Luminescence

LUMINESCENCE (from Latin lumen-light), 'cold light', caused not by an increase in temperature (thermal or temperature radiation), but by various other causes. Depending on the method of excitation of luminescence, several types are distinguished. These are: 1) photoluminescence - luminescence caused by preliminary illumination; 2) electroluminescence - luminescence under the influence of electrical effects, in particular cathodoluminescence - luminescence under the action of cathode rays; 3) chemiluminescence - luminescence under the influence of chemical processes, for example, luminescence accompanying the oxidation of phosphorus, luminescence of rotten wood; various types of biological luminescence (fireflies, _v luminescent bacteria); 4) triboluminescence - luminescence under the influence of mechanical effects (for example, luminescence when breaking chalk). Of all these types of luminescence, photoluminescence is the most studied. In many cases, the luminescence that arose under the influence of illumination practically ceases simultaneously with the cessation of the latter. In these cases, we are dealing with fluorescence. In other cases, luminescence can last for many hours after the cessation of illumination - this phenomenon is called phosphorescence. In both cases, the so-called Stokes law is observed, according to which the wavelength of the emitted light is generally greater than the wavelength of the exciting light, i.e., the spectral composition of the luminescent light is shifted relative to the composition of the exciting light in the 'red' direction. Fluorescence is observed in gases, liquids and solids, phosphorescence - exclusively in solids. Fluorescence is particularly characteristic and easily excited in solutions of various dyes - fluorescein, eosin and others. At the same time, it is remarkable that fluorescence is already observed at completely negligible concentrations of the dye (approximately 10~⁶ g/cm²). In recent times, fluorescence has begun to be used for analytical purposes. Thus, most alkaloids give a characteristic and strong fluorescence even in very strong dilutions. For example, quinine (sulfate) can be detected by its blue fluorescence at a dilution of 1:10⁸. Thus, by the intensity and color of fluorescence, one can monitor the extraction of various alkaloids. Similarly, when testing the purity of pharmaceutical preparations, the fluorescence method provides very valuable services. Among other things, it is possible to distinguish novocaine, cocaine* stovaine, etc., by fluorescence. The theoretical interpretation of luminescence phenomena, especially photoluminescence, has made great progress thanks to the development of the quantum theory (see Quantum theory) and the theory of atomic structure (see Atom). In particular, the process of fluorescence of gases has received a very detailed explanation. According to modern concepts, molecules or atoms of a substance that have absorbed a light quantum pass into a special excited state, in which one of the external electrons moves to a more distant orbit. If the substance is in a gaseous state at sufficient rarefaction, so that collisions between molecules are relatively rare, then such an excited molecule is able to remain in a state characterized by excess energy for a very short, but finite period of time (approximately 10^-7 sec.). After this time, the electron spontaneously returns to the normal stable orbit, and the excess energy is radiated in the form of fluorescence light. With an increase in gas pressure, the probability of collisions between molecules increases, and it may happen that the excited molecule will experience a collision before the electron has time to return to the normal orbit. In this case, the transition from the excited to the normal state will no longer be accompanied by light emission, and the excess energy is most often used for a chemical reaction (for example, for the decomposition of the colliding molecule) or can pass into the kinetic energy of the translational motion of the colliding molecules, i.e., ultimately - into heat. Thus, with an increase in gas pressure, fluorescence should weaken, which is actually observed (quenching of fluorescence). In liquids, fluorescent molecules undergo numerous impacts from the molecules of the solvent. Nevertheless, in this case, it has also been directly established by experiment that excited molecules are 'delayed' in this state for periods of time on the order of 10^-7 sec. Thus, it turns out that those parts of fluorescent molecules (usually very complex), where the fluorescence process itself takes place, are somehow protected from the deactivating influence of the solvent molecules. And only with an increase in concentration, when collisions between fluorescent molecules become more frequent, the same processes of fluorescence quenching are observed as in gases. In the case of chemiluminescence, the mechanism of luminescence is completely analogous to the one considered; only here the excitation of molecules is not due to the absorption of light, but due to the thermal effect of the reaction. For example, if the reaction proceeds according to the type A + B → AB + Q, then the energy Q, released during this reaction, is initially concentrated in the AB molecule, which as a result comes into an excited state and, under certain conditions, can give up its excess energy by emitting light. The mechanism of phosphorescence is not yet fully clear. It is very probable that this phenomenon is associated with the existence of so-called 'metastable' states, i.e., such excited states from which a molecule cannot spontaneously return to the normal state. To return to the normal state, a metastable molecule must experience some disturbance from outside, for example, an impact. The lifetime of such molecules can be very long.

