technique as the spontaneously emitted photons are recorded as signal after incident laser light has been absorbed by a species. Most commonly, the wavelength integrated fluorescence is detected leading to a fluorescence excitation spectrum that is equivalent to an absorption spectrum. Additional information can be obtained from the wavelength-dispersed fluorescence. In gerneral, LIF possesses intrinsically low noise allowing the detection of very low concentrations in confined volumes. High spatial resolution is usually achieved by right-angle collection of the fluorescent light with a large-aperture lens. An absolute measurement of absorbance requires detailed knowledge of the fluorescence process and careful calibration of the detection system (fluorescence quantum yield, geometrical setup, spectral response of the detector, *etc.*). Using amplified photodetectors, laser-induced fluorescence can be measured linearly over a large dynamic range from single-photon counting to saturation of the photodetector. The measurement is virtually background free if stray light from the incident laser is suppressed effectively, *e.g.* with an optical filter. If the fluoresecence lifetime is long compared to the laser pulse length, stray light can also be discriminated against by time gating of the signal. If the fluorescence lifetime exceeds the time between collisions, quenching of the fluorescence can occur, reducing the signal 