Singlet Oxygen Generation via UV-A, -B, and -C Photoexcitation of Isoprene-Oxygen (CH-O) Encounter Complexes in the Gas Phase.

The formation of singlet oxygen O provided by the photoexcitation of the encounter complexes of isoprene with oxygen (CH-O) 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 O is found to be a one-photon process. In the UV-C region (253-278 nm) the quantum yield of O is measured. This yield of O is governed mainly by photoexcitation of O molecules to the Herzberg III (Δ) state via enhanced absorption by CH-O collision complexes. So excited triplet O gives rise to singlet oxygen because of triplet-triplet annihilation in the collisions with unexcited O molecules. In the UV-B (308 nm) region the appearance of O is attributed to the excitation of a double spin-flip (DSF) transition in complex CH-O. In the UV-A region (355 nm) besides DSF the O-assisted T ← S excitation of isoprene to the triplet state takes place, which is a sensitizer of O formation. The contribution of the encounter complexes CH-O to the production of singlet oxygen and to the lifetime of isoprene in the Earth's troposphere are estimated.