OH Formation in the Photoexcitation of NO2 beyond the Dissociation Threshold in the Presence of Water Vapor

The pulsed-laser-excitation/resonance fluorescence technique was used to assess the efficiency of OH formation following photoexcitation of NO 2 at discrete wavelengths beyond the photodissociation threshold in the presence of water vapor: NO 2 * + H 2 O → HONO + OH. Excitation at wavelengths between 432 and 449 nm was found to lead to OH production via a facile sequential two-photon absorption by NO 2, leading to O( 1 D) and thus to OH in the presence of H 2 O, i.e., NO 2 + h ν → NO 2 *, NO 2 * + h ν → NO 2 **, NO 2 ** → NO + O( 1 D), O( 1 D) + H 2 O → 2OH. The cross section for the transition NO 2 ** ← NO 2 * was found to be similar to that for the NO 2 * ← NO 2 transition at 435 nm. At 532 nm, the two-photon process is not sufficiently energetic to form O( 1 D), and OH is not observed. An upper limit of approximately 7 × 10 -5 is found for the reactive quenching of NO 2 * by water vapor relative to collisional quenching. The atmospheric relevance of OH formation via NO 2 excitation is discussed.