Measurement of hydroxyl radical (OH) concentration in an argon RF plasma jet by laser-induced fluorescence

Abstract

The concentration of hydroxyl (OH) radicals in a plasma pencil, an atmospheric RF plasma jet ignited in argon, was measured by laser-induced fluorescence calibrated by Rayleigh scattering on ambient air. A suitable excitation scheme for this discharge was suggested based on laser excitation to the lowest vibrational level of the A 2Σ+ state. Effects of spectral overlap between the laser and absorption line, fluorescence saturation, temporal evolution of fluorescence radiation, rotational energy transfer and rotational distribution on the diagnostic method were analysed. The maximum OH concentration was approximately 5 × 1020 m−3. The maximum was reached when the measurement point localized in the effluent of the plasma pencil was inside but close to the tip of the visible active discharge.