Quantitative Imaging of Hydrogen Peroxide and Hydroperoxyl Radical in Low Temperature Plasmas by Photofragmentation Laser-Induced Fluorescence

Hydrogen peroxide (H₂O₂) and hydroperoxyl radical (HO₂) are key reactive species in plasma assisted processes such as decontamination, biomedical treatments, and combustion. Imaging measurements of H₂O₂ and HO₂ in a plasma are challenging because these molecules cannot be detected directly by laser-induced fluorescence due to their predissociative electronically excited states. We present recent developments that enable quantitative imaging of these species in plasmas using photofragmentation laser-induced fluorescence (PF-LIF). In this pump-probe technique, the target molecule, H₂O₂ or HO₂, is photodissociated by a UV pump laser to produce OH fragments. These photofragments are then detected with OH-LIF imaging using an overlapping probe laser beam for LIF excitation. A challenge in implementing the PF-LIF technique is achieving species specificity in plasmas that contain both H₂O₂ and HO₂ because both molecules dissociate to form OH. Previously, there was no way to differentiate between contributions of these species to the resulting PF-LIF signal. In this work, we developed an approach to distinguish between these two species using differences in the photodissociation dynamics of H₂O₂ and HO₂ that result in different rotational energy distributions of their photofragments. By probing two different rotational transitions of the OH photofragments, we demonstrate quantitative 2D imaging measurements of H₂O₂ and HO₂ distributions in the effluent of the COST reference microplasma jet.