Measurements and kinetic modeling of O2 vibrational Kknetics in O2-Ar mixtures partially dissociated by a Ns pulse discharge

Abstract

Abstract Vibrational kinetics of O2 is studied during the O atom recombination in an O2-Ar mixture, partially dissociated by a burst of ns discharge pulses in a heated plasma flow reactor. The time-resolved temperature in the discharge afterglow is determined by Rayleigh scattering. Time-resolved O atom number density is measured by ps Two-Photon absorption Laser Induced Fluorescence (TALIF), calibrated in xenon. Time-resolved vibrational level populations of molecular oxygen, O2(v=8-20), are measured by ps Laser Induced Fluorescence (LIF), with the absolute calibration by NO LIF. Time-resolved ozone number density is monitored by broadband UV absorption. The results are compared with the predictions of a state-specific kinetic model. The experimental data indicate a rapid initial decay of O2(v) populations generated by electron impact in the discharge, due to the vibration-translation (V-T) relaxation by O atoms. This is followed by a slower population reduction, on the time scale much longer compared to that for V-T relaxation or vibration-vibration (V-V) exchange. Both O atoms and the O2(v) populations decay on the same time scale, indicating that chemical reactions initiated by the O atom recombination result in the generation of vibrationally excited O2 molecules. These trends are reproduced by the kinetic model, which shows that the reaction of O atoms with ozone is the dominant pathway of O2(v) generation at the present conditions. The predicted relative O2(v) populations are close to the experimental results, but absolute number densities differ from the experimental data. This is likely due to uncertainties in the absolute calibration of LIF measurements and in the spectroscopic model used in the data reduction. The present work demonstrates the capability for the absolute, time-resolved measurements of vibrationally excited O2 in recombining gas flows, to quantify the energy partition in the recombination reactions.