Luminescence is the emission of light by certain materials in a relatively cold state. It differs from the radiation of hot bodies, such as burning wood or coal, molten iron and wire, heated by electric current. Luminescence radiation is observed:
- in neon and fluorescent lamps, televisions, radars and screens of fluoroscopes;
- in organic substances such as luminol or firefly luciferin;
- in some pigments used in outdoor advertising;
- with lightning and northern lights.
In all these phenomena, light radiation is not the result of heating the material above room temperature, therefore 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 absorption of energy by a material, for example, from a source of ultraviolet or X-ray radiation, electron beams, chemical reactions, etc. This leads to atoms of a substance in 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 luminescence efficiency depends on the degree of conversion of the excitation energy into light. The number of materials with sufficient efficiency for practical use is relatively small.
Luminescence and glow
The excitation of luminescence is not associated with 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 even at room temperature, this is sufficient for radiation to occur 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, atomic collisions can be so strong that the electrons separate from them and recombine, emitting light. In this case, luminescence and incandescence become indistinguishable.
Luminescent pigments and dyes
Conventional pigments and dyes have a color, as they reflect that part of the spectrum that is complementary to the absorbed. A small portion of the energy is converted to heat, but no noticeable radiation occurs. If, however, the luminescent pigment absorbs daylight in a certain part of the spectrum, it can emit photons other than reflected ones. This occurs as a result of processes inside the dye or pigment molecule, due to which ultraviolet can be converted into visible, for example, blue light. Such luminescence methods are used in outdoor advertising and in washing powders. In the latter case, the “clarifier” remains in the fabric not only to reflect white, but also to convert ultraviolet radiation to blue, compensating for yellowness and enhancing whiteness.
Early research
Although lightning, the northern lights and the dim glow of fireflies and mushrooms have always been known to mankind, the first studies of luminescence began with synthetic material when Vincenzo Cascariolo, an alchemist and shoemaker from Bologna (Italy), heated a mixture of barium sulfate (in the form of barite, in 1603). heavy spar) with coal. The powder obtained after cooling emitted a bluish glow at night, and Cascariolo noticed that it could be restored by exposing the powder to sunlight. The substance was called “lapis solaris,” or sun stone, because alchemists hoped that it could turn metals into gold, the symbol of which is the sun. The afterglow caused the interest of many scientists of that period, who gave the material other names, including “phosphorus”, which means “light carrier”.
Today, the name “phosphorus” is used only for a chemical element, while microcrystalline luminescent materials are called phosphor. The "Phosphorus" Cascariolo appears to have been barium sulfide. The first commercially available phosphor (1870) was "Balman's paint" - a solution of calcium sulfide. In 1866, the first stable zinc sulfide phosphor was described - one of the most important in modern technology.
One of the first scientific studies of luminescence, which manifests itself in rotting wood or flesh and in fireflies, was performed in 1672 by the English scientist Robert Boyle, who, although he did not know 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 such chemical agents as alcohol, hydrochloric acid and ammonia;
- radiation requires access to air.
In the years 1885-1887, it was observed that crude extracts obtained from West Indian fireflies (firecrackers) and folad mollusks produce light when mixed.
The first effective chemiluminescent materials were non-biological synthetic compounds, such as luminol, discovered in 1928.
Chemoluminescence and Bioluminescence
Most of the energy released in chemical reactions, especially oxidation reactions, takes the form of heat. In some reactions, however, part of it is used to excite electrons to higher levels, and in fluorescent molecules until chemiluminescence (CL) occurs. Studies show that CL is a universal phenomenon, although the luminescence intensity is so low that the use of sensitive detectors is required. There are, however, some compounds that exhibit bright CL. The most famous of them is luminol, which, when oxidized with hydrogen peroxide, can give 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 energy, since less than 1% of the molecules emit light. In the 1960s, it was discovered that oxalic acid esters, oxidized in anhydrous solvents in the presence of highly 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 an emitting state. All bioluminescent reactions known today are catalyzed by oxidation reactions occurring in the presence of air.
Thermostimulated Luminescence
Thermoluminescence does not mean temperature radiation, but amplification of the light radiation of materials whose electrons are excited by heat. Thermostimulated luminescence is observed in some minerals, and especially in crystalline phosphors, after they were excited by light.
