CONGRESSES AND CONFERENCES

SESSION OF THE DIVISION OF PHYSICAL AND MATHEMATICAL SCIENCES OF THE USSR ACADEMY OF SCIENCES DEVOTED TO ISSUES OF LUMINESCENCE OF SOLIDS AND LIQUIDS

On October 5–10, 1944, within the walls of the Physical Institute of the Academy of Sciences, a conference was held on questions of the luminescence of solid and liquid bodies, convened by the Division of Physical and Mathematical Sciences of the USSR Academy of Sciences. The conference was attended by representatives of the Academy of Sciences—its Divisions of Physical and Mathematical, Chemical, and Geological Sciences—of the universities of Moscow, Leningrad, and Tomsk, of numerous scientific and research institutes (GOI, VEI, Gintsvetmet, Giredmet, the Roentgen Institute, the Institute of Plastics, the Institute of Aircraft Equipment, VIEM, and others), of industry (the Electrolamp Plant, NKAP plants, and others), as well as representatives of consumer organizations. In all, 320 persons representing 113 institutions were registered as present at the conference. Almost all sessions took place in an overcrowded hall, and at times, when the most pressing questions were being discussed, the conference hall of FIAN could not accommodate all who wished to attend.

What accounts for this undoubted interest in questions of luminescence—an interest justifying the convocation of a conference that was the third on an international scale after the Warsaw congress of 1936 (convened, also to a considerable extent on the initiative of Soviet physicists) and the discussion on questions of luminescence held at the Faraday Society in 1938?

The rapid development of the theory of luminescence in recent years is explained above all by the fact that an understanding of this phenomenon, even in its most general features, became possible only from the time of the discovery of the quantum properties of light and matter, i.e., only on the basis of the most recent achievements of physical science. In addition, serious technical applications of luminescence could be realized only on the basis of new physical and technical achievements in other fields. The development of luminescence had been hindered, as Academician S. I. Vavilov noted in opening the conference, by the absence of a proper theoretical core and of major technical applications. Now both exist, and the circle of people working on luminescence has expanded considerably.

However, up to the present time there is no unanimity regarding the definition of the concept luminescence. The old definition of luminescence as an excess over the thermal radiation of a body, given by Wiedemann, is correct but suffers from excessive breadth. S. I. Vavilov proposed to restrict it by defining luminescence as an excess over the thermal radiation of a body in the case where this excess radiation possesses a finite duration of \(10^{-10}\) sec or more.

The agenda of the conference included the following principal sections: 1) Luminescence of solutions. 2) Luminescence of crystalline bodies. 3) Lumines-

cent analysis. 4) Luminescent light sources. 5) Practical application of luminous compositions. In all, 23 main reports were heard.

1. The large introductory report by S. I. Vavilov was devoted to a survey of the present state of the theory of photoluminescence of solutions. The study of solutions is of interest from several points of view. First of all, the luminescence of solutions is one of the simplest and most convenient objects for establishing the general properties of photoluminescence. Further, the study of the photoluminescence of solutions can provide new information on the structure and properties of dissolved molecules. Finally, the investigation of the luminescence of solutions is a means of studying the liquid state of matter.

The features of the spectral properties of the photoluminescence of solutions make it little like the luminescence of atoms and molecules in rarefied gases and vapors, and allow one to speak of a distinctive spectroscopy of photoluminescence. One of the fundamental regularities of photoluminescence spectra is the well-known Stokes law. A deeper analysis of this regularity enabled S. I. Vavilov to formulate this law not in the form of a purely spectral dependence, but in a broader and more general spectro-photometric form.

Under the condition that preliminary or stepwise excitation is eliminated, photoluminescence can preserve a constant quantum yield if the exciting wave is converted, on average, into a longer one than itself. Conversely, the yield of photoluminescence decreases sharply in the reverse conversion of long waves into short ones.

