Quantum Yield and Mechanism of Singlet Oxygen Generation via UV Photoexcitation of O2–O2 and N2–O2 Encounter Complexes
The mechanism and spectral dependence of the quantum yield of singlet oxygen O2(a 1Δg) photogenerated by UV radiation in gaseous oxygen at elevated pressure (32–130 bar) have been experimentally investigated within the 238–285 nm spectral region overlapping the range of the Wulf bands in the absorption spectrum of oxygen. The dominant channel of singlet oxygen generation with measured quantum yield up to about 2 is attributed to the one-quantum absorption by the encounter complexes O2–O2. This absorption gives rise to oxygen in the Herzberg III state O2(A′ 3Δu), which is assumed to be responsible for singlet oxygen production in the relaxation process O2(A′ 3Δu, υ) + O2(X 3Σg–) → O2({a 1Δg" {b 1Σg+}) + O2({a 1Δg, υ = 0" {b 1Σg+, υ = 0}) with further collisional relaxation of b to a state. This mechanism is deduced from the analysis of the avoiding crossing locations on the potential energy surface of colliding O2–O2 pair. The observed drop of the O2(a 1Δg) yield near spectral threshold for O2 dissociation is explained by the competition between above relaxation and reaction giving rise to O3 + O (O + O + O2) supposed in literature. The quantum yield of O2(a 1Δg) formation from encounter complex N2–O2 measured at λ = 266 nm was found to be the same as that for O2–O2.