Strategy for single-shot CH₃ imaging in premixed methane/air flames using photofragmentation laser-induced fluorescence

Single-shot imaging of methyl radical (CH₃) in premixed methane/air flames is demonstrated using photofragmentation laser-induced fluorescence (PF-LIF) technique. A pump-probe strategy was adopted with the pump laser at 212.8 nm photolyzing CH₃, and with the probe laser at 426.8 nm detecting the photolyzed CH (X ²Π) fragments. Spatially resolved spectrograph of the PF-LIF signal from a stable laminar flame was recorded across the reaction zone to investigate potential interferences. The results indicate that the single-photon channel, CH₃ + 212.8 nm → CH (X ²Π) + H₂, dominates the photofragmentation process. The CH₂ radical was excluded from being an interfering precursor of the CH (X ²Π) fragments owing to its relatively low concentration and small absorption cross section. Naturally present CH in the flame was identified as the main interference, but was conservatively estimated to account for only less than 4% of the total PF-LIF signal. Signal-to-noise ratio of around 10 was realized for single-shot imaging of natural CH₃ in turbulent jet flames.