The Mechanism of the Solvent Perturbation of the aΔg→ X3Σg-Radiative Transition of O2

The rate constants k b - a and k a - X of the b 1 Σ g + → a 1 Δ g and a 1 Δ g → X 3 Σ g - emissions of O 2 have been determined in liquid CCl 4, C 2 Cl 4, and C 4 Cl 6 . The ratios k a - X / k b - a range from 4.4 × 10 -4 (CCl 4 ) to 8 × 10 -4 (C 4 Cl 6 ). In addition, rate constants k a - X have been determined in several solvents and in the binary solvent mixtures H 2 O/acetone, acetone/C 6 H 6, and CH 3 OH/CHCl 3 . k a - X depends for H 2 O/acetone and CH 3 OH/CHCl 3 in a strongly anomalous way on the bulk polarizability P of the solvent, demonstrating that no general smooth correlation of k a - X with P exists. Our results confirm the perturbation model developed by Minaev. According to this model, the collision-enhanced b → a radiative transition lends intensity to the transition a → X. Both radiative transitions are bimolecular processes. For the pure solvents, the second-order rate constants correlate roughly with the square of the molar refraction R of the solvent. If the effects of the solvent refractive index, of the collision frequency, and of the dependence of the probability P a - X of the collision-induced radiative transition per collision on the size of the collider, are removed, a direct and linear proportionality of the transition moment of the collision-induced emission with the collider's molecular polarizability is discovered. For mixtures, k a - X is additively composed of the contributions of the individual components. These results explain for the first time consistently and quantitatively the solvent effects on k a - X .