Improvement of two-photon laser induced fluorescence measurements of H- and O-atoms in premixed methane/air flames

Two-photon LIF is used to measure the concentration of H- and O-atom in low-pressure premixed methane/air flames at different equivalence ratios. New experimental evidences of photochemical effects, interference and collisional energy transfer upon excitation at 205 and 226 nm are shown. At 205 nm, CH3 photodissociation is shown to be an important photolytic source of H atoms in the reaction zone of the methane flames. At 226 nm, an efficient energy transfer between O(P-3) and N-2 is established from the observation of O-resonant emissions from the second positive system of N-2. The subsequent rate of O(P-3) depletion appears to be essentially ''controlled'' by the O(P-3) concentration and is probably only minimally temperature dependent. Finally quenching rate coefficients for O(P-3) by water and nitrogen are deduced from quenching rates measurements performed in CH4/O-2/N-2 and CH4/O-2/Ar flames.