Photochemical effects in two-photon-excited fluorescence detection of atomic oxygen in flames

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Abstract
This paper describes photochemical effects observed using 226-nm two-photon-excited fluorescence detection to measure the atomic oxygen concentration in hydrogen-oxygen flames. In a study of a lean atmospheric-pressure flame, we observed artificially high atomic-oxygen concentration levels in the postflame gases using all but the most gentle excitation conditions (intensities greater than ∼0.1 GW/cm2). A similar study of a lean low-pressure (72-Torr) flame showed little evidence of photochemical production of atomic oxygen. Using a second laser system in a pump-probe configuration, with the probe laser monitoring the atomic oxygen concentration in a very lean flame while the pump laser was scanned across molecular-oxygen Schumann-Runge bands at 221 nm, we demonstrated that excess atomic oxygen concentrations can be produced by single-photon excitation of these bands in vibrationally excited oxygen molecules present in the flame. This production mechanism explains at least part of the artificially high concentration levels observed in the atmospheric-pressure flame.