Meetings and Conferences

The Third All-Union Conference on Luminescence and the Application of Luminous Compositions

On June 15–20, 1951, the Third Conference on Luminescence and the Application of Luminous Compositions was held in Moscow, convened by the Commission on Luminescence of the Division of Physical and Mathematical Sciences of the Academy of Sciences of the USSR. The conference was dedicated to the memory of Academician S. I. Vavilov, the creator and head of the Soviet school of luminescence, the initiator of the establishment of the Commission, and the organizer of all the All-Union conferences on luminescence. Conducted according to a plan drawn up by S. I. Vavilov himself, the conference summarized the results of work on luminescence and the application of luminous compositions carried out in the Soviet Union over the three years that had passed since the convening of the second conference.

Like the previous conferences, the third conference attracted the attention of broad circles of physicists, chemists, lighting engineers, physicians, biologists, geologists, and representatives of other specialties. The lively discussions of the reports read at the conference testified to the ever-growing interest in questions of luminescence and its applications.

The conference opened with a major report by V. L. Levshin, “Sergei Ivanovich Vavilov—Creator and Head of the Soviet School of Luminescence.” Briefly reviewing the results of S. I. Vavilov’s work on luminescence, the speaker showed how great the contribution of this eminent scientist was to the development of the study of luminescence. S. I. Vavilov established, first of all, the fundamental laws associated with the energetics of the luminescence process; it was shown that the absolute value of the luminescence yield—the efficiency of conversion of absorbed energy into radiation energy—can be sufficiently high; it was shown that the quantum yield of luminescence, i.e., the ratio of the number of emitted quanta to the number of absorbed quanta, remains constant over a broad range of wavelengths of the exciting light and drops sharply (in full agreement with thermodynamics) in the long-wavelength region, where the absorbed quanta prove to be smaller than the emitted ones (Vavilov’s law); the influence of various factors on the light yield was investigated and scientifically classified. S. I. Vavilov’s studies in the field of the energetics of luminescence made it possible to give an exhaustive spectrophotometric formulation of one of the fundamental laws of luminescence—the Stokes law. Studies of the influence of the concentration of the luminescent substance in solutions on their luminescent properties led S. I. Vavilov to create a unified theory of all concentration phenomena in luminescence, a theory based on concepts of inductive resonance coupling of excited and unexcited molecules and fully consistent with experiment.

The second large cycle of S. I. Vavilov’s work was devoted to the study of the polarization of the luminescence of solutions, an important characteristic determined by the anisotropic properties of luminescing molecules and their behavior in a medium. In the first works of S. I. Vavilov and of the author of the report, the foundations of the theory of polarized luminescence were laid and a new method for investigating the liquid state was given. Especially interesting and important in this cycle are S. I. Vavilov’s works on the study, by the polarization method, of the properties of elementary molecular emitters. The method of polarization diagrams proposed by S. I. Vavilov makes it possible simply to determine the multipolarity of these emitters. The discovery by S. I. Vavilov of polarization spectra of luminescence, i.e., the dependence of the observed degree of polarization on the wavelength of the exciting light, made it possible to substantiate an entirely new method for studying the structure of molecules.

An important role in the development of the doctrine of luminescence was played by S. I. Vavilov’s works on the study of the laws of decay of luminescence, on the elucidation of the relationship between long-duration and short-duration afterglow of organoluminophores, on the study of the luminescence of uranyl compounds and rare earths, etc. The systematic and profound study of the phenomenon of luminescence enabled S. I. Vavilov to give a strict, scientific definition of the very concept of luminescence.

S. I. Vavilov’s role in the development of luminescence is not limited to his own investigations. He created the Soviet school of luminescence, which works fruitfully and rightly occupies a leading place in world science in this field.

Engaged in the profound fundamental questions of the theory of luminescence, S. I. Vavilov constantly pointed to the need for an ever broader introduction of the phenomenon of luminescence into the national economy. On his initiative, luminescent lamps were developed and introduced into production; luminescent analysis was developed; varied applications of luminous compositions were worked out, etc. S. I. Vavilov was the organizer and ideological inspirer of work on luminescence and its application being carried out in the USSR. The grave loss suffered by our Soviet science and the public with the death of S. I. Vavilov is especially heavy for all those working in the field of luminescence.

