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 NO2 at discrete wavelengths beyond the photodissociation threshold in the presence of water vapor: NO2* + H2O --> 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 NO2, leading to O(D-1) and thus to OH in the presence of H2O, i.e., NO2 + hv --> NO2*, NO2* + hv --> NO2**, NO2** --> NO + O(D-1), O(D-1) + H2O --> 2OH. The cross section for the transition NO2** <-- NO2* was found to be similar to that for the NO2* <-- NO2 transition at 435 nm. Al 532 nm, the two-photon process is trot sufficiently energetic to form O('D), and OH is not observed. An upper limit of approximately 10(-5) is found for the reactive quenching of NO2* by water vapor relative to collisional quenching. The atmospheric relevance of OH formation via NO2 excitation is discussed.