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Luminescence

Luminescence is a cold glow of bodies that is not associated with their heating to high temperatures. Luminescence can be caused by the action of light on bodies (photoluminescence), electron flow ( cathode luminescence), chemical reaction energy (chemoluminescence), etc. The glow of certain bacteria, fish , and insects is called bioluminescence. Luminescence, which disappears simultaneously with the cessation of excitation of the phosphor (a substance that can luminesce), is called fluorescence, and lasting for a noticeable period of time after cessation of excitation - phosphorescence. See also Fluorescent analysis, Fluorescence microscopy.

Luminescence (from the Latin. Lumen, luminis - radiation, light) - excess radiation of a substance above its normal at a given temperature of thermal radiation and longer than the period of oscillation of light waves. The cause of luminescence is any effect on the substance, leading to the transition of its molecules in an excited state. A part of the energy absorbed by the substance is then released as the energy of the luminescent glow (this part, expressed as a percentage, is called the luminescence energy yield). According to the method of excitation, luminescence is subdivided into photoluminescence (excited by light), chemiluminescence (excited by the energy of exogenous chemical reactions; a special case of it is bioluminescence, for example, the glow of fireflies). In addition, X-ray luminescence, radioluminescence, cathodoluminescence, electroluminescence are known. By duration, the luminescence is divided into fluorescence, at which the luminescence lasts almost as long as the stimulating effect acts, and phosphorescence, when the luminescence continues even after the excitation effect ceases.

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The property of luminescing may have solid, liquid and gaseous substances. Under the action of ultraviolet radiation invisible to the eye, photoluminescent many substances, producing an ultraviolet or visible luminescence of various colors. Under the action of visible light, mainly violet, blue and green, only certain classes of compounds can noticeably photoluminesce. These include aromatic compounds with double conjugated bonds, dyes, porphyrins and their derivatives. The luminescence property is used in luminescence analysis (see).

The luminescence during photoluminescence can be observed as shown in the figure. If instead of the observer's eye you install a photocell with an amplifier, you get a fluorimeter. The fluorimeter is also suitable for the quantitative determination of the concentration of a luminescent substance in a solution or in a transparent solid phase when the luminescence excitation light is absorbed by no more than 20–30%.

photoluminescence
Photoluminescence (observation scheme); 1 - source of exciting light; 2 - the eye of the observer; 3 - light filter that transmits only light that excites luminescence; 4 - a transparent light filter only for the luminescence light; 5 and 6 - lenses; 7 - fluorescent object.

Luminescence light can be decomposed into a spectrum, which, like the absorption spectrum, is determined by the molecular composition of the substance. Registration of luminescence spectra is performed by spectrofluorimeters. For gases, spectra are usually linear, and for complex compounds in solution or in the solid state, these are broad diffuse bands (or band) that are shifted to the long-wavelength part of the spectrum with respect to the absorption band. The intensity of luminescence can significantly decrease and even drop to zero when some extinguisher compounds are added to a solution or suspension that contains a fluorescent substance due to the interaction of the quencher molecules with the molecules of the luminescent substance as a result of either collisions or the formation of complexes. The transition of a luminescent compound into an aggregated state (taking place in concentrated solutions or in multicomponent biological objects) significantly changes all the luminescent characteristics of a substance. Luminescence finds practical application in production, physics, chemistry, geology, biology, forensic examination, medicine and the food industry.