The radiative lifetimes of the b1Σ+ and a1Δ states of O2, S2 and SO have been evaluated by perturbation expansions including 3Σg−, 1Δg, 1Σg+, 1Πg, 3Πg states for the homonuclear systems and 3Σ− (2), 1Δ, 1Σ+ (2), 1Π (2), 3Π (2), 3Σ+ states for SO; all wavefunctions are MRD CI expansions up to 5000 terms. The mixing coefficients are obtained from the spin-orbit operator in the Breit-Pauli form evaluating all one- and two-particle terms explicitly. In O2 and S2 the radiative lifetime of the b1Σg+ state is found to be largely determined by the spin-contributions to the magnetic moment in the magnetic dipole operator. The calculated value of 11.65 s for O2 is in excellent agreement with the measured value of 12 s; the calculations predict a lifetime of 3.4 s for b1Σg+ in S2. The calculated lifetime corresponding to the b1Σg+ −a1Δg transition is 720 s in good accord with the experimental intensity determination (400 s within a factor of two). The intensity for the a1Δg −X3Σg− transition is dominated by the orbital angular momentum term in the magnetic-dipole operator and arises from 1Δg −1Πg and X 3Σg− −3Πg transitions present in the perturbed X 3Σ− and a1Δg wavefunctions. Calculated values are τ(O2) = 5400 s relative to a measured value of 3900 s, and τ = 350 s for S2 as a prediction. The absence of the inversion favors electric (rather than magnetic) dipole processes in SO. The b1Σ+ −X3Σ− transition borrows its intensity predominantly from terms connecting b1Σ+ −21Σ+ and X 3Σ− −23Σ− which occur as perturbers in the pure spin wavefunctions. The calculated b1Σ+ lifetime is 13.6 ms in fair accord with the recently measured 7 ± 2 ms. For τ (a1Δ) the calculations predict intensity borrowing from A3Π−X 3Σ− and C 3Π−X 3Σ− as well as 1Δ−1Π and 1Δ−21Π dipole transitions resulting in a value of 450 ms. The decrease in lifetime from the first- to the second-row molecules is quantitatively demonstrated to arise from increased spin-orbit interaction, while a different mechanism is responsible for the change in b1Σ+ lifetimes from homonuclear to heteronuclear systems. Finally, all calculations demonstrate that spin-forbidden radiative transition probabilities can be obtained quite effectively by modern-day quantum chemical calculations.
