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 $2.25\times {10}^{-10}$ cm⁻¹ with a 100 second acquisition, equivalent to $5.5\times {10}^{12}{cm}^{-3}$ of HO₂ (under ideal conditions). Concentrations in the range of (3.1–7.8) × 10¹³ cm⁻³ were inferred in the 4 mm wide effluent of the plasma jet. (paper)