Prof. T. P. Kravets, Corresponding Member of the Academy of Sciences of the USSR, delivered a vivid report on the life and work of S. I. Vavilov. Having dwelt in particular detail on the first years of S. I. Vavilov’s scientific activity, the speaker showed how the talented pupil absorbed the best traditions of his teacher, the outstanding Russian physicist P. N. Lebedev. By a series of examples T. P. Kravets illustrated the significance of the conditions in which a scientist is formed, and of his first scientific impressions, for the entire subsequent creative scientific activity. After briefly considering the main lines of S. I. Vavilov’s scientific work, the speaker gave a concise and expressive characterization of S. I. Vavilov as a remarkable teacher, historian and popularizer of science, scientific director of a number of institutions, and then president of the Academy of Sciences, an outstanding state and public figure.

The agenda of the conference, which included 50 reports read at 11 sessions, contained the following main sections: 1) molecular luminescence of substances in the gaseous, liquid, and solid states, 2) luminescence of crystallophosphors, 3) luminescent analysis and other applications of luminescence, and 4) luminescent light sources and their application.

1. Two reports of the first section were devoted to the study of the luminescence of vapors of complex organic molecules. Questions of the luminescence of molecules in the gaseous state, considered for the first time at a conference on luminescence, are extremely important for understanding the processes of intramolecular transformation of excitation energy,

since in vapors it is possible to observe the behavior of molecules isolated from the medium. The productivity of studies of luminescence of simple and complex aromatic compounds was especially clearly shown in the report of B. S. Neporent. Having investigated the continuous absorption and emission spectra of vapors of complex molecules, the author convincingly showed that the diffuse character of the spectra is determined mainly by intramolecular interactions, and not by the influence of the medium, as might have been thought on the basis of the study of spectra of solutions. Analysis of the width and shape of absorption and emission bands led the author to the conclusion that, according to the character of the spectra, molecules can be divided into two main categories, to which one or another value of the probability of intramolecular redistribution of energy over the degrees of freedom can be assigned. At a relatively small value of this probability (weak coupling of vibrational and electronic states), the band width does not depend on the frequency of the electronic transition, and the absorption and emission spectra turn out to be mirror-symmetric on the frequency scale (Levshin’s rule). With strong coupling of the vibrational and electronic states, the band width of the emission is proportional to the square of the frequency of the electronic-vibrational transition, and the spectra are mirror-symmetric on the wavelength scale. The speaker particularly emphasized that, when studying all the properties of complex aromatic molecules determined by their vibrational energy, the molecules should be regarded as classical statistical systems with a large number of degrees of freedom.

In the report of Academician A. N. Terenin and A. V. Karyakin, sensitized fluorescence in vapors of organic compounds was considered; it is observed in mixtures of vapors of fluorescing organic compounds through the enhancement of the fluorescence of one of the components of the mixture upon addition of vapors of another. The authors succeeded in showing that the appearance of fluorescence of acridine, observed by them when naphthalene vapor was added to acridine vapor, is not connected with the effect of ignition (increase in yield) of fluorescence upon the addition of foreign vapors, studied by B. S. Neporent, as a result of the removal of excess vibrational energy from the excited molecule that has absorbed a quantum of light. This circumstance allowed the authors to conclude that the energy of naphthalene molecules excited as a result of absorption of light is transferred to acridine molecules, which by themselves absorb the exciting light rather weakly. The existence of transfer of excitation energy from naphthalene molecules to acridine molecules is confirmed by the experiments performed by the authors on selective quenching of the fluorescence of vapors by oxygen.

In the work of M. D. Galanin, “Quenching by Absorbing Substances and Sensitized Fluorescence of Solutions,” the ideas of S. I. Vavilov on the migration of excitation energy as a result of inductive resonance interaction of excited and unexcited molecules received further development. These ideas were laid by S. I. Vavilov at the foundation of his theory of concentration phenomena in luminescence, in which the interaction of chemically identical molecules was considered. In subsequent works by S. I. Vavilov and the author of the report, it was shown that transfer of excitation energy is also possible between dissimilar molecules, and the interacting molecules may differ strongly from one another in chemical nature. The necessary condition is only the corresponding arrangement of the absorption and emission bands. If an insufficiently, but strongly absorbing substance is added to a fluorescing solution, then, along with the weakening of luminescence caused by “trivial” causes (inactive absorption of the exciting light, reabsorption of fluorescence light), a weakening is observed that is connected with transfer

energy of excitation from the fluorescent molecule that has absorbed light to the nonfluorescent one. In the work under discussion, the criterion for the presence of such quenching was the measurement on a fluorometer of the reduction in the duration of the excited state. The observed regularities received a theoretical explanation in the work. If the interacting molecules are different, but both are capable of fluorescing, then the transfer of excitation energy should lead to sensitized fluorescence of the solution. A careful analysis of the entire set of facts observed in the experiment made it possible for the speaker to assert that such sensitization by inductive energy transfer does indeed take place.

