Spin-forbidden radiative decay involving quasidegenerate states. Application to the B 1Σ+→a 3Π transition in MgO

The spin–orbit induced interactions among the low-lying X, B 1Σ+, a 3Π, A 1Π, and c 3Σ− states of MgO are studied with emphasis on the experimentally observed B 1Σ+→a 3Π spin–forbidden dipole-allowed radiative transition. A dressed diabatic states approach is used in which the spin–orbit induced perturbation of the (zeroth order) a 3Π0+ and a 3Π1 fine structure states is partitioned into a contribution from the zeroth order X 1Σ+0+ and A 1Π1 states and contributions from all other remaining states of 1Σ+0+ and 1Π1 symmetry. This perturbation–partitioning approach extends a recently developed technique for determining spin–orbit perturbed wave functions directly in the CSF basis based on first order perturbation theory [J. Chem. Phys. 83, 1168 (1985)] to situations involving quasidegenerate electronic states. The wave functions in question are expanded in second order configuration state function spaces of between 60 000–100 000 terms. The parallel [ μ∥(B 1Σ+0+, v=0; a 3Π0+ v=n)] and perpendicular [ μ⊥(B 1Σ+0+, v=0; a 3Π1, v=n)] components of the spin–forbidden dipole-allowed transition moment for the B 1Σ+, v=0→a 3Π, v=n transition were obtained. It was found that μ⊥(B 1Σ+0+, v=0; a 3Π1, v=n) peaks at n=1 and that for 0≤n≤3 we have μ⊥(B 1Σ+0+,v=0; a 3Π1, v=n) >μ∥ (B 1Σ+0+,v=0; a 3Π0+,v=n)]. The spin–orbit induced mixings of the zeroth order X 1Σ+0+ and A 1Π1 states with the a 3Π0+ and a 3Π1 states are responsible for the preponderance of the observed B→a (0,0) and (0,1) transition moments.