Detection of HO2 in an atmospheric pressure plasma jet using optical feedback cavity-enhanced absorption spectroscopy

Cold non-equilibrium atmospheric pressure plasma jets are increasingly applied in material processing and plasma medicine. However, their small dimensions make diagnosing the fluxes of generated species a challenge. Here we report on the detection of the hydroperoxyl radical, HO2, in the effluent of a plasma jet by the use of optical feedback cavity-enhanced absorption spectroscopy. The spectrometer has a minimum detectable absorption coefficient alpha(min) of 2.25 x 10(-10) cm(-1) with a 100 second acquisition, equivalent to 5.5 x 10(12) cm(-3) of HO2 (under ideal conditions). Concentrations in the range of (3.1-7.8) x 10(13) cm(-3) were inferred in the 4 mm wide effluent of the plasma jet.