A. V. Karyakin spoke about his study of the quenching by oxygen of the fluorescence of acridine derivatives in the adsorbed state. This quenching is observed only for those acridine derivatives which, when adsorbed, exhibit afterglow indicating the presence of a metastable level close to the principal excited one. Quenching proceeds in two stages: it is only slightly effective at low pressures of \(O_2\); at higher pressures the quenching efficiency is equal to unity. The lifetimes of the excited state, calculated from the quenching, are sufficiently close to the values measured directly on a fluorometer.

A number of reports concerned the establishment of a phenomenological relation between the chemical structure of organic molecules and their spectra and ability to luminesce—one of the fundamental problems of luminescence, the solution of which is impossible without the accumulation of a large amount of experimental data.

In the report by V. L. Levshin on work carried out by him jointly with T. M. Tarasova, the influence of temperature on the absorption and emission of complex molecules was considered. This investigation was carried out for a number of acridine derivatives. Study of the spectra of these compounds showed that the character of the spectrum depends substantially on the symmetry of the molecular structure. Symmetrical molecules exhibit two absorption maxima in the visible and far ultraviolet regions of the spectrum. In unsymmetrical molecules an additional intermediate band appears. As the temperature is raised this intermediate band is rapidly weakened. The far ultraviolet band, upon cooling, broadens toward longer wavelengths, with additional vibrational maxima being revealed. The absorption band in the visible region increases in intensity and broadens toward longer wavelengths as the temperature is lowered. Molecules with two acridine nuclei have absorption and luminescence spectra with two maxima. Analysis of these spectra makes it possible to resolve them into two pairs of curves having the form of the spectra of molecules with one acridine nucleus. This allowed the authors to draw the conclusion that the action of the acridine nuclei is additive with respect to absorption. The change in the yield of luminescence with temperature is determined to a significant extent, as the authors showed, by the viscosity of the solvent.

E. V. Shpol’skii and A. A. Il’ina investigated the fluorescence spectra and phosphorescence spectra of polycyclic aromatic hydrocarbons of the pyrene series in frozen solutions at the temperature of liquid air and liquid nitrogen. For comparison, they studied the spectra of aromatic hydrocarbons not belonging to the pyrene derivatives but containing the same number of benzene rings in a different arrangement (1,2-benzanthracene, 1,2,5,6-dibenzanthracene). The spectra consist of a series of narrow bands exhibiting a regular arrangement. In the case of 3,4-benzpyrene, the character of the spectrum undergoes a noticeable change upon excitation in the long-wavelength absorption band, coinciding with the short-wavelength fluorescence band: the bands become narrower, and the entire spectrum is shifted

into the long-wave region. This phenomenon recalls the change in the character of the fluorescence spectrum of aniline vapors, observed by A. N. Terenin, A. T. Vartanyan, and B. S. Neporent, upon excitation by different wavelengths.

A detailed consideration of the effect of the solvent on the spectra and fluorescence yield of salicylic acid and vitamin B was carried out in the work of N. A. Izmailov and V. A. Kremer. Their investigation showed that the spectra and fluorescence yield depend on the ratio of the undissociated and dissociated forms of the substance, the equilibrium between which is the result of the influence of the basicity of the solvent. In addition, the solvent affects the fluorescence of each of the limiting forms, and this influence is different for the ion and for the undissociated compound. The fluorescence and absorption spectra in inert solvents differ sharply from the spectra in alcohol; however, after the addition of insignificant amounts of alcohol, the spectra acquire the form characteristic of alcoholic solutions. This phenomenon is observed not only in alcoholic solutions but also in solutions in other polar solvents.

The report of B. A. Pyatnitskii was devoted to the study of the long-lived afterglow of organic molecules caused by metastable energy states. The author studied the phosphorescence of benzene and its simplest derivatives at low temperatures. The intensity of the emission and the lifetime of the molecule in the metastable state were investigated as functions of temperature; moreover, it was possible to establish comparatively simple analytic expressions describing the course of the experimental dependences.

A number of reports read at the conference testified to significant successes in the development of the theory of luminescence of molecular crystals.

A. S. Davydov briefly set forth the theory of luminescence of molecular crystals developed by him, based on ideas of resonant interaction between the molecules forming the crystal. This interaction leads to a change in the absorption and luminescence spectra of isolated molecules, causes migration of excitation energy in the crystal, and accounts for the occurrence of specific “crystalline” excited states. Investigation of the changes in the spectra upon transition from the isolated molecule to the crystal may serve as an additional means for studying the structure of molecular crystals. Ideas of resonant interaction make it possible to explain a number of properties of the luminescence of crystals, in particular the absence of concentration quenching in many molecular crystals. Consideration of crystalline excited states makes it possible to investigate the anisotropic absorption and emission of light by molecular crystals.

