Direct observation of OH radicals after 565 nm multi-photon excitation of NO2 in the presence of H2O

The study of a controversial reaction, NO 2 ∗ + H 2 O as a potential new source of OH radicals in the atmosphere, has been performed using the coupling of a FAGE instrument to a laser photolysis cell. No OH radicals were observed using an unfocused excitation laser beam at 565 nm, but OH formation was observed by focusing the beam even though this reduces the instrument sensitivity. The dependence of the OH-fluorescence signal with the laser energy shows that the OH radicals originate from a complex mechanism including multiphoton absorption of NO2.