Plasma-liquid interfacial layer detected by in situ Raman light sheet microspectroscopy

In situ Raman microscopy has been adapted to study the plasma–water interface by applying a light sheet technique. The Raman modes of the –OO stretch of H₂O₂, symmetric stretch (v ₁) of NO₃ ⁻, and –OH bend of water were measured simultaneously. By modulating the volume of water under detection, both the bulk liquid and interface regions have been probed with micrometer depth resolution. The plasma was a DC glow discharge generated in atmospheric-pressure air with a water cathode. In the bulk liquid, the molar concentration of aqueous NO₃ ⁻ increased at a linear rate of 48 µM min⁻¹, whereas aqueous H₂O₂ growth stopped at about 5 mM. The concentrations of H₂O₂ and NO₃ ⁻ both increased when measuring at depths less than about 20 µm from the interface. The depth profile of NO₃ ⁻ concentration was reconstructed, showing that the interfacial layer of NO₃ ⁻ has a depth of 28 µm. The shape of the meniscus may influence the interpretation of this depth. Previous models of plasma-water interfaces have predicted interfacial layers of similar depth for short-lived aqueous species such as OH but not for long-lived species such as NO₃ ⁻. (paper)