An experimental investigation of the luminescence and absorption of light in crystals of organic compounds was carried out by A. F. Prikhotko. Comparison of the absorption spectra of crystals with the absorption spectra of vapors and cooled solutions of a number of organic compounds—benzene, naphthalene, anthracene, phenanthrene, and naphthacene—made it possible to discover the appearance in the crystal of new “purely crystalline” levels, responsible for the appearance in the spectra of crystals of sharply polarized absorption bands absent from the absorption spectra of vapors and cooled solutions. Weakly polarized bands can be brought into correspondence with series in the spectra of vapors. The study of luminescence spectra showed that, whereas in vapors or solutions radiation occurs from “molecular” levels, in crystals the most intense radiation is due to transitions from “purely crystalline” levels. Investiga-

the polarization of absorption lines and the intensity of absorption and luminescence bands makes it possible to draw conclusions as to whether particular lines belong to the substance under study itself, or to impurities contained in it. In the latter case, apparently, resonance lines due to crystalline levels should be absent.

A. N. Sevchenko reported on the spectra and decay of the photoluminescence of uranium compounds. He investigated the infrared absorption spectra of a number of uranium salts; analysis of the spectra made it possible to show that in some compounds the uranyl ion has a bent form, whereas in other crystals all three atoms forming the ion are arranged along one straight line. Using uranyl nitrate as an example, the speaker showed that luminescence spectra depend extremely strongly on the amount of crystallization water contained in the crystal. An equally sharp change in the structure of the spectra was observed when ordinary crystallization water was replaced by heavy water. This made it possible to conclude that the crystal lattice plays an essential role in forming the structure of the spectra: only an insignificant part of the lines is due to transitions between electron-vibrational levels of the uranyl ion. These data allowed the speaker to subject to criticism all proposed schemes of energy levels of uranium compounds up to the present time. The work also studied the temperature dependences of the yield and lifetime of the excited state for various uranium salts and solutions. In most of the cases investigated, shortening of the lifetime of the excited state with increasing temperature occurs in parallel with a drop in the yield, which indicates that quenching proceeds, according to S. I. Vavilov’s classification, according to the scheme of quenching of the second kind, i.e. the excited molecules are quenched.

Investigating the influence of hydrolysis products on the luminescence spectra of crystals of uranium salts, B. E. Gordon came to the conclusion that the spectra of frozen solutions of uranyl salts with anions incapable of complex formation are a superposition of two spectra, \(UC_2OH^+\) and \(UO_2^{++}\). In the spectra of uranyl sulfate, hydrolysis develops so strongly that practically only the luminescence of \(UO_2OH^+\) is observed. The author considers that at room temperature in solutions only the \(UO_2OH^+\) ion luminesces, and that in many cases the luminescence of crystals of uranium salts may be regarded as the luminescence of crystal phosphors activated by an impurity of the basic salt carried along during crystallization. In contrast to A. N. Sevchenko, the speaker believes that anhydrous salts which do not contain addition products do luminesce.

An interesting case of intramolecular transformation of excitation energy was considered in the report by A. N. Sevchenko and A. G. Morachevsky, “Investigation of the photoluminescence of intracomplex compounds of europium and samarium.” In these compounds absorption is carried out by the organic part of the complex (benzoylacetone, benzoylmethane), while the characteristic line spectrum indicates that the atoms of the rare earths are responsible for the emission.

Having synthesized a number of such compounds, the authors investigated their absorption and emission spectra, the laws of decay of luminescence, as well as temperature quenching and the shortening of the lifetime of the excited state with increasing temperature. Investigation of the dependence of the quantum yield of luminescence on the wavelength of the exciting light showed its constancy, i.e. the transfer of excitation energy from the organic part to the rare-earth atom occurs with equal probability, independently of the excitation level of the organic part of the complex.

1. Sixteen reports were devoted to questions of the luminescence of crystalline phosphors.

V. V. Antonov-Romanovskii, in his report “On the bleaching action of ultraviolet light on phosphors,” presented a number of new facts indicating that exciting light can exert on crystalline phosphors both flashing-up and quenching actions, united by the general term “bleaching action.” The relation between one or the other action is determined by the wavelength of the exciting light; however, in all cases it is necessary to take into account both the bleaching action and the storage of light sum, determined to a significant degree by the equilibrium between the exciting and bleaching actions. The assumption of the existence of a bleaching action enabled the author to interpret the observed anomalies in the degree of filling, at different temperatures, of trapping levels of different depth. The author considers it possible to explain, on the basis of the assumption of the bleaching action of the exciting light, the peculiar character of the glow-flare curves of certain phosphors, consisting in the presence of a sharply expressed maximum on the flare curve, substantially exceeding in magnitude the value of the stationary brightness.

