Luminescence: types, methods, application. Thermally stimulated luminescence is what?

Luminescence is the emission of light by certain materials in a relatively cold state. It differs from the radiation of hot bodies, for example burning wood or coal, molten iron and wire, heated by electric current. Emission of luminescence is observed:

• In neon and fluorescent lamps, televisions, radar and screens of fluoroscopes;
• In organic substances, such as luminol or luciferin in fireflies;
• In some pigments used in outdoor advertising;
• With lightning and aurora borealis.

In all these phenomena, light radiation is not the result of heating the material above room temperature, so it is called cold light. The practical value of luminescent materials lies in their ability to transform invisible forms of energy into visible radiation.

Sources and process

The phenomenon of luminescence occurs as a result of the absorption of energy by the material, for example, from a source of ultraviolet or x-ray radiation, electron beams, chemical reactions, etc. This leads the atoms of matter into an excited state. Since it is unstable, the material returns to its original state, and the absorbed energy is released in the form of light and / or heat. Only external electrons are involved in the process. The effectiveness of luminescence depends on the degree of conversion of excitation energy to light. The number of materials with sufficient efficiency for practical use is relatively small.

Luminescence and incandescence

The excitation of luminescence is not due to the excitation of atoms. When hot materials begin to glow as a result of incandescence, their atoms are in an excited state. Although they vibrate already at room temperature, this is enough that the radiation occurs in the far infrared region of the spectrum. With increasing temperature, the frequency of electromagnetic radiation shifts to the visible region. On the other hand, at very high temperatures, which are created, for example, in shock tubes, collisions of atoms can be so strong that the electrons separate from them and recombine, emitting light. In this case, luminescence and incandescence become indistinguishable.

Fluorescent pigments and dyes

Conventional pigments and dyes have color, since they reflect that part of the spectrum that is complementary to the absorbed. A small part of the energy is converted into heat, but no noticeable radiation occurs. If, however, the luminescent pigment absorbs daylight in a certain portion of the spectrum, it can emit photons that are different from the reflected ones. This occurs as a result of processes inside the dye molecule or pigment, due to which the ultraviolet can be converted into visible, for example, blue light. Such methods of luminescence are used in outdoor advertising and in washing powders. In the latter case, the "clarifier" remains in the tissue not only to reflect white, but also to convert the ultraviolet radiation into blue, compensating for the yellowness and enhancing whiteness.

Early research

Although lightning, the northern lights and the dim glow of fireflies and mushrooms were always known to mankind, the first studies of luminescence began with synthetic material, when in 1603 Vincenzo Cascariolo, an alchemist and shoemaker from Bologna (Italy), heated a mixture of barium sulphate (barite, Heavy spar) with coal. Powder obtained after cooling, at night emitted a bluish glow, and Cascariolo noticed that it can be restored by exposing the powder to sunlight. The substance was called "lapis solaris", or a sunstone, because the alchemists hoped that it could turn metals into gold, the symbol of which is the sun. The afterglow aroused the interest of many scientists of the period who gave the material and other names, including "phosphorus", which means "carrier of light".

Today the name "phosphorus" is used only for a chemical element, while microcrystalline luminescent materials are called a phosphor. "Phosphorus" Cascario, apparently, was barium sulphide. The first commercially available phosphor (1870) was "Balmain's paint" - a solution of calcium sulphide. In 1866, the first stable phosphor from zinc sulphide was described - one of the most important in modern technology.

One of the first scientific studies of luminescence, manifested in the decay of wood or flesh and fireflies, was performed in 1672 by the English scientist Robert Boyle, who, although he did not know about the biochemical origin of this light, nevertheless established some of the basic properties of bioluminescent systems:

• The glow is cold;
• It can be suppressed by chemical agents such as alcohol, hydrochloric acid and ammonia;
• Radiation requires access to air.

In 1885-1887 it was observed that crude extracts obtained from West-Indian fireflies (fire-flake click-beans) and from mollusks of folad, when mixed, produce light.

The first effective chemiluminescent materials were non-biological synthetic compounds, such as luminol, discovered in 1928.