This proposition can be proved thermodynamically in the most general form. The constancy of the emission spectrum upon excitation by any wavelengths and the existence, established by V. L. Levshin, of mirror symmetry of absorption and emission spectra are likewise fundamental properties of the photoluminescence of solutions. The independence of the luminescence spectrum from the wavelength of the exciting light makes it possible to introduce, in the field of solution luminescence, entirely new concepts of the yield spectrum and polarization spectrum, i.e. the dependences of these quantities on the wavelength of the exciting light. These spectra can be placed, in characterizing a luminescent substance, alongside the customary emission and absorption spectra.

The concept of luminescence yield, i.e. of the ratio of luminescence energy to absorbed energy, is, in the case of solutions, considerably simpler and more definite than for rarefied vapors, again in view of the invariance of the emission spectrum. However, the fundamental question of why some molecules luminesce while others do not has not yet been solved even in the most general terms. Apparently, its solution is impossible without detailed structural analysis. The influence on luminescence yield of various factors—concentration of luminescing molecules, solvent, temperature, foreign impurities, etc.—has been clarified much better. It has proved possible to classify quenching processes and to construct a theory of quenching by foreign impurities, as well as a theory of concentration quenching, based on the concept of energy transfer as a result of quantum-mechanical resonance.

Turning to questions connected with the polarization of photoluminescence, S. I. Vavilov described an elegant polarization method for determining the multipole character of emitters, developed by him recently. Observations of the polarization of luminescence of solutions also make it possible to judge the behavior of complex molecules in solution—providing new information on the properties of the liquid state. Of great interest is the dependence of polarization on concentration—concentration depolarization. The theory of this phenomenon developed by S. I. Vavilov, based on the concept of the quantum-mechanical process of energy transfer from excited molecules to unexcited ones, makes it possible to explain the empirical dependences observed here.

In conclusion, S. I. Vavilov dwelt on questions concerning the lifetime of excited states and the laws of decay of photoluminescence, pointing to the rationality of dividing the phenomena of photoluminescence into spontaneous, forced, and recombination processes on the basis of the mechanism of luminescence-

tion. Of particular interest are metastable states with subsequent forced transitions. They arise as a result of internal relations in the molecule upon excitation and, only after arising, begin to depend on the external medium.

A. N. Terenin delivered a report of great interest on the influence of the medium on the photoluminescence of the simplest aromatic compounds. The influence of the medium is appropriately considered in two limiting cases: 1) in the case of luminescence of molecules in the gaseous state in the presence of a foreign gas, and 2) in the case of molecules dissolved in a solid phase. A. N. Terenin spoke first of all about a very interesting phenomenon, discovered and studied in his laboratory by B. S. Neporent (GOI). It consists in the ignition, i.e., an increase in the yield of photoluminescence when foreign impurities are added to a gas. The magnitude of this effect increases upon transition to excitation by shorter-wavelength rays, and in the region of diffuse bands associated with the photochemical decomposition of molecules as a result of predissociation. The phenomenon may be interpreted as stabilization of an excited molecule that is at the threshold of decomposition, by removal of the excess vibrational energy. It may be assumed that the deactivation of excited molecules of aromatic compounds is connected with the development of a special kind of deformation vibrations of the molecule—torsional vibrations, leading to disruption of the planar configuration of the aromatic molecule. The role of such torsional vibrations must be especially great in the case of aromatic compounds, since here any deformation of the type of a skewing or rotation of one part of the molecule relative to another diminishes the resonance of the electronic system. Thus, the connection between electronic and vibrational states is especially strong in the case of aromatic molecules. It is evident that the most effective stabilization of an excited molecule will occur in solid solutions, where the mobility of the molecular skeleton is strongly restricted. Indeed, in a number of cases one can observe an intensification or even the appearance of luminescence when nonfluorescent compounds are introduced into solid solutions.