Phenomena of quenching of the luminescence of crystalline phosphors, important for understanding the mechanism of the phosphorescence process, were considered in the reports of F. I. Vergunas and F. D. Klement.

F. I. Vergunas studied temperature quenching and photoconductivity of zinc sulfide phosphors. Having studied phosphors activated with zinc, copper, and manganese, she established that the activation energy necessary for quenching luminescence practically does not depend on the wavelength of the exciting light in the case of ZnS · Zn phosphors. At the same time, the activation energy in ZnS · Cu and ZnS · Mn phosphors depends on \(h\nu_{\text{ex}}\) and has one value under excitation by light with wavelength \(365\ \mathrm{m}\mu\), and two values under excitation by light in the region of the intrinsic absorption of zinc sulfide. The two different values of the activation energy are explained by the presence of two possible ways of converting excitation energy into heat: in the process of migration from the lattice to the activator (external quenching), and inside the activator (internal quenching). A parallel study of the photoconductivity of the same phosphors showed that the magnitude of the photoconductivity does not change in the region of temperature quenching of luminescence. This enabled the author to draw the conclusion that internal quenching exists in phosphors.

The report of F. D. Klement was devoted to establishing the connection between spectral properties and quenching phenomena in crystalline phosphors. The principal factors causing luminescence quenching (an increase in temperature and in activator concentration), as the speaker showed, have the same effect on the spectral properties of crystalline phosphors. This made it possible for the author to bring together the various kinds of temperature quenching and, in particular, to consider that concentration quenching is connected with a change (usually a decrease) in the activation energy of the radiationless transition. The change in activation energy manifests itself experimentally in a lowering of the temperature at which temperature quenching begins. This is in agreement with the observed decrease in this phenomenon and the accompanying phenomenon of a reduction of the optimal activator concentrations when the temperature of the phosphor is raised. The author showed that the degree to which phosphors are subject to quenching is connected with their crystallochemical structure. The fewer the lattice disturbances introduced by the activator, the higher the value of the optimal concentration.

N. A. Tolstoi reported on work carried out by him jointly with V. A. Arkhangel’skaia and A. M. Bonch-Bruevich and concerning

studies of the initial stages of relaxation of excited states in crystallophosphors and semiconductors. The main task that the authors set for themselves was to obtain sufficiently detailed phenomenological data on the initial stages of the kinetics of luminescence and induced conductivity under optical and electron excitation. Having investigated the regularities of the initial stages of the rise and decay of the glow of zinc-sulfide phosphors with different copper contents under different types of excitation, the authors concluded that these regularities do not fit into a simple bimolecular scheme. The authors established a nonspecificity of stationary excited states, showing that the entire variety of relaxation curves observed experimentally can be reduced to a distinction in the volume densities of excitation. With increasing temperature, the fractional-hyperbolic law of decay degenerates into a law close to exponential and almost independent of the excitation intensity. A comparison of the kinetics of photoluminescence and photoconductivity of cadmium sulfide single crystals showed that in some simple cases photoconductivity and luminescence are related to one another as a function and its time derivative. Of interest is the introduction into the circle of phenomena being compared of the little-studied phenomenon of cathodoconductivity.

X-ray luminescence of zinc-sulfide phosphors was the subject of study in the work of A. I. Rusanova. In the spectra of phosphors containing, alongside the emission band of the principal activator, an emission band belonging to zinc, the latter is excited more strongly by X-rays, whereas the band of the principal activator is excited more strongly by ultraviolet light. The yield of X-ray luminescence determined in the work proved to be close to 25%. In the course of the work, special attention was paid to the crystal-chemical structure of luminophors, and a new class of luminophors—zinc oxysulfides—was discovered.

E. K. Putseiko investigated photoelectric sensitivity and light absorption in cadmium sulfide crystals, paying special attention to elucidating the nature of the narrow selective maximum of photoelectric sensitivity located near 520 mμ. The intrinsic absorption of crystals in this region is small. The author interpreted this maximum as being due to the transition of electrons from local impurity-type levels into the conduction band. The presence of these levels in cadmium sulfide crystals was detected by the condenser and added-backlighting method. The differential photoelectric method made it possible to detect these levels in the absorption spectrum of the crystals.

