Quantitative femtosecond, two-photon laser-induced fluorescence of atomic oxygen in high-pressure flames.

Quantitative femtosecond two-photon laser-induced fluorescence of atomic oxygen was demonstrated in an H ₂ /air flame at pressures up to 10 atm. Femtosecond excitation at 226.1 nm was used to pump the 3pP3 _{J ^' =0,1,2} ←←2pP3 _{J ^'' =0,1,2} electronic transition of atomic oxygen. Contributions from multiphoton de-excitation, production of atomic oxygen, and photolytic interferences were investigated and minimized by limiting the laser irradiance to ∼10 ¹¹   W/cm ² . Quantitative agreement was achieved with the theoretical equilibrium mole fraction of atomic oxygen over a wide range of fuel-air ratios and pressures in an H ₂ /air laminar calibration burner.