Brewster angle-cavity ringdown spectroscopy (BA-CRDS) for measuring plasma species in liquids

Cavity ringdown spectroscopy (CRDS) is an ultra-sensitive absorption spectral technique, which is widely used for absolute measurement of trace gases in diverse environments including plasmas. In addition to quantification of gas molecules in plasmas, measuring gas molecules in liquids is also important yet remains 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 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, which is sufficiently sensitive for measurements of various plasma species in liquids. We present our recent developments in this technology, with a goal to bring a powerful tool to the low temperature plasma community for quantification of reactive plasma species in multiphases or other complex settings.