The mechanism of luminescence of alkali-halide phosphors was considered in two reports.

In the report of M. P. Kasha, “On the mechanism of luminescence of X-rayed crystals of alkali-halide compounds in the ultraviolet region,” it was shown that color centers in these crystals can be divided into hole and electron centers ($D$- and $E$-centers). Hole centers account for the absorption in the long-wave ($D_1$) and short-wave ($D_2$ and $D_3$) ultraviolet parts of the spectrum. These ideas concerning the nature of color centers enabled the author to explain the whole set of diverse phenomena that he observed in alkali-halide crystals.

I. A. Parfianovich considered the mechanism of the flash and phosphorescence of alkali-halide phosphors excited by X-rays. Studies of the curves of temperature bleaching showed that these phosphors

possess a number of local levels located at different depths, with the shallower ones giving rise to phosphorescence, and the deeper ones to flashes, which can be realized both optically and thermally (in some cases only optically). Phosphors containing electrons at flash levels are colored. In parallel with the luminescence of the light sum of the flash, bleaching of the phosphors takes place. This serves as evidence that the flash levels in alkaline-earth phosphors are connected with color centers. The author emphasized that all data on alkaline-earth phosphors undoubtedly testify in favor of the recombination nature of their luminescence, and once again pointed out the erroneousness of the views of Randall and Wilkins, who considered the luminescence of these phosphors to be metastable.

Questions of the methodology for studying the luminescence of crystallophosphors were considered in two reports.

M. N. Alentsev described an ingenious method for measuring the absolute yield of ultraviolet luminescence, developed by him jointly with L. A. Vinokurov. Based on the law of the independence of the quantum yield of photoluminescence from the wavelength of the exciting light, discovered by S. I. Vavilov, this method makes it possible simply and rapidly to determine an important characteristic of phosphors with ultraviolet radiation. As is known, until now the determination of the absolute quantum yield of luminescence has been a rather complicated problem.

A new method for studying the kinetics of luminescence and photoconductivity was proposed by N. A. Tolstoy. The method consists in studying the first derivative with respect to time of the corresponding relaxation curve. Differentiation of the electrical signal, identical in shape to the curve under study, is carried out by means of a differentiating \(RC\)-circuit connected between two cathode followers. The differentiation method makes it possible: 1) to study the rate of the very initial stages of relaxation curves, 2) to determine rapidly the \(\tau\) of exponential processes, 3) to determine rapidly the parameters of processes proceeding according to a fractional-hyperbolic law, 4) to judge the parallelism of the processes of luminescence and photoconductivity.

At the conference a report was delivered by S. M. Ryvkin on the possibility of extending the method of N. A. Tolstoy and P. P. Feofilov (the “taumeter” method) toward short times. Having analyzed the taumeter method, S. M. Ryvkin showed that, using this method, one can with sufficient accuracy study exponential processes (or processes approximated by exponential ones) whose \(\tau\) has a value substantially smaller than the formation time of the exciting light \(\Pi\)-pulse. Without special experimental artifices the author succeeded in determining relaxation times of the order of \(3 \cdot 10^{-7}\) sec.

A special place on the agenda was occupied by questions, considered for the first time in sufficient detail at a conference on luminescence, concerning the nature of luminescence centers in crystallophosphors. The lively discussion on reports dealing with these questions testified to their topicality and to the significant successes achieved by Soviet researchers in this field. The main issue in the discussion was whether luminescence centers are a quasi-molecule, i.e., a complex chemical formation, or simply atoms or ions of the activator metal.

The first point of view was developed in the report of M. A. Konstantinova-Shlezinger, “Luminescence centers and factors affecting the process of phosphor luminescence,” based on the results of work carried out by her jointly with N. A. Gorbacheva, E. T. Teremetskaya, and Z. A. Trapeznikova. These resul-

... led the authors to the conviction that the luminescence of crystal phosphors is caused, as a rule, by quasimolecules formed by the activator with the anions surrounding it. Such a point of view makes understandable a number of facts known from the experimental study of phosphors and their properties: 1) the change in the luminescence spectrum with fluxes, 2) the difference in the luminescence spectra of one and the same activator in different bases, 3) the dependence of the phosphor’s luminescence spectrum on the method of preparing the base, etc. From the opposite point of view, in the authors’ opinion, these facts require a special explanation in each case. As a consequence of the ideas set forth in the report, there follows a close interrelation between the luminescent properties of the phosphor and the rigidity of the crystal lattice. This is confirmed, in particular, by the fact that the given temperatures at which temperature quenching begins are almost identical for all the phosphors investigated.