Another group of questions touched upon by A. N. Terenin in his report is connected with the sensitized phosphorescence of a number of aromatic compounds excited in the absorption region of the solvent. Excitation of this region of the spectrum in the gas phase leads to predissociation of the solvent. In the condensed phase the absorbed energy is transferred to molecules capable of luminescing and is emitted by them with their characteristic spectrum. The speaker noted that this phenomenon of sensitized phosphorescence is observed for molecules capable of association by means of hydrogen bonds. Probably, for phosphorescence to appear, a closer connection of the molecule with its surroundings is necessary than the forces of van der Waals interaction. Such a connection may be a hydrogen bond, by means of which a molecule dissolved in the solid phase is retained in the structural network of the glassy medium. The hydrogen bond affects resonating structures and, by introducing a factor of perturbation, violates the selection rules of electronic transitions that are valid for a free molecule.

Next, the report of P. P. Feofilov (GOI), “Anisotropy of molecules and polarization of luminescence,” was heard. Analyzing polarization spectra (the dependence of luminescence polarization on the wavelength of the exciting light) and comparing them with the spectral change of the dichroism of oriented molecules, the speaker showed that, in order to interpret polarized luminescence, it is necessary to assume the existence in the molecule, alongside the emission oscillator, of a set of absorbing oscillators forming definite rigid angles with the axes of the molecule. Comparison of the polarization and absorption spectra of a large number of organic dyes shows that, in a first approximation, each separate band in the absorption spectrum may be described by an individual linear electronic oscillator. A joint analysis of the spectra of polarization, absorption, and dichroism may prove extremely useful—

... for elucidating the features of the electronic structure of complex organic molecules.

L. A. Tummerman (FIAN) reported in his paper on very interesting phenomena. Studying the laws of quenching of the photoluminescence of complex molecules in solution by means of a fluorometer, the speaker found that at low temperatures the quenching of the luminescence of a number of dyes ceases to be exponential over the entire interval, and the exponential portion of the decay curve is preceded by a “dark pause,” equal, in order of magnitude, to the decay time. The peculiarity of this dark pause consists, among other things, in the fact that it can be greatly shortened and even destroyed by very small additions of quenching impurities. Direct fluorometric data on the dark pause are confirmed by indirect polarization measurements. It is not at present possible to give a satisfactory physical interpretation of the observed phenomena.

A. N. Zaidel (LGU) reported on work on the study of the luminescence of solutions of rare-earth salts, which forms part of the research in the field of spectroscopy of rare earths carried out by the speaker jointly with A. N. Filippov and Ya. I. Larionov. The large body of experimental material accumulated as a result of this work made it possible to systematize the spectra and to express a number of judgments about the nature of the luminescing centers. Thus, it was found that non-fluorescing solutions of EuCl₃ begin to luminesce upon addition of traces of the SO₄ ion. The same was also observed with certain other rare earths. These phenomena can be explained if one assumes that the fluorescence center is a formation containing, in addition to the rare-earth ion, also an anion. It is possible that such a formation is simply an undissociated molecule or a still more complex compound, not dissociating even upon dilution.

A considerable part of the paper by A. A. Shishlovskii (KGU) on the photoluminescence of liquid and solid solutions of ions of heavy metals was also devoted to clarifying the nature of the fluorescence carrier. Comparison of the spectra of certain luminescing compounds under various conditions (in particular, Tb++ in solutions and crystals) made it possible to conclude that in some cases the luminescence of the ion is involved, while in others—that of a more complex compound formation.

