The formation of O and H radicals in a pulsed discharge in atmospheric pressure helium with water vapour admixtures

The spatio-temporal distribution of O and H radicals in a 90 ns pulsed discharge, generated in a pin-pin geometry with a 2.2 mm gap, in He + H2O (0.1% and 0.25%), is studied both experimentally and by 1D fluid modelling. The density of O and H radicals as well as the effective lifetimes of their excited states are measured using picosecond resolution two-photon absorption laser induced fluorescence. Good agreement between experiments and modelling is obtained for the species densities. The density of O and H is found to be homogenous along the discharge axis. Even though the high voltage pulse is 90 ns long, the density of O peaks only about 1 mu s after the end of the current pulse, reaching 2 x 10(16) cm(-3) at 0.1% H2O. It then remains nearly constant over 10 mu s before decaying. Modelling indicates that the electron temperature (Te) in the centre of the vessel geometry ranges from 6 to 4 eV during the peak of discharge current, and after 90 ns, drops below 0.5 eV in about 50 ns. Consequently, during the discharge (<100 ns), O is predominantly produced by direct dissociation of O-2 by electron impact, and in the early afterglow (from 100 ns to 1 mu s) O is produced by dissociative recombination of O-2 (+). The main loss mechanism of O is initially electron impact ionisation and once T (e) has dropped, it becomes mainly Penning ionisation with He-2* and He* as well as three-body recombination with O+ and He. On time scales of 100-200 mu s, O is mainly lost by radial diffusion. The production of H shows a similar behaviour, reaching 0.45 x 10(16) cm(-3) at 1 mu s, due to direct dissociation of H2O by electron impact (<100 ns) followed by electron-ion recombination processes (from 200 ns to 1.5 us). H is dominantly lost through Penning ionisation with He* and He-2* and by electron impact ionisation, and by charge exchange with O+. Increasing concentrations of water vapour, from 0.1% to 0.25%, have little effect on the nature of the processes of H formation but trigger a stronger initial production of O, which is not currently reproduced satisfactorily by the modelling. What emerges from this study is that the built up of O and H densities in pulsed discharges continues after electron-impact dissociation processes with additional afterglow processes, not least through the dissociative recombination of O-2 (+) and H-2 (+).