Time-resolved absolute OH density of a nanosecond pulsed discharge in atmospheric pressure He-H₂O: Absolute calibration, collisional quenching and the importance of charged species in OH production

The time-resolved OH density in a nanosecond pulsed filamentary discharge in an atmospheric pressure He-H₂O(0.05%) mixture is measured using laser induced fluorescence. The lifetime of the excited OH(A) state is found to be strongly time dependent during the plasma pulse, with the shortest decay time occurring at the moment the plasma is switched off. The measured LIF intensity is corrected for this time-resolved quenching and calibrated using Rayleigh scattering. Time-resolved electron density , He (³S1) metastable density , gas temperature and optical emission are presented and used in the interpretation of the observed time dependence of the OH density. Based on these experimental data, it is shown that for the present discharge conditions, OH is mainly produced by charge transfer reactions to water followed by dissociative recombination of the water ion. In addition, two often encountered issues with the calibration of OH LIF are highlighted with an example.