Solvent and Heavy-Atom Effects on the O2(X3Σg-) → O2(b1Σg+) Absorption Transition.

The effect of 16 liquid solvents on both the spectrum and molar absorption coefficient of the X3Σg- → b1Σg+ transition in molecular oxygen has been examined. The ability to monitor this weak transition using air or oxygen saturated samples at atmospheric pressure was facilitated by the rapid and efficient O2(b1Σg+) → O2(a1Δg) transition, which allowed the use of O2(a1Δg) phosphorescence as a sensitive probe of O2(b1Σg+) production. The results of these O2(a1Δg) phosphorescence experiments are consistent with the results of independent experiments in which the O2(a1Δg) thus produced was "trapped" via a chemical reaction. The data recorded were used to calculate rate constants for the O2(b1Σg+) → O2(X3Σg-) radiative transition, a parameter that is otherwise difficult to directly obtain from such a wide range of solvents using O2(b1Σg+) → O2(X3Σg-) phosphorescence. The data show that the response of the O2(b1Σg+) → O2(X3Σg-) radiative transition to solvent is not the same as that of the O2(b1Σg+) → O2(a1Δg) and O2(a1Δg) → O2(X3Σg-) radiative transitions, both of which have been extensively examined over the years. However, our data are consistent with a theoretical model proposed by Minaev for the effect of solvent on radiative transitions in oxygen and, as such, arguably provide one of the final chapters in describing a system that has challenged the scientific community for years.