Quantitative imaging of H₂O₂ and HO₂ in a cold plasma jet by photofragmentation laser-induced fluorescence

Abstract

Accurate determination of the concentrations of reactive oxygen and nitrogen species (RONS) in low-temperature plasmas is critical to understand the interactions between the plasma and treatment targets. While laser-induced fluorescence (LIF) is commonly used to measure plasma species, hydrogen peroxide (H₂O₂) and the hydroperoxyl radical (HO₂), two important RONS, are not directly accessible by LIF due to their predissociative electronically excited states. Instead, photofragmentation laser-induced fluorescence (PF-LIF) can be used, where H₂O₂ or HO₂ is photodissociated by a pump laser to produce OH molecules, which are then detected by LIF using a probe laser. However, differentiating the PF-LIF signals of HO₂ and H₂O₂ remains challenging as both species produce OH photofragments. This work demonstrates a method for quantitative PF-LIF imaging measurements of HO₂ and H₂O₂ concentrations using the COST reference microplasma jet. By leveraging the different photodissociation dynamics of HO₂ and H₂O₂, we separate their individual contributions to the PF-LIF signal. The presented method involves combining calibrated signals from rotationally excited OH molecules resulting from H₂O₂ photofragmentation with signals from OH molecules in the rotational ground state. Applicable to any steady-state reactive flow, this method can be used not only in plasma applications, but also in the fields of combustion diagnostics and catalysis.