Absolute Density Measurement of OH Radicals in an Atmospheric Pressure Flame Using Time-Resolved Laser-Induced Fluorescence

We determined the absolute density of OH radicals in a propane-air flame using laser-induced fluorescence (LIF). To obtain the quantum efficiency of the fluorescence, the quenching lifetime of the A2Σ+ (υ'=0) level in the flame was measured with a time-correlated single-photon counting method using a picosecond dye laser. The efficiency of the fluorescence detection system was calibrated by a Rayleigh scattering measurement. The measured quenching rate, 5.6×108 s-1, was in agreement with a previous value deduced from the quenching cross sections at a high temperature. The number density obtained in the present work is consistent with the results of the absorption measurement.