Rotational line strengths in 3Σ+–3Σ− electronic transitions. The β3Σu+ – X3Σg− and A3Σu+ – X3Σg− systems of molecular oxygen

Rotational line strengths are given for 3Σ+(int) – 3Σ−(int) transitions arising from spin–orbit coupling. Observed branch intensities for the forbidden β3Σu+ – X3Σg− transition of O2 may be explained by assuming spin–orbit mixing of β3Σu+ with the B3Σu− and E3Σu− states. On the other hand, observed branch intensities for the Herzberg I A3Σu+ – X3Σg− transition of O2 may be explained only by assuming mixing with 3Σ and 3Π states. In neither case do earlier formulae, derived assuming a single 3Π perturber, apply.