Production and reactivity of the hydroxyl radical in homogeneous high pressure plasmas of atmospheric gases containing traces of light olefins

A photo-triggered discharge has been used to study the production kinetic mechanisms and the reactivity of the hydroxyl radical in a N-2/O-2 mixture ( 5% oxygen) containing ethane or ethene for hydrocarbon concentration values in the range 1000-5000 ppm, at 460 mbar total pressure. The discharge ( current pulse duration of 60 ns) has allowed the generation of a transient homogeneous non-equilibrium plasma, and the time evolution of the OH density has been measured ( relative value) in the afterglow ( up to 200 mu s) by laser induced fluorescence ( LIF). Experimental results have been explained using predictions of a self-consistent 0D discharge and plasma reactivity modelling, and reduced kinetic schemes for OH have been validated. It has been shown that recombination of H- and O-atoms, as well as reaction of O with the hydroperoxy radical HO2, plays a very important role in the production of OH radicals in the mixture with ethane. H is a key species for production of OH and HO2 radicals. As for ethane, O, H and HO2 are key species for the production of OH in the case of ethene, but carbonated radicals, following the partial oxidation of the hydrocarbon molecule by O, also play a non-negligible role. The rate constant for O- and H-atom recombination has been estimated to be 3 x 10(-30) cm(6) s(-1) at near ambient temperature, consistent with LIF measurements on OH for both mixtures with ethane and ethene.