Characterization of plasma-liquid interactions using the nitrate anion as a probe molecule for in situ Raman spectroscopy

When put in contact with water, plasmas generated in air at atmospheric pressure induce a cascade of processes in the liquid to produce aqueous species such as OH, H₂O₂, NO₂^-, and NO₃^-. One limitation to understanding the complex liquid electrochemistry is that experimental characterization is generally performed ex situ, removed from the plasma reactor and after treatment. To perform direct measurements of physical and chemical properties in the plasma-liquid interfacial region, we have developed in situ spontaneous Raman microspectroscopy using a light-sheet method, which previously showed that the plasma-produced concentrations of aqueous NO₃^- can reach an order of 10 mM. In this study, we use the nitrate anion as a probe molecule of the aqueous environment, tracking the spectral width of the N-O symmetric stretch mode (v₁) of NO₃^-. To do so, we introduce NaNO₃ into water at concentrations higher than 10 mM. Far from the plasma-liquid interface, we observe spectral broadening upon plasma generation by up to 4 cm^-1 compared to bulk water before plasma treatment. This broadening occurs gradually, reaching a maximum after about 15 minutes and then relaxing after the plasma is switched off. However, near the interface, the spectrum narrows by up to 3 cm^-1 compared to bulk water prior to plasma initiation. These opposing spectral changes must be due to different mechanisms, which we will consider in this presentation.