Opposite views were expressed by A. A. Bundel in the report “On the State of Activators in Crystal Phosphors.” The speaker indicated that there are three points of view on the state of the activator in a crystal phosphor: 1) at the luminescence center the atom of the activator is bound to the flux anion, 2) the luminescence centers are chemical complexes with the activator as the central atom, 3) the luminescence center is a superstoichiometric atom of the activator. Having pointed out a number of facts contradicting the first two points of view, the speaker came to the conclusion that at the present time the most probable conception should be considered to be that of luminescence centers as superstoichiometric atoms.

A. A. Chepnev noted in his report the important role played in the formation of a luminophore by the dispersed state of the activator. If, in the process of formation of a luminophore, the activator does not pass into a finely dispersed state, then the luminescence yield falls sharply, since coarsely dispersed colloidal particles do not luminesce. The flux plays a very important role in this, since it can facilitate the separation of the activator in finely dispersed form. Having thoroughly investigated zinc-sulfide phosphors activated with copper, the speaker established that in the case when copper precipitates in the form of large, nonluminescing particles, possibilities arise for the appearance of zinc luminescence.

In the report of N. I. Ivanova “On the Temperature Conditions for the Formation of Crystal Phosphors,” the formation of phosphors was considered as a process of diffusion of the activator into the lattice of the base substance. The investigation, carried out on halide salts of metals of groups I and II, showed that this diffusion takes place at temperatures close to the temperature of disruption of the base lattice. The dependence of the temperature of phosphor formation on the kind of activator is insignificant. If the activator enters the lattice isomorphously, then the formation of the phosphor is greatly facilitated, and in a number of cases the phosphor may arise simply through the joint dissolution of the components of the mixture.

The nature of short-duration luminescence was investigated in the work of K. V. Shalimova. Having studied the absorption and emission spectra and the duration of the excited state of alkali-halide phosphors activated with thallium, as well as zinc oxide and zinc sulfide, the speaker came to the conclusion that many properties of phosphors can be explained if the luminescence is correlated with transitions between energy levels of the atoms or ions of the activator. With increasing concentration of the activator, a complication of the spectra is observed, which the author explains by the splitting of levels and by the lifting of the prohibition from the corresponding transitions under the action of the electric field of the lattice, whose symmetry is disturbed when a large amount of activator is introduced.

MEETINGS AND CONFERENCES

The absorption bands of excess metal atoms in sublimated salt films were the subject of a report by P. N. Kokhanenko. The author observed narrow absorption bands of films obtained by sublimating salts or by treating sublimed metallic films. These bands were attributed by the author to metal atoms in excess of stoichiometry. The presence of these excess metal atoms determines the ability of the films to luminesce; however, apparently not all excess atoms produce luminescence centers.

1. Two sessions were devoted to questions of luminescence analysis and other applications of luminescence.

In a survey report, M. A. Konstantinova-Shlezinger briefly reviewed the principal results of work on luminescence analysis carried out both in the USSR and abroad over the last three years. The speaker noted the ever broader application of luminescent methods both for chemical luminescence analysis and for monitoring production processes, as well as for solving other diverse problems posed by science and technology.

A. V. Karyakin spoke on the application of fluorescence for the defectoscopy of machine parts. In creative collaboration with industrial workers, the author succeeded in developing a simple method that makes it possible to detect surface defects on parts made of ferromagnetic materials, nonferrous metals and alloys, plastics, on parts made of austenitic steels, and on cutting tools with hard-alloy plates. The method makes it possible to estimate the depth of cracks.

A. A. Ilyina described the spectral-fluorescent method he developed for determining carcinogenic substances in resinous products, based on the observation in luminescence spectra of bands characteristic of one of the most active carcinogenic substances—3,4-benzpyrene. The presence of this substance can be detected in resin down to concentrations of 0.001–0.002%.

In three separate reports, questions related to luminescent methods for the study of petroleum and its fractions were considered. This field appears to be one of the most developed and promising areas of application of luminescent analysis methods. Here, as F. M. Effen­diev indicated in his report, the basic principles of the luminescent method of petroleum investigation are clearly formulated. The reports demonstrated the effectiveness of combining luminescent methods with chromatographic methods of separating substances in an adsorption column.

The application of luminescence analysis to problems of biology was the subject of a report by M. N. Meisel, “Luminescence-Microscopic Analysis of the Functional State of Living Matter.” The speaker showed that the luminescence-microscopic method makes it possible to recognize and identify, in living cells and tissues and also in preparations isolated from them, a number of biologically important substances. Intravital fluorochroming of biological objects makes it possible to reveal definite structures of living matter and to judge their functional significance. The fluorochroming techniques developed by the author make it possible, in a number of cases, to recognize living and dead microbial cells.

