Brewster angle-cavity ringdown spectroscopy for low temperature plasma diagnostics*

Measuring gas molecules in liquids is one of the challenges in diagnostics of low temperature plasma interacting with a liquid. We report on the development of a Brewster angle-cavity ringdown spectroscopy (BA-CRDS) system for low temperature plasma measurements in multiphases. The system can measure gas species in solutions, with a detection limit (minimum detectable absorbance) of 9.1×10-5, which is equivalent to a detection limit of 0.04 parts per billion for measuring OH radicals in water at 308 nm. With further developments, the detection limit can be potentially up to 10-6 or lower.  In this exploratory study, absorption cross sections of HgBr2 and H2O2 in aqueous phase at 256 nm are measured. Furthermore, temporal profiles of absorbance from distilled water, HgBr2, and H2O2 solutions when interacting with a helium atmospheric plasma jet are individually characterized at different plasma powers, gas flow rates, and/or solute concentrations. The observed linear temporal profiles of absorbance from the plasma-interacted water suggest formation of H2O2 from plasma-generated OH radicals, while the nonlinear temporal profiles from the plasma-treated HgBr2 solutions reveal possible removal of HgBr2 by OH radicals. Our result demonstrates that the new BA-CRDS system is a powerful tool for quantification of reactive plasma species in multiphases or other complex settings.