Flame radicals, fragments of molecules with high reactivity, control the process of combustion to a high degree. Therefore, an accurate knowledge of the number densities of these species is very important. Laser-induced-fluorescence (LIF) spectroscopy is one of the most sensitive optical techniques for accurate determination of concentrations and temperatures. However, when LIF is applied for quantitative diagnostics in industrial combustion processes, in which high pressures (1 bar < p < 10 bar) and high temperatures are typical, several problems associated with the LIF method limit the accuracy of the derived data. Collisional-induced processes reduce the fluorescence yield considerably, 2 or 3 orders of magnitude are typical for atmospheric pressure. Therefore, time-resolved picosecond LIF spectroscopy is applied for the diagnostics. However, the quantitative interpretation of picosecond LIF intensities still requires acccurate knowledge of quenching rates for the relevant pressures, temperatures and combustion species. In addition, the intensity and the time-evolution of the fluorescence signal depends very sensitively on the direction of polarization of the exciting laser beam1 and must also be considered in the data analysis.

© 1995 Optical Society of America

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