Novel plasma diagnostics for low-temperature plasma in contact with liquids: Brewster angle-cavity ringdown spectroscopy

Application of low temperature plasma (LTP) emanates from interactions of the LTP agents such as reactive species (RS) with a targeted medium. The spatial profile and temporal dynamics of RS in situ are of vital importance for LTP diagnostics. Yet, it is one of the most challenging tasks, especially when LTP interacts with a wet medium, e.g., a medical solution, mainly because traditional spectroscopic techniques such as optical emission spectroscopy suffer from large optical losses. We report on the development of a Brewster angle-cavity ringdown spectroscopy (BA-CRDS) system for LTP diagnostics. The system can be used to measure various plasma species in solutions such as OH and HOO radicals. The detection limit, i.e., the minimum detectable absorbance, is 1.2×10^-5, equivalent to 6 ppb OH radicals in water at 308 nm. The absorption cross section of HgBr₂ in aqueous phase is measured to be (1.8±0.1)×10^-18 cm², close to the reported data in gas phase (1.4 ×10^-18 cm²). The temporal behavior of distilled water, when interacting with a helium plasma jet, is characterized at different plasma power and gas flow rates. The temporal behaviors of other solutions, e.g., HgBr₂, Methanol, etc., are also explored. Some reaction pathways leading to the removal of substances such as HgBr₂ by OH radical and the formation of H₂O₂ in the solutions are proposed. These results demonstrate that the BA-CRDS system is a powerful tool for quantification of RS in aqueous phase with high detection sensitivity.