Brewster angle-cavity ringdown spectroscopy for low temperature plasma measurements in multiphases

Abstract

We report on the development of a Brewster angle-cavity ringdown spectroscopy (BA-CRDS) system for low temperature plasma diagnostics. 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 higher reflectivity ringdown mirrors and improved designs of a Brewster angle cell, 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 to be (1.8±0.1)×10-18 cm2 and (5.2±0.5)×10-20 cm2, respectively. 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.