Formation and consumption of HO₂ radicals in ns pulse O₂–He plasmas over a liquid water surface

Abstract

HO2 radicals are an important precursor in the formation of H2O2, 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 HO2 have been studied in a controlled environment, in ns pulse discharge O2-He plasmas in contact with a liquid water surface. For this, time-resolved, absolute number densities of HO2 in O2-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 HO2 number density is inferred from the CRDS data using a spectral model exhibiting good agreement with previous measurements of absolute HO2 absorption cross sections. HO2 is generated during the discharge burst and decays in the afterglow between the bursts. The HO2 number density is also measured vs. the O2 fraction in the mixture. Comparison with the kinetic modeling predictions demonstrates good agreement with the data and identifies the dominant HO2 generation and decay processes. HO2 in the plasma is formed predominantly by the recombination of H atoms, generated by the electron impact dissociation of water vapor, with O2 molecules. Reactions with O atoms and OH radicals are among the main HO2 decay processes in the afterglow. HO2 is also detected when O2 is not present in the mixture. In this case, it is generated primarily by the recombination of OH radicals, via the formation of H2O2. The results demonstrate that CRDS can also be used for HO2 and other plasma chemical reaction product measurements in atmospheric pressure plasma jets impinging on a liquid water surface in ambient air.

Plasma‐enhanced evaporation and its impact on plasma properties and gaseous chemistry in a pin‐to‐water pulsed discharge

“…On the other hand, experimental diagnostics on the interaction process between plasma and liquid is even more considerably challenging, because of the high complexity of this multiphase plasma system wherein physicochemical processes cover a very broad scale in time and space, not to mention its enriched gas background due to evaporation effect [26,27] . In recent years, advanced optical diagnostics, such as laserinduced fluorescence (LIF) spectroscopy and absorption spectroscopy, have been applied to track the production of reactive species including OH, H, O, and HO 2 in the gaseous plasma above the solution surface or droplet surface [28][29][30] . However, quantitative diagnostic studies on the important evaporation phenomenon in the plasma-liquid interaction process are still scarce, not to mention its important impact on the physicochemical properties in this multiphase system from an experimental view.…”

Spatially- and time-resolved measurements of HO2 radicals in a ns pulse atmospheric pressure plasma jet

“…This is done to minimize the interference from the water vapor absorption and dust along the optical path in the ambient air, and to protect the high-reflectivity CRDS mirrors. The mirrors (R = 0.99999, Layertec GmbH, radius of curvature of 1 m) and mounts are the same as in our previous work [13]. During the operation, the mirrors are purged with dry air (see figure 1(c)).…”

“…From a practical standpoint, it would be beneficial to generate HO 2 in the plasma sustained near the liquid water surface, using the evaporating water as one of the reactants, as was done previously at P = 0.11 bar [13]. Although the same approach can also be employed at atmospheric pressure, this would necessitate the use of a significantly smaller electrode gap, limiting the optical access for the laser diagnostics.…”

“…Although the same approach can also be employed at atmospheric pressure, this would necessitate the use of a significantly smaller electrode gap, limiting the optical access for the laser diagnostics. In [13], the time-resolved HO 2 number density was measured in the O 2 -He flow over the liquid water, excited by a ns pulse, dielectric barrier discharge in a plane-to-plane geometry. Kinetic modeling was used to analyze the plasma chemical reactions and determine the dominant HO 2 generation and decay pathways.…”

Brewster angle-cavity ringdown spectroscopy for low temperature plasma measurements in multiphases

“…CRDS overcomes the limitations of the three techniques and has been applied to measure OH radicals in several types of LTPs [13][14][15][16][17][18][19][20]. In addition to OH radicals, several other plasma-generated species involved in plasma processes under various conditions have also been measured by CRDS [21][22][23][24][25][26]. However, when it comes to the complex settings of LTP, e.g.…”