Measurements of collisional quenching of hydrogen atoms in an atmospheric-pressure hydrogen oxygen flame by picosecond laser-induced fluorescence

Abstract

Atomic hydrogen has been probed in a hydrogen/oxygen flame at atmospheric pressure by use of two-photon, picosecond Laser-Induced Fluorescence (LIF). The fluorescence was detected and temporally resolved by a streak camera with a temporal resolution of a few ps which allowed determination of the collisionally quenched lifetime of then = 3 level of atomic hydrogen. The measurements were performed for three flame stoichiometries,φ = 1.4, 2.0 and 3.5, and were spatially resolved with respect to the reaction zone. The evaluated lifetime was found to vary rapidly in the reaction zone from 60 to 105 ps just above and then levelled out at 90 ps throughout the post-flame region. Power dependence measurements of the fluorescence signal indicated the presence of other phenomena, such as saturation, ionization and photodissociation. Since an inverted population is created between then = 3 and then = 2 level, Stimulated Emission (SE) of considerable magnitude occurred and was studied temporally resolved using the streak camera. The LIF and SE signal strengths for individual laser pulses were recorded in order to analyse a possible anti-correlation between the LIF and the SE.