Comparison of nanosecond and picosecond excitation for two-photon laser-induced fluorescence imaging of atomic oxygen in flames.

Two-photon laser-induced fluorescence (LIF) imaging of atomic oxygen is investigated in premixed hydrogen and methane flames with nanosecond and picosecond pulsed lasers at 226 nm. In the hydrogen flame, the interference from photolysis is negligible compared with the LIF signal from native atomic oxygen, and the major limitations on quantitative measurements are stimulated emission and photoionization. Excitation with a nanosecond laser is advantageous in the hydrogen flames, because it reduces the effects of stimulated emission and photoionization. In the methane flames, however, photolytic interference is the major complication for quantitative O-atom measurements. A comparison of methane and hydrogen flames indicates that vibrationally excited CO2 is the dominant precursor for laser-generated atomic oxygen. In the methane flames, picosecond excitation offers a significant advantage by dramatically reducing the photolytic interference. The prospects for improved O-atom imaging in hydrogen and hydrocarbon flames are presented.