Laser-Induced Fluorescence of Hydroxyl (OH) Radical in Cold Atmospheric Discharges

Abstract

The application of laser-induced fluorescence (LIF) to measurement of absolute concentration of hydroxyl radicals in cold atmospheric discharges is described. Though only the case of OH is presented, the method can be directly applied to other molecules as well. Starting from the rate equations for the LIF process, the main formulas for two-and multilevel excitation scheme are derived. It is also shown how to use partially saturated LIF in practice, enhancing the signal-to-noise ratio. Practical tips for automating the data evaluation are given, allowing processing large data sets, particularly suitable for planar measurements. Gas temperature estimation from fluorescence on different rotational absorption lines is shown as an attractive method for obtaining temperature maps with high spatial resolution. The important aspects of calibration are discussed, particularly the overlap of the laser line with the selected absorption line and the measurement of the Rayleigh scattering for sensitivity calibration, together with the common sources of errors. The application of OH(A, v 0 =0 X, v 00 = 0) excitation scheme to the effluent of atmospheric pressure plasma jet ignited in argon and of OH(A, v 0 =1 X, v 00 = 0) to the plasma of coplanar surface barrier discharge in air and in water vapor is shown.