Laser-induced fluorescence of the CH molecule in a low-pressure flame

The laser-induced fluorescence from A²Δ(υ′ = 0)→X²Π(υ″ = 0)] band of the CH radical was studied in a low-pressure (20 torr) methane-oxygen flame (φ = 1.06). A time-resolved fluorescence technique was used to measure the relative CH concentration profile and the quenching of the A²Δ excited state through the flame. The pressure dependence of the quenching was also measured and used to determine an effective quenching cross section of 6 Ų in the CH₄-O₂ flame. Analysis of the fluorescence spectra scanned at different delays after the laser excitation, according to a pseudo-three-level model, yields a rotational energy transfer (RET) rate in the A²Δ(υ′ = 0) electronic state which is a factor of four faster than the electronic quenching rate of 1.57 × 10⁷ sec^-1 in the flame at 2000 K.