Questions of the methodology of luminescent investigations as applied to problems of medicine and biology were treated in the report by S. K. Rozental, who described a simple apparatus developed by him for observing the luminescence of both macro- and micro-objects. E. B. Rabkin spoke on the application of luminescence in

practice of medical and physiological research.

P. P. Voronov’s report on production methods of using luminous substances in light industry was accompanied by effective demonstrations. The speaker demonstrated luminous fabrics, plastics, and various products made from them.

E. M. Mandelberg described the successes achieved over the past three years in the application of luminescence in the visual arts. During these years the workshop of luminescent-decorative painting under his direction moved from experimental-research work to production activity.

V. I. Veinberg spoke about the new possibilities created as a result of using the phenomenon of luminescence in the production of motion pictures; his report was accompanied by a demonstration of films shot with the use of luminescent methods.

1. Two sessions devoted to luminescent light sources and their applications were held jointly with the Commission on Lighting Engineering of the Department of Technical Sciences of the Academy of Sciences of the USSR.

In a major report, R. A. Nilender gave a survey of work on luminescent lamps and phosphors carried out at MELZ in 1948–1950. The speaker indicated that, as a result of creative cooperation between the plant’s workers and the workers of scientific research institutes, it had been possible to create a new, improved formulation of phosphors for fluorescent lamps of various types (single-component phosphors based on halophosphates). The plant had carried out extensive work on creating the design and manufacturing technology of lamps that made it possible, within three years since the previous conference, to pass from the experimental production of fluorescent lamps to the mass production of these economical light sources.

The reports by R. G. Smirnova and F. M. Pekerman, presented by them on behalf of a group of workers from MELZ and scientific institutes, were devoted to the development and study of phosphors based on phosphates for fluorescent lamps. The authors developed in detail a number of formulations for preparing a single-component phosphor based on calcium halophosphate, activated by antimony and manganese and possessing luminescent properties that permit its use in fluorescent lamps in place of the previously used two-component mixture. The luminescent properties of halophosphate phosphors proved to be very sensitive to the chemical composition of the base, with optimal properties being obtained at a quite definite chemical composition. An interesting result of the work was the discovery of a new class of phosphors based on antimony pentoxide. The study of the physical properties of halophosphate phosphors allowed the authors to conclude that the glow of these phosphors is, to a significant extent, sensitized luminescence: the excitation energy absorbed by one of the activators (antimony) is transferred to the second activator (manganese).

I. I. Kalinichenko reported on behalf of a group of plant and institute workers on a phosphor with ultraviolet radiation for erythemal lamps. This phosphor, which is a mixture of calcium and zinc phosphates activated by thallium, possesses ultraviolet emission with a maximum at about 300 mμ and a high quantum yield. Tests of the phosphor in fluorescent lamps showed that with its aid an effective source of erythemal radiation can be created.

A brief communication on the biological significance of erythema lamps was made by D. N. Lazarev.

In the report by D. A. Shklover methods and measurements of the optical characteristics of the radiation of luminophores, luminescent lamps, and cathode-ray tubes were presented. The speaker described a number of objective photoelectric instruments that make it possible to determine the brightness of the glow of luminophores, color, luminous intensity, and brightness of luminescent lamps, as well as an instrument for the automatic recording of radiation spectra on the screen of a cathode-ray tube with a long afterglow.

A. S. Shaikevich, in his report, considered the favorable effect of luminescent lighting on visual functions and labor productivity. In the report by Ya. B. Zilberblat, on the basis of studying the influence of ambient temperature on the characteristics of luminescent lamps, the question was considered of the possibility of using luminescent lamps for the outdoor lighting of cities.

N. V. Gorbachev spoke about the use of luminescent lamps and materials in installations for architectural lighting and about the use of luminous compositions in the design of architectural illumination.

All the reports concerning the production and application of luminescent lamps testified to the firm place occupied by these new light sources in the national economy.

In closing the conference, the chairman of the Commission on Luminescence, V. L. Levshin, noted the considerable successes achieved over the past three years in the development of both the theory of luminescence and its diverse applications. The personnel working in the field of luminescence have grown significantly and become stronger. The Soviet school of luminescence founded by S. I. Vavilov continues to work intensively and fruitfully, successfully carrying out the task set by Comrade I. V. Stalin before all our science: to overtake and surpass the achievements of foreign science.

The conference adopted a resolution summing up the results of the work on luminescence carried out in the USSR in recent years and outlining the main paths for the development of this field of knowledge.

P. Feofilov