s state. The corresponding state with Ω=3/2 has recently been discovered by Fink and Shestakov and is found to undergo a strong nonadiabatic interaction with the X2 2Π3/2 state. Two other related states A3 4Π5/2 and A4 4Π1/2 are predicted by the present calculations, but have not yet been verified experimentally. Similarly, the L1 2Φ7/2 and L2 2Φ5/2 states found in the present work have also not yet been observed.