The mechanism of the solvent perturbation of the a1Δg→X3Σg- radiative transition of O2

The rate constants k(b-a) and k(a-X) of the b(1)Sigma(g)(+) --> a(1)Delta(g) and a(1)Delta(g) --> X(3)Sigma(g)(-) emissions of O-2 have been determined in liquid CCl4, C2Cl4, and C4Cl6. The ratios k(a-X)/k(b-a) range from 4.4 x 10(-4) (CCl4) to 8 x 10(-4) (C4Cl6). In addition, rate constants k(a-X) have been determined in several solvents and in the binary solvent mixtures H2O/acetone, acetone/C6H6, and CH3OH/CHCl3. k(a-X) depends for H2O/acetone and CH3OWCHC4 in a strongly anomalous way on the bulk polarizability P of the solvent, emonstrating 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 k(a-X)(c) 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 Pa-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).