Collision-induced b1Σg+–a1 Δg, b1Σg+–X3 Σg- and a1Δg–X3Σg - transition probabilities in molecular oxygen

Multireference configuration interaction (MRCI) calculations have been performed for the dipole transition moments of the Noxon band, b 1 Σ g + –a 1 Δ g , and for the red and IR atmospheric emission bands, b 1 Σ g + –X 3 Σ g - and a 1 Δ g –X 3 Σ g - , in O 2 in collision complexes with different solvent molecules. The spin–orbit coupling between the b 1 Σ g + and X 3 Σ g - (M S =0) states does not change after collision so the a–X transition borrows intensity from the collision-induced Noxon band b–a, and the b–X band borrows electric dipole transition probability from the collision-induced difference dipole moments of the b and X states. The calculations show that the b–a, a–X and b–X transition probabilities are enhanced by ca. 10 5 , 10 3 and 1.2 times by O 2 +H 2 collisions. An additional order of magnitude enhancement for all three transitions is possible for solvent molecules with larger polarizability than that of molecular hydrogen. The collision-induced b–a transition dipole moment depends not only on the intermolecular distance, but also on the internuclear O–O distance. Since a–X and b–a transition moments are directly connected (D a,X =0.013iD b,a ), the strong dependence on the O 2 bond length leads to an additional enhancement of the (0,1) vibronic band intensity of the a 1 Δ g –X 3 Σ g - transition. The dipole moments of the b and X states are also shown to depend on the O 2 bond length in the collision complex, which leads to additional collision-induced enhancement of the (0,1) band of the b–X system.