Formation mechanism of vibrationally excited O2 molecules in the multiphoton absorption of NO2

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Abstract
A pump–probe two‐color experiment has been performed to elucidate the formation mechanism of O2 X 3Σ−g in the multiphoton absorption of NO2 over the region 460–540 nm. A probe dye laser was employed to excite O2 X 3Σ−g into the B 3Σ−u state and the UV emission intensity of Schumann–Runge bands was measured under the various experimental conditions. The maximum vibrational level of O2 X 3Σ−g formed is v‘max =24 which corresponds to Evib=30 968 cm−1. The rotational distribution of O2 X 3Σ−g (v‘=24) was almost of Boltzmann with Trot=1300 K at low pressures. The isotopic 1:1 mixture of N16O2 and N18O2 has been photolyzed to test whether the O2 molecules are formed by unimolecular dissociation or through chemical reactions. From the product branching ratio of 16O2:16O18O:18O2 and the maximum vibrational levels observed, the vibrationally excited O2 molecules are concluded to be mainly generated by the chemical reaction of O(1D)+NO2→O2+NO, ΔH=32 000 cm−1. The O(1D) atoms are formed by a sequential three‐photon absorption of NO2, where the initial two‐photon absorption occurs through the 1 2B2←X̃ 2A1 transition in a cyclic manner and a certain collision‐induced process takes place in a dense system of the predissociative states locating in 11 900–19 400 cm−1 above the dissociation threshold of NO(2Π)+O(3P).