Fluorescence

General information

A fluorescent Molécule (fluorophore or fluorochrome) has the property to absorb luminous energy (light of excitation) and to quickly restore it in the form of fluorescent light (light of emission). Once the energy of the Photon absorptive, the molecule is then in an electronically excited state, generally a state Singulet, which one notes S1*. The return at the fundamental state can then be made various manners. One of them is the emission of a photon, it is the phenomenon of fluorescence. The Wavelength re-emitted by the excited molecule can be of the same (fluorescence of resonance) or larger wavelength wavelength. The fact that the wavelength of emission is larger comes owing to the fact that, in the liquid media in particular, the molecule low turns over at the fundamental state starting from the level of vibration of the excited state (rule of Kasha' S). This difference is called displacement of Stokes.

This displacement of the spectrum of emission towards more raised wavelengths, described by the law of Stokes, is essential for the separation and the detection of the light of fluorescence, specific signal delivered by the fluorophore.

There exists a great choice of fluorochromes, each one being able to be characterized by its spectra of excitation and emission.

The principle of fluorescence is used in the confocaux microscopes with laser scanning and the fluorescence microscopes.

The phenomenon of fluorescence is not limited to the emission in the visible spectrum, but also relates to the emission of x-rays (X-ray fluorescence).

History of fluorescence

Around the year 1000 existed at the Empereur of China, a magic table on which an ox appeared each evening. It was the first example, in the history, of a material manufactured by the Man, able to emit luminescent light. This process was found involuntarily by a shoe-maker at the end of the 16th century.

Characteristics of the fluorophores

The various characteristics of the fluorophores are:

Wavelengths: those which correspond to the peaks of the spectra of excitation and emission,
Coefficient of extinction (or molar absorption): it connects the quantity of absorptive light, for a given wavelength, with the concentration of the fluorophore in solution (M^-1 cm^-1)
Quantum yield: effectiveness relative of the fluorescence compared with the other ways of de-energizing (= many emitted photons/many absorptive photons)
Lifespan to the excited state: it is the characteristic duration during which the molecule remains in an excited state before turning over in its basal state (psec). This duration is comparable to the half-life of the excited state.
Photoblanchiment (photobleaching): when the molecule is in an excited state, there exists a certain probability so that it takes part in chemical reactions (one speaks then about photochemical reactions), in particular with oxygen in the form of free radicals. The fluorochrome loses its properties of fluorescence then. In other words, when one excites a solution of fluorescent molecules, a certain proportion of them is destroyed at every moment and consequently the intensity of fluorescence decrease during time. This phenomenon can be awkward, in particular in microscopy of fluorescence, but it can also be made profitable photoblanchiment to measure molecular mobility by the method of Redistribution of fluorescence after (FRAP) or of FLIP (Loss of fluorescence during photoblanchiment local).

Daily applications

Various applications

Owing to the fact that fluorescence generally results in the visible emission of light starting from an invisible energy source (Ultraviolet S), the fluorescent objects appear more luminous than of the same dyed, but nonfluorescent objects. This property is used by anti-collision paintings of orange color which one paints, for example, certain parts of the Avion S, but also with a simple esthetic aim (clothing, etc)

Fluorescence is also used in the case of the black Lumière, source of light primarily made up of ultraviolet close relations, who emphasizes the fluorescent white and objects when it is emitted in the half-light, in order to create a special environment.

Fluorescence is also used in Imagerie by x-rays because it makes it possible to convert the X-rays into visible light for the eye or a Capteur CCC.

Fluorescent tube

The fluorescent Tube (official name: luminescent tube ) is another well-known application. These tubes are used for the lighting Industriel and Domestique (called " néons" by error: because the gas Néon would emit a red light). They contain gases, generally vapors of mercury to low pressure or argon, which emit a light invisible Ultraviolet you. The interior wall is covered with a mixture of fluorescent powders which re-emit this light in the visible field while approaching the white. These tubes offer much better a electric Rendement than a Lamp incandescent traditional, i.e. they emit more lumen S by Watt and, therefore heat much less. Today, the form can change and the electronic which them control allows an output still improved. One finds thus lamps called to energy saving being able to replace the traditional lamps advantageously.

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