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The formation of singlet oxygen 1O2 provided by the photoexcitation of the encounter complexes of isoprene with oxygen (C5H8-O2) in the gas phase within the spectral region 253.5-355 nm has been observed at the elevated pressure of oxygen. Singlet oxygen has been detected with its NIR luminescence centered near 1.27 μm. The photogeneration of 1O2 is found to be a one-photon process. In the UV-C region (253-278 nm) the quantum yield of 1O2 is measured. This yield of 1O2 is governed mainly by photoexcitation of O2 molecules to the Herzberg III (3Δu) state via enhanced absorption by C5H8-O2 collision complexes. So excited triplet O2 gives rise to singlet oxygen because of triplet-triplet annihilation in the collisions with unexcited O2 molecules. In the UV-B (308 nm) region the appearance of 1O2 is attributed to the excitation of a double spin-flip (DSF) transition in complex C5H8-O2. In the UV-A region (355 nm) besides DSF the O2-assisted T1 ← S0 excitation of isoprene to the triplet state takes place, which is a sensitizer of 1O2 formation. The contribution of the encounter complexes C5H8-O2 to the production of singlet oxygen and to the lifetime of isoprene in the Earth's troposphere are estimated.

The mechanism and spectral dependence of the quantum yield of singlet oxygen O(2)(a (1)Δ(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 O(2)-O(2). This absorption gives rise to oxygen in the Herzberg III state O(2)(A' (3)Δ(u)), which is assumed to be responsible for singlet oxygen production in the relaxation process O(2)(A' (3)Δ(u), υ) + O(2)(X (3)Σ(g)(-)) → O(2)({a (1)Δ(g)}, {b (1)Σ(g)(+)}) + O(2)({a (1)Δ(g), υ = 0}, {b (1)Σ(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 O(2)-O(2) pair. The observed drop of the O(2)(a (1)Δ(g)) yield near spectral threshold for O(2) dissociation is explained by the competition between above relaxation and reaction giving rise to O(3) + O (O + O + O(2)) supposed in literature. The quantum yield of O(2)(a (1)Δ(g)) formation from encounter complex N(2)-O(2) measured at λ = 266 nm was found to be the same as that for O(2)-O(2).