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.