Absolute OH density determination by laser induced fluorescence spectroscopy in an atmospheric pressure RF plasma jet

In this paper, the ground state OH density is measured in high pressure plasma by laser-induced fluorescence (LIF) spectroscopy. The OH density determination is based on the simulation of the intensity fraction of fluorescence from the laser-excited level of OH (A) in the total detected LIF signal. The validity of this approach is verified in an atmospheric pressure Ar + H2O plasma jet sustained by a 13.56 MHz power supply. The transition line P1(4) from OH(A, v ′ = 1, J ′ = 3) → OH(X, v′′ = 0, J ′′ = 4) is used for the LIF excitation. The absolute OH density is determined to be 2.5 × 10 m−3 at 1 mm away from the jet nozzle. It corresponds to a dissociation of 0.06% of the water vapor in the working gas. Different mechanisms of OH (X) production in the core of the plasma jet are discussed and analyzed.