KrF laser-induced photobleaching effects in O(2) planar laser-induced fluorescence signals: experiment and model.

KrF excimer lasers are often employed as high-power excitation sources in planar laser-induced fluorescence (PLIF) imaging experiments to measure the distributions of O(2), OH, and H(2)O-all important species in combustion phenomena. However, due to the predissociative nature of these molecules, the high laser pumping rates typically required in such PLIF experiments may significantly deplete the ground-state population. The proper interpretation of the ground-state number density and/or the temperature from the fluorescence signals then requires the inclusion of photobleaching effects. We compare the results of a five-level rate-equation model incorporating photobleaching effects to the time-resolved PLIF signals from O(2) as obtained in the products of a fuel-lean CH(4) air flame. The results indicate that the fluorescence signals in a typical predissociated PLIF imaging experiment are subject to significant amounts of photobleaching. In an effort to provide a convenient way to account for photobleaching, a simple three-level model is developed. This model provides an analytic solution that describes satisfactorily the time-integrated fluorescence signal when compared with both the five-level model and the measurements. The results also indicate that at the low laser irradiances required to minimize the effects of photobleaching, the correspondingly low fluorescence signal levels make the acquisition of single-shot PLIF images a challenge to currently available camera systems.