Laser-induced fluorescence

Laser-induced fluorescence (LIF) is a resonant absorption–emission interaction process between irradiated laser photons and atoms or molecules. Due to the resonant character of the excitation process, minor combustion species at very low concentration levels (ppm or sub-ppm) can be probed using single-photon absorption processes, or by tuning the exciting laser to a wavelength where the species absorbs two photons simultaneously (two-photon LIF). Quantitative concentration measurements are feasible, but hindered by inter- or intramolecular energy transfer processes. Additionally, the temperature can be measured by determining the population distribution of the molecules, using Boltzmann's statistics. In many practical combustion systems the combustion fluid of interest (e.g., fuels or fuel components) does not give any LIF signal at all, or provides too-strong fluorescence contributions that are composed of signals from numerous species. Here, quantitative interpretation of the signals is typically not feasible. For the quantitative analysis of non- or weakly fluorescing fluids, it is advantageous to add well-characterized fluorescing tracers to the fluids (tracer LIF). The appropriate choice of the best suitable tracer system strongly depends on the particular application case. Recent laser developments have provided an increase of repetition rates into the multi-kHz regime. Using these high repetition rates, integral time scales of typical turbulent flames can be resolved in the measurements. In combination with intensified complementary metal-oxide semiconductor (CMOS) cameras, cinematographic imaging of flame dynamics becomes feasible, providing a complementary view compared to statistically independent sampling. This chapter includes a brief introduction to the most recent developments in the field of LIF at high repetition rates (high-speed LIF), and also provides detailed information on suitable instrumentation for LIF applications, in particular for high-speed LIF.