Imaging of Methyl Radical in a Plasma Jet by Photofragmentation Laser-Induced Fluorescence

The methyl radical (CH₃) is a key intermediate species in plasma assisted processes such as catalysis and combustion. For example, in oxidative coupling of methane, the initial extraction of a hydrogen atom from methane results in the formation of methyl, which can subsequently react to form desired products such as C₂ species. However, methyl cannot be detected directly by laser-induced fluorescence. We present a technique for imaging the two-dimensional distribution of methyl near catalytic surfaces, using photofragmentation laser-induced fluorescence (PF-LIF). In this technique, methyl is photodissociated by a 213 nm laser beam to produce CH fragments. These photofragments are then detected with LIF imaging by exciting a transition in the B-X band of CH with a second laser at 390 nm. Collisional B-A electronic energy transfer then populates the A-state of CH, after which fluorescence is detected in the A-X band at 430 nm with a gated ICCD camera and suitable band pass filter. This non-resonant detection scheme enables interrogation near surfaces. The PF-LIF diagnostic is calibrated by producing a known amount of methyl through photodissociation of acetone. We demonstrate the PF-LIF approach by imaging the methyl distribution in a nanosecond pulsed helium plasma jet with a coflow containing hydrocarbon fuels.