O Atom Generation and Decay in O 2 -Ar Mixtures Dissociated by a Ns Pulse Discharge in a Heated Plasma Flow Reactor

Kinetics of O atom generation and decay is studied in O2-Ar mixtures in a heated plasma flow reactor with optical access for laser diagnostics. The O atom number density is measured by fs Two-Photon Absorption Laser Induced Fluorescence (TALIF), calibrated in xenon. In addition, an alternative calibration approach is used, where most of O2 molecules in the reactant mixture (0.125% O2-Ar) dissociate into O atoms, due to the rapid energy transfer from excited Ar atoms, such that their number density approaches twice the original O2 number density. The present method is not affected by the uncertainty in the O / Xe two-photon absorption cross section ratio and can be used to assess the applicability of O TALIF calibration in Xe for fs TALIF. Time resolved, absolute O atom number densities are measured in preheated O2-Ar mixtures partially dissociated by a burst of ns discharge pulses (up to 300 pulses/burst at 80 kHz) and in the afterglow. The measurements are made at the furnace temperature of T=400 K, P=200 Torr, and O2 mole fractions ranging from 0.125% to 20%. The O atom number densities in mixtures with O2 mole fractions higher than 0.125% are obtained by scaling the respective fluorescence yields.