Quantum Yield and Mechanism of Singlet Oxygen Generation via UV Photoexcitation of O₂–O₂ and N₂–O₂ Encounter Complexes

Abstract

The mechanism and spectral dependence of the quantum yield of singlet oxygen O₂(a ¹Δ_g) 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′ ³Δ_u), which is assumed to be responsible for singlet oxygen production in the relaxation process O₂(A′ ³Δ_u, υ) + O₂(X ³Σ_g^–) → O₂({a ¹Δ_g}, {b ¹Σ_g⁺}) + O₂({a ¹Δ_g, υ = 0}, {b ¹Σ_g⁺, 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 ¹Δ_g) 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 ¹Δ_g) formation from encounter complex N₂–O₂ measured at λ = 266 nm was found to be the same as that for O₂–O₂