Low temperature plasma diagnostics using Brewster angle-cavity ringdown spectroscopy

We report on the development of a Brewster angle-cavity ringdown spectroscopy (BA-CRDS) system for low temperature plasma diagnostics. The system can measure gaseous species in solutions, with a detection limit of 1.2×10^-5 (the minimum detectable absorbance), which is equivalent to a detection limit of 6 ppb for measuring OH radicals in water at 308 nm. In the initial test, we measured the absorption cross section of HgBr₂ in aqueous phase to be (1.8±0.1)×10^-18 cm², consistent with the reported data in gas phase (1.4 ×10^-18 cm²). We also used the system to characterize the temporal behavior of a helium plasma jet treated distilled water at different plasma power and gas flow rates. Some reaction pathways leading to the removal of HgBr₂ by OH radical and formation of H₂O₂ in the solutions are proposed. Results demonstrate that the BA-CRDS system is a powerful tool for quantification of reactive plasma species in multiphase systems or other complex settings, with high detection sensitivity.