Solvent and Heavy-Atom Effects on the O₂(X³Σ_g^-) → O₂(b¹Σ_g⁺) Absorption Transition

The effect of 16 liquid solvents on both the spectrum and molar absorption coefficient of the X³Σ_g^- → b¹Σ_g⁺ 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(b¹Σ_g⁺) → O₂(a¹Δ_g) transition, which allowed the use of O₂(a¹Δ_g) phosphorescence as a sensitive probe of O₂(b¹Σ_g⁺) production. The results of these O₂(a¹Δ_g) phosphorescence experiments are consistent with the results of independent experiments in which the O₂(a¹Δ_g) thus produced was "trapped" via a chemical reaction. The data recorded were used to calculate rate constants for the O₂(b¹Σ_g⁺) → O₂(X³Σ_g^-) radiative transition, a parameter that is otherwise difficult to directly obtain from such a wide range of solvents using O₂(b¹Σ_g⁺) → O₂(X³Σ_g^-) phosphorescence. The data show that the response of the O₂(b¹Σ_g⁺) → O₂(X³Σ_g^-) radiative transition to solvent is not the same as that of the O₂(b¹Σ_g⁺) → O₂(a¹Δ_g) and O₂(a¹Δ_g) → O₂(X³Σ_g^-) 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.