Laser-induced fluorescence
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Laser-induced fluorescence (LIF) is a spectroscopic method.
The preferred specie is excited with help of a laser. The wavelength is often selected to be the one at which the specie has its largest cross section. The excited specie will after some time, usually in the order of few nano seconds to micro seconds, de-excite and emit light at a wavelength larger than the excitation wavelength. This light, fluorescence, is measured.
Two different kinds of spectra exists, LIF-spectra and excitation scans.
The LIF-spectra are performed with a fixed lasing wavelength, as above and the fluorescence spectrum is analyzed. Excitation scans on the other hand collects all fluorescent light, without measuring the wavelength. Instead the lasing wavelength is changed.
The advantage over absorption spectroscopy is that it is possible to get two- and three-dimensional images since fluorescence takes place in all directions (i.e. fluorescence signal is not polarized). The signal-to-noise ratio of fluorescence signal to very high, providing a good sensitivity to the process. It is also possible to distinguish between more species, since both the lasing wavelength can be tuned to a particular excitation of a given specie.
LIF is useful to study the electronic structure of molecules and their interactions. It has also been successfully applied for quantitative measurement of concentrations in fields like combustion, plasma, spray and flow phenomena at as low as nanomolar concentration.