Photoluminescence
Photoluminescence, which occurs under the influence of electromagnetic radiation incident on a substance, can be produced 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, as a rule, is equal to or greater than the wavelength of the exciting light (i.e., equal to or less energy). This difference in wavelength is due to the conversion of incoming energy into vibrations of atoms or ions. Sometimes, with intense exposure to a laser beam, the emitted light may 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 phosphors glow brightly in the invisible region behind the violet part of the spectrum, and thus discovered UV radiation. The conversion 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 the process of ionization followed by recombination of electrons and ions, resulting in luminescence. It finds application in fluoroscopes used in X-ray diagnostics, and in scintillation counters. The latter register and measure gamma radiation directed to a disk coated with a phosphor, which is in optical contact with the surface of the photomultiplier.
Triboluminescence
When crystals of certain substances, such as sugars, are crushed, sparks are visible. The same is observed for 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 surfaces and during crystallization. The light radiation then occurs by discharge - either directly, between fragments of molecules, or through the excitation of luminescence of the atmosphere near a separated surface.
Electroluminescence
Like thermoluminescence, the term electroluminescence (EL) includes various types of luminescence, a common feature of which is that light is emitted during an electric discharge in gases, liquids, and solid materials. In 1752, Benjamin Franklin established the luminescence of lightning caused by electric discharge through the atmosphere. In 1860, a discharge lamp was first demonstrated at the Royal Society of London. It produced bright white light during a high voltage discharge 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 using a phosphor.
EL observed at electrodes during electrolysis is due to ion recombination (therefore, this is a kind of chemiluminescence). Under the influence of an electric field, light is emitted in thin layers of luminescent zinc sulfide, which is also called electroluminescence.
A large amount of materials emits a glow under the influence of accelerated electrons - diamond, ruby, crystalline phosphorus and some complex salts of platinum. The first practical application of cathodoluminescence is an oscilloscope (1897). Similar screens using advanced crystalline phosphors are used in televisions, radars, oscilloscopes, and electron microscopes.
Radioluminescence
Radioactive elements can emit alpha particles (helium nuclei), electrons and gamma rays (high-energy electromagnetic radiation). Radiation luminescence is the luminescence excited by a radioactive substance. When alpha particles bombard crystalline phosphorus, tiny flickers are visible under the microscope. This principle was used by the English physicist Ernest Rutherford to prove that the atom has a central nucleus. 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, such as tritium or radium. Impressive natural luminescence is the northern lights: radioactive processes on the sun throw huge masses of electrons and ions into space. When they approach the Earth, its geomagnetic field directs them to the poles. Gas discharge processes in the upper atmosphere create the famous auroras.
Luminescence: physics of the process
The emission of visible light (i.e., with wavelengths between 690 nm and 400 nm) requires an excitation energy, the minimum of which is determined by Einstein's law. Energy (E) is equal to Planck's constant (h) times the frequency of light (ν) or 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 is through electron volts (1 eV = 1.6 × 10 -12 erg) - from 1.8 to 3.1 eV.
The excitation energy is transferred to the electrons responsible for luminescence, which jump from their main 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 through exposure to accelerated particles, such as electrons, positive ions or photons.
Often the excitation energy is much higher than that required to raise the electron to the radiation level. For example, the luminescence of phosphor crystals in television screens is produced by cathode electrons with an average energy of 25,000 electron-volts. Nevertheless, the color of luminescent light is almost independent of particle energy. It is affected by the level of the excited state of energy of the crystalline centers.
Fluorescent lamps
The particles that cause luminescence are the external electrons of atoms or molecules. In fluorescent lamps, for example, a mercury atom is excited by an energy of 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 emitted in the form of ultraviolet light with a wavelength of 185 nm. The transition between a different level and a base level produces ultraviolet radiation at 254 nm, which, in turn, can excite other phosphors 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 into monochromatic UV light.
At high pressure, the frequency increases. The spectra no longer consist of a single spectral line of 254 nm, and the radiation energy is distributed over the 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 visible bluish-green light, and when a part of the radiation is transformed into red light using a phosphor, the result is white.
When gas molecules are excited, their luminescence spectra show wide bands; not only electrons rise to higher energy levels, but also the vibrational and rotational movements of atoms as a whole are excited. This is because the vibrational and rotational energies of the molecules are 10 -2 and 10 -4 of the transition energies, which, when folded together, form many 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 the 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 well 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 method of its production. 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.
An electron, however, can be raised to a higher energy level (called the “excited triplet state”) with its spin reversed. In quantum mechanics, transitions from triplet states to singlet states are forbidden, and, therefore, their lifetime is much longer. Therefore, luminescence in this case has a much longer period: phosphorescence is observed.