Imaging of Hydrogen Peroxide and Methyl in Nanosecond Pulsed Plasmas by Photofragmentation Laser-Induced Fluorescence

Hydrogen peroxide (H₂O₂) and the methyl (CH₃) are key reactive species in plasma assisted processes such as surface decontamination, biomedical treatments, catalysis, and combustion. Imaging of H₂O₂ and CH₃ is challenging because they cannot be detected directly by laser-induced fluorescence. We present a technique for imaging 2D distributions of these species in plasmas using photofragmentation laser-induced fluorescence (PF-LIF). The target molecule, H₂O₂ or CH₃, is photodissociated by a UV pump laser to produce OH or CH fragments, respectively. These photofragments are then detected with LIF imaging using an overlapping probe laser beam for LIF excitation. For H₂O₂ PF-LIF measurements, the probe laser excites transitions in the A-X(1,0) band of OH, and fluorescence is detected from A-X(1,1) and (0,0) bands. For CH₃ PF-LIF measurements, transitions in the B-X band of CH are excited, and fluorescence from the overlapping A-X(0,0), A-X(1,1), and B-X(0,1) bands is detected with the A-state populated by collisional B-A electronic energy transfer. These non-resonant detection schemes enable interrogation near surfaces. We demonstrate PF-LIF imaging measurements of H₂O₂ and CH₃ mole fractions in nanosecond pulsed DBD plasmas with mixtures of He-H₂O-O₂ and CH₄-CO₂-He, respectively.