Absolute OH density and gas temperature measurements by laser induced fluorescence in a microsecond pulsed discharge generated in a conductive NaCl solution

Electrical discharges in bubbles in liquid water have been extensively investigated for many applications including water treatment and often the OH radical is considered to be a key enabler of many applications. Nonetheless, to date, no OH density measurements have been reported for discharges in bubbles or in liquids in general. In this paper, we report an investigation of discharges in bubbles by laser-induced fluorescence (LIF). The plasma was generated by applying a microsecond voltage pulse to a needle electrode submerged in an aqueous NaCl solution in a Hele-Shaw cell. To determine the gas temperature and OH radical density, a LIF model resolving the energy transfer at the rotational level was implemented. Gas compositions at different gas temperature were obtained with a 0-D kinetics model. The LIF model also enabled us to explain the non-equilibrium rotational distribution of the OH ground state by LIF. Gas temperatures and OH radical densities up to 4900 ± 800 K and 3.2 ± 0.5 × 1017 cm-3 are consistent with conditions favoring thermally driven reactions in the plasma-filled vapor bubble.