Formation and consumption of HO 2 radicals in ns pulse O 2 –He plasmas over a liquid water surface

Hydroperoxyl (HO 2 ) radicals are an important precursor in the formation of hydrogen peroxide (H 2 O 2 ), a key species in plasma-liquid interactions, such that their formation and consumption pathways need to be understood. In this work, the generation and decay of HO 2 have been studied in a controlled environment, in ns pulse discharge O 2 –He plasmas in contact with a liquid water surface. For this, time-resolved, absolute number densities of HO 2 in O 2 –He mixtures excited by a repetitive ns pulse discharge are measured in situ by cavity ring down spectroscopy (CRDS). The discharge cell with external electrodes to generate the plasma and a water reservoir are integrated into the CRDS cavity. The high-reflectivity cavity mirrors are purged with helium to protect them from water vapor condensation. The experimental results are obtained at near room temperature, both during the discharge pulse burst and in the afterglow. The HO 2 number density is inferred from the CRDS data using a spectral model exhibiting good agreement with previous measurements of absolute HO 2 absorption cross sections. HO 2 is generated during the discharge burst and decays in the afterglow between the bursts. The HO 2 number density is also measured vs. the O 2 fraction in the mixture. Comparison with the kinetic modeling predictions demonstrates good agreement with the data and identifies the dominant HO 2 generation and decay processes. HO 2 in the plasma is formed predominantly by the recombination of H atoms, generated by the electron impact dissociation of water vapor, with O 2 molecules. Reactions with O atoms and hydroxyl (OH) radicals are among the main HO 2 decay processes in the afterglow. HO 2 is also detected when O 2 is not present in the mixture. In this case, it is generated primarily by the recombination of OH radicals, via the formation of H 2 O 2 . The results demonstrate that CRDS can also be used for HO 2 and other plasma chemical reaction product measurements in atmospheric pressure plasma jets impinging on a liquid water surface in ambient air.