Mechanisms for the Excitation of Singlet Molecular Oxygen.

Available from UMI in association with The British Library. Requires signed TDF. A discharge-flow system with chemiluminescence detection has been used to study a number of reactions which could potentially form O_2(a ^1Lambda_{g}) as one of the products. The main aim of the work was to investigate the reaction between O atoms and vibrationally excited hydroxyl radicals. In the system used to study this reaction, a number of interfering reactions could also occur. Therefore, the reactions of H atoms and NO _{rm x}^ecies with O_2 and O_3 were also investigated. In the reaction of NO with O_3 , it was shown that the fraction of reaction leading to O_2(a^1Lambda _{g}) accounted for less than 3 times 10^{-5 } of the total reaction. The rate constant for the total reaction was measured using emission from excited NO_2 at lambda ~eq 1300 nm to monitor (NO), and a value of (1.9 +/- 0.2) times 10^{-14 } cm^3molecule ^{-1} s^{-1 } was obtained. The rate constants for the reactions of N atoms with O_2 and O_3 were measured and found to be (8.8 +/- 0.4) times 10^ {-17} and (1.0 +/- 0.2) times 10^{ -16} cm^3molecule ^{-1}s^{-1 }, respectively. In these reactions, emission from O_2(a^1Lambda _{g}) was observed, but only in the presence of H-atom impurities. In the reaction of N atoms with O atoms, novel emissions were observed. The emissions were shown to be part of the Ogawa bands of NO(b^4Sigma ^- to a^4 Pi). Vibrational constants were calculated and emission intensities shown to correlate with calculated Franck-Condon factors. In the reactions of H atoms with O_2 and O_3, O_2 (a^1Lambda_{g} ) was observed over and above that expected from its generation in the H + HO_2 reaction. In both of these systems the formation of O_2 (a^1Lambda_{g} ) was attributed to the reaction of O atom with vibrationally excited OH. It was shown that the fraction of reaction leading to the excited species is at least 2%, and may be sufficiently efficient to be a major source of O_2(a^1Lambda _{g}) in the Earth's atmosphere.