Chemi- and bioluminescence

Most of the energy released in chemical reactions, especially oxidation reactions, has the form of heat. In some reactions, however, part of it is used to excite electrons to higher levels, and in fluorescent molecules before the onset of chemiluminescence (CL). Studies show that CL is a universal phenomenon, although the intensity of luminescence is so small that it requires the use of sensitive detectors. There are, however, some compounds that demonstrate a bright CL. The most famous of these is luminol, which, when oxidized with hydrogen peroxide, can produce strong blue or blue-green light. Other strong CL-substances are lucigenin and lofin. Despite the brightness of their CL, not all of them are effective in converting chemical energy into light, since less than 1% of molecules emit light. In the 1960s it was found that oxalic acid esters oxidized in anhydrous solvents in the presence of strongly fluorescent aromatic compounds emit bright light with an efficiency of up to 23%.

Bioluminescence is a special type of CL catalyzed by enzymes. The luminescence yield of such reactions can reach 100%, which means that each molecule of the reacting luciferin passes into a radiating state. All bioluminescent reactions known today are catalyzed by oxidation reactions taking place in the presence of air.

Thermally stimulated luminescence

Thermoluminescence means not thermal radiation, but an increase in the light emission of materials whose electrons are excited by heat. Thermally stimulated luminescence is observed in some minerals, and especially in crystallophosphors after they have been excited by light.

Photoluminescence

Photoluminescence, which occurs under the influence of electromagnetic radiation incident on the substance, can be carried out in the range from visible light through ultraviolet to X-ray and gamma radiation. In luminescence caused by photons, the wavelength of the emitted light is, as a rule, equal to or greater than the wavelength of the exciting (ie equal to or lesser energy). This difference in wavelength is due to the transformation of incoming energy into vibrations of atoms or ions. Sometimes, with intense exposure to a laser beam, the emitted light can have a shorter wavelength.

The fact that PL can be excited by ultraviolet radiation was discovered by the German physicist Johann Ritter in 1801. He noticed that the phosphors brightly glow in the invisible region behind the violet part of the spectrum, and thus discovered the UV radiation. The transformation of UV into visible light is of great practical importance.

Gamma and X-rays excite crystalline phosphors and other materials to the state of luminescence by an ionization process followed by recombination of electrons and ions, as a result of which luminescence occurs. It finds use in fluoroscopes used in X-ray diagnostics and in scintillation counters. The latter detect and measure gamma radiation directed at a disc coated with a phosphor that is in optical contact with the surface of the photomultiplier.

Triboluminescence

When crystals of some substances, for example sugar, are crushed, sparks are visible. The same is observed in many organic and inorganic substances. All these types of luminescence are generated by positive and negative electric charges. The latter are produced by mechanical separation of the surfaces and in the process of crystallization. Light radiation then occurs by a discharge - either directly, between fragments of molecules, or through excitation of the luminescence of the atmosphere near the separated surface.

Electroluminescence

Like thermoluminescence, the term electroluminescence (EL) includes different types of luminescence, the common feature of which is that light is radiated by electric discharge in gases, liquids and solids. In 1752, Benjamin Franklin established the luminescence of lightning, caused by an electric discharge through the atmosphere. In 1860, a discharge lamp was demonstrated for the first time in the Royal Society of London. It produced bright white light when high voltage was discharged through carbon dioxide at low pressure. Modern fluorescent lamps are based on a combination of electroluminescence and photoluminescence: mercury atoms in a lamp are excited by an electric discharge, the ultraviolet radiation emitted by them is converted into visible light by means of a phosphor.

EL observed in electrodes during electrolysis is due to ion recombination (hence, it is a kind of chemiluminescence). Under the influence of an electric field in light layers of luminescent zinc sulphide, light is emitted, which is also called electroluminescence.

A large number of materials emit glow under the influence of accelerated electrons - diamond, ruby, crystalline phosphorus and some complex platinum salts. The first practical application of cathodoluminescence is the oscilloscope (1897). Similar screens using improved crystalline phosphors are used in TVs, radar, oscilloscopes and electron microscopes.

