Abstract
A laser-saturated fluorescence technique which can be used to measure accurate OH radical concentrations over wide ranges of flame pressure, composition, and temperature has been developed. The balanced cross-rate model is used to analyze the fluorescence data. The basic premise of the model is that the population of the upper and lower rotational levels which are coupled by the laser is approximately constant during the laser pulse. Using the balanced cross-rate model to calculate concentrations, OH number densities calculated from saturated fluorescence agree to within 20% with number densities calculated from independent absorption measurements over a wide range of flame conditions. In addition, the ratio of number densities calculated from saturated fluorescence and absorption is constant over an order of magnitude range of flame pressure, demonstrating the insensitivity of the saturated fluorescence signal to collisional quenching rates. The estimated OH detection limit of the fluorescence system is 10 12–10 13 molecules/cm 3.
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