Fluorescence

Fluorescence is a subtype of photoluminescence / luminescence. After excitation of material by light, fluorescence can occur by spontaneous emission. Usually, the photons emitted by the material are of longer wavelength, i.e. lower energy, than the incident photons. Physical systems capable of fluorescence are called fluorophores. Light-excited states have typical lifetimes characteristic of certain materials. While phosphorescence, a related type of radiation, often shows afterglow, fluorescent radiation is depleted after only a few micro-, nano-, or picoseconds. A special form of fluorescence is the so-called laser-induced fluorescence (LIF), which is used in many medical, bio- and environmental analytical applications.

Using LIF, it is possible to perform trace analyses with high detection sensitivity. Even single molecules can be measured and detected. Samples of any phase can be measured. Especially the analysis of biological samples, such as cell structures or tissue, is common (e.g. combined with microscopes). Molecules are excited to higher energy levels by a laser. Photons (fluorescence) are emitted during relaxation to the ground state. If the excitation is performed by short pulsed lasers in the (sub-) nanosecond range, it is possible to measure the lifetime of the excited states. The intensity of fluorescence strongly depends on absorption properties of the irradiated material – the more absorption, the more fluorescence. This means that the measurement of the sample-specific absorption spectrum serves as the basis for selecting the excitation wavelength. Industrially, LIF is particularly interesting for the analysis of substance concentrations in gas and liquids. Specifically, OH radicals or nitrogen oxides can be detected and quantified.