Detection of aqueous H₂O₂ and NO₃^- at plasma-water interface by in situ Raman spectroscopy

Using a light sheet technique, in situ Raman microspectroscopy has been applied to investigate the liquid side of the plasma-water interface with micrometer depth resolution [1]. During plasma operation, simultaneous measurements were performed to track the Raman spectra of the –OO stretch mode of H₂O₂, symmetric stretch (v₁) of NO₃^-, and –OH bend of water. The light sheet was positioned just below a glow discharge was generated in atmospheric-pressure air facing a water cathode. Far from the interface region, aqueous NO₃^- was produced at a rate of 48 µM/minute, but the aqueous H₂O₂ concentration stabilized at about 5 mM. When approaching the interface to within several tens of microns, the concentrations of both species increase. These measurements reveal the existence of an interfacial layer of excess NO₃^- concentration extending 28 µm in depth, although this determination is subject to some interpretation due to the presence of the meniscus. Interfacial layers of such depth have been modeled for transient species such as OH but not for NO₃^-, a stable product of plasma-activated water.