E. Shpolsky. Luminescence of plants, bacteria, fungi, and animals. In the animal world, luminescence is observed in representatives of almost all animal groups. Since the time of Aristotle and Pliny, the glow of decaying wood in the dark has been known. In the middle of the 19th century, this phenomenon was described and studied by Heller and Ludwig, who established that its cause lies in the production of light by fungi living in the wood. It has been established (by Molisch and others) that about 30 species of fungi produce light. The production of light occurs inside cells provided they are fully viable, in a moist environment and with a sufficient amount of oxygen. Luminescence is very common in bacteria (about 35 species). An attempt to combine them into one genus, Photobacterium, proved unsuccessful. The luminescence of bacterial cultures explains the glow of decaying meat and dead animals, known since the time of Aristotle, as well as the occasional glow observed in open wounds. Among the artificial media necessary for bacterial luminescence, peptone, carbohydrates (glucose), or substances of the lecithin group are noted. Oxygen is absolutely necessary for luminescence, even in the smallest doses, and water. Salts are also a necessary component of the medium for cultivating bacteria, playing mainly an osmotic rather than a chemical role. Very favorable results are obtained with a 3% content of NaCl in the medium, but it can be successfully replaced by a number of other salts: KNO₃, KCl, etc., and even equivalent amounts of sugar. Narcotizing substances in large doses stop luminescence, while small amounts of ether, various alcohols, and also KCN in a concentration of ^t/_{10-^t/125 somewhat activate luminescence (a stimulating effect on the catalytic processes underlying the phenomenon being studied).-The active reaction of the medium is very important for luminescence. A slightly alkaline reaction is most favorable. The lower temperature limit for bacterial luminescence is -10°, the upper corresponds to the upper limit of enzymatic processes (about 40-50°). The optimum varies for different species, averaging around 20°. Bright sunlight stops luminescence, affecting the red part of the spectrum; violet rays, on the contrary, somewhat stimulate luminescence. Ultraviolet rays and Ra rays do not disturb luminescence. In protozoa, luminescence has been described in a number of species of Radiolaria and Dinoflagellata, contained in enormous quantities in seawater; this explains the well-known phenomenon of periodic 'sea glowing.' Luminescence has been described for a number of groups of crustaceans, myriapods, and insects; among the latter, beetles hold a special place. Luminescent beetles (from the families Malacodermidae and Elateridae) served as the subject of major works by Dubois and Garvey, who studied the physiology of luminescence. In contrast to bacteria and fungi, where continuous luminescence occurs, in almost all animals, short-term flashing luminescence is encountered, which is a response to irritation. Luminescence can be caused by the action of various factors-osmotic, mechanical, electrical, etc. In contrast to bacteria, where luminescence is always intracellular, in animals, extracellular luminescence of secreted substances also occurs. Physically, the luminescence of plants and animals is characterized by a continuous but very short spectrum with a wavelength ranging from 0.41 μ to 0.72 μ and an extremely small production of heat; the efficiency of luminescence is almost 100%. The intensity of the light itself is also minimal. The light produced during luminescence can be polarized, capable of causing fluorescence, phosphorescence, and exerting a photochemical effect on a photographic plate and participating in the formation of chlorophyll in leaves, as well as causing phototropic bending of plants. Various authors have described different colors of luminescence. The dependence of luminescence on the presence of O2, known since the time of Spallanzani, led to the belief that luminescence appears as a result of an oxidative process. The assumption that chemiluminescence occurs here acquired a high degree of reliability after Radzishewsky succeeded in obtaining light production in vitro in an alkaline medium when oxygen acts on a number of substances: aldehydes, fatty acids, polyhydric alcohols, phosphatides, etc. The chemical mechanism of luminescence was finally elucidated by the major works of Dubois and Garvey, who studied dried luminescent organs of animals or their secretions and showed that luminescence occurs during the interaction of two substances: luciferin and the enzyme luciferase. Luciferin (photogen by some authors), apparently a protein-like substance, is formed inside cells as a result of a purely biological process from proluciferin under the action of the enzyme coluciferase (photogenesis). It can withstand heating up to 70-100°. Luciferase (an enzyme belonging to the group of oxidases) can only withstand heating up to 60° and, when acting on luciferin, can be replaced by a number of oxidizers: permanganate, hydrogen peroxide, etc. The process of luminescence itself, therefore, does not have a vital character. Luminescence can be achieved with the smallest traces of oxygen; according to Harvey's data, positive results are obtained if one part of O2 is to 3,700,000 parts of water. An increase in temperature by 10° increases the rate of the luciferin-luciferase reaction by 3-4.5 times. The intensity of luminescence is proportional to the partial pressure of oxygen. Both luciferin and luciferase have a high degree of specificity, and luminescence does not occur when these substances from different species interact. The constancy of luminescence, which continues, as for example in bacteria, without any interruption for many days, indicates that in this case there is a mobile equilibrium of reacting substances, and there is a reduction of oxidized luciferin-oxyluciferin with its conversion back to its original state. According to Harvey, the chemical reactions occurring during luminescence can be represented as follows: if LH2 denotes luciferin, and L-oxyluciferin, then LH2 + O + Luciferasa → L + H2O + Luciferasa. The existing equality of energy release and absorption explains the absence of CO2 formation (O is bound to H) and heat. At the same time, the original substances are restored and again enter into the reaction. Luminescence is thus an extremely economical form of light production.-The morphology of light production cannot be considered sufficiently studied. Regarding the biological significance of luminescence, there is a certain variety of views in the literature. Its significance for the sexual process (attraction and finding of mates) in beetles and some worms can be considered established. Similarly, the protective role of luminescence is insufficiently studied, although it is probable.-Some authors (Trojan, Heller) completely deny the biological significance of luminescence, considering it only as a means for cells to get rid of decay products. According to Mangold, the author of an extensive review on luminescence, its origin is exactly that, and in the process of evolution, in some forms, luminescence acquired secondary biological significance.-The extreme economy of bioluminescence has long given rise to attempts at its technical application. Molisch, Dubois, and others constructed bacterial lamps-glass vessels filled with broth with luminescent bacteria; however, these attempts did not find practical application, as the light turned out to be very weak.}

S. Zalkind.