Radioluminescence

Radioactive elements can emit alpha particles (helium nuclei), electrons and gamma rays (high-energy electromagnetic radiation). Radiation luminescence is a luminescence excited by a radioactive substance. When alpha particles are bombarded with crystalline phosphorus, tiny flickering is seen under the microscope. This principle was used by the English physicist Ernest Rutherford to prove that the atom has a central core. Self-luminous paints, used for marking watches and other instruments, operate on the basis of radar. They consist of a phosphor and a radioactive substance, for example tritium or radium. Impressive natural luminescence is the northern lights: radioactive processes on the Sun throw huge masses of electrons and ions into the space. When they approach the Earth, its geomagnetic field directs them to the poles. Gas-discharge processes in the upper layers of the atmosphere create the famous polar lights.

Luminescence: the physics of the process

The emission of visible light (ie, with wavelengths between 690 nm and 400 nm) requires an excitation energy, the minimum of which is determined by Einstein's law. The energy (E) is equal to the Planck constant (h) multiplied by the light frequency (ν) or by its speed in vacuum (s) divided by the wavelength (λ): E = hν = hc / λ.

Thus, the energy required for excitation ranges from 40 kilocalories (for red) to 60 kilocalories (for yellow) and 80 kilocalories (for violet) per mole of substance. Another way of expressing energy - through electron-volts (1 eV = 1.6 × 10 ^-12 ergs) - from 1.8 to 3.1 eV.

The excitation energy is transferred to electrons responsible for luminescence, which jump from their basic energy level to a higher one. These states are determined by the laws of quantum mechanics. Different mechanisms of excitation depend on whether it occurs in single atoms and molecules, in combinations of molecules or in a crystal. They are initiated by the action of accelerated particles, such as electrons, positive ions or photons.

Often the excitation energy is much higher than necessary to raise the electron to the level of radiation. For example, glow of phosphor crystals in television screens is produced by cathode electrons with average energies of 25,000 electron-volts. Nevertheless, the color of fluorescent light is almost independent of the energy of the particles. It is affected by the level of the excited state of the energy of the crystalline centers.

Fluorescent lamps

The particles, due to which luminescence arises, are the outer electrons of atoms or molecules. In fluorescent lamps, for example, a mercury atom is excited under the influence of energy 6.7 eV or more, raising one of the two external electrons to a higher level. After its return to the ground state, the difference in energy is radiated in the form of ultraviolet light with a wavelength of 185 nm. The transition between another level and the base produces ultraviolet radiation at 254 nm, which, in turn, can excite other luminophores that generate visible light.

This radiation is especially intense at low mercury vapor pressures (10 ^-5 atmospheres) used in low-pressure discharge lamps . Thus about 60% of the electron energy is converted to monochromatic UV light.

At high pressures, the frequency increases. The spectra no longer consist of a single spectral line of 254 nm, and the radiation energy is distributed over spectral lines corresponding to different electronic levels: 303, 313, 334, 366, 405, 436, 546 and 578 nm. High-pressure mercury lamps are used for illumination, since 405-546 nm correspond to the visible bluish-green light, and when a part of the radiation is transformed into red light with the help of a phosphor, the result is white.

When the gas molecules are excited, their luminescence spectra show broad bands; Not only electrons rise to higher energy levels, but at the same time vibrational and rotational motions of atoms in general are excited. This is because the vibrational and rotational energies of the molecules are 10 ^-2 and 10 ^-4 from the transition energies, which combine to form a set of slightly different wavelengths that make up one band. In larger molecules, there are several overlapping bands, one for each type of transition. The radiation of molecules in the solution is predominantly ribbon-like, which is caused by the interaction of a relatively large number of excited molecules with solvent molecules. In molecules, as in atoms, external electrons of molecular orbitals participate in luminescence.

Fluorescence and phosphorescence

These terms can be distinguished not only on the basis of the duration of the glow, but also on the way it is produced. When an electron is excited to a singlet state with a residence time of 10 ^-8 s from which it can easily return to the ground state, the substance emits its energy in the form of fluorescence. During the transition, the spin does not change. The base and excited states have a similar multiplicity.

The electron, however, can be raised to a higher energy level (called the "excited triplet state") with the inversion of its spin. In quantum mechanics, transitions from triplet states to singlet states are forbidden, and, consequently, their lifetime is much longer. Therefore, luminescence in this case has a much longer time: phosphorescence is observed.