To this same section may be assigned two papers on chemiluminescence which deserved attention at the meeting: the paper by Professor A. G. Gurvich (VIEM) on the physico-chemical foundations of mitogenetic radiation and its study was listened to with great interest. This radiation is nothing other than ultraviolet chemiluminescence of very short wavelengths and negligible intensity. The wavelength of this radiation does not exceed 260 mμ. Thus, in the chemical reaction determining this radiation, energy of the order of 100–150 kcal must be liberated. According to Frankenburger’s hypothesis, such energy can be liberated in acts of recombination of atoms or free radicals. Free radicals may be formed in hydrolytic processes in the liquid phase. Such events are rare; however, the number of free atoms and radicals may be regarded as sufficient, taking into account the negligible intensity of mitogenetic radiation. To explain the characteristic spectra of radiation, Frankenburger put forward an additional hypothesis, according to which the recombination energy is absorbed by molecules present in the reacting mixture and is emitted by them with specific wavelengths. A. G. Gurvich cited a number of data that confirm Frankenburger’s hypothesis. The second hypothesis is subject to direct experimental verification. Upon addition to the solution of a substance known not to take part in the reaction, the spectrum shows the entire system of bands characteristic of that substance (thus, if to the urea–urease system, which is an emitter of mitogenetic radiation, glucose is added, then

new emission bands characteristic of glucose are observed. Thus, mitogenetic radiation may be regarded as sensitized luminescence. An energy calculation of certain hypothetical processes, carried out by the speaker, showed that Frankenburger’s hypothesis concerning the recombination of radicals as the cause of mitogenetic radiation is quite acceptable from the energetic point of view as well.

B. Ya. Sveshnikov (GOI) delivered a report on chemiluminescence in solutions. He indicated the principal paths being followed at present in the study of chemiluminescence. These are: 1) establishing the connection between the structure of molecules and their capacity for chemiluminescence, and 2) studying the kinetics of the process.

The speaker reviewed work on chemiluminescence and, on the basis of his own investigations in the field of phthalic cyclohydrazides, put forward hypotheses concerning the carrier of chemiluminescence and the mechanism of the reactions that accompany luminescence. Chemiluminescence is closely connected, as the speaker noted, with oxidation reactions, and therefore any attempt to understand its mechanism must contribute to the understanding of this most important process in nature.

A report by T. I. Weinberg (given on his own behalf and on behalf of Prof. V. V. Vargin) (GOI) was devoted to questions of glass luminescence. In many respects the luminescence of glasses occupies an intermediate position between the luminescence of solutions and that of solid crystalline bodies. The speaker reviewed work in the field of glass luminescence, dwelling in greater detail on work on the study of the luminescence of uranium and manganese glasses carried out at the State Optical Institute.

1. Three reports (V. L. Levshin, V. V. Antonov-Romanovsky, and D. I. Blokhintsev), devoted to the physics of crystalline luminescent substances, the most important in practical terms, showed that in the field of understanding the processes of phosphorescence there have been undoubted successes, connected to a considerable extent with the work of Soviet authors.

V. L. Levshin gave a critical review of the state of the question of the luminescence of crystalline substances. The speaker drew a sharp distinction between two kinds of luminescence encountered in crystalline substances.

The first, which he called “luminescence of discrete centers,” takes place entirely within individual centers—ions or molecules; the second—“crystalline luminescence”—throughout the entire volume of the crystal. Only the second luminescence is directly connected with the crystalline state of the substance; the first may also occur in vitreous substances, in liquids, and in gases.

The speaker pointed to the possibility of the simultaneous development of both kinds of luminescence in one and the same substance and cited several cases of luminescence that may be interpreted in this way.

In the second part of his communication the speaker considered the mechanism of “crystalline luminescence.” He proposed the following scheme of the process.

Absorption occurs by undeformed sites of the lattice located in the immediate vicinity of activator ions. Upon excitation, the electron is transferred to the conduction band. Some of the electrons, having entered the conduction band, move rapidly through the crystal and recombine with the vacant sites of excited centers; the energy released is taken up by the nearest activator ions and is emitted by them in the form of “instantaneous luminescence.” A second portion of the electrons in the conduction band, before recombination, undergoes one or several localizations on local attachment levels, also caused by activator ions and situated comparatively close ($L \simeq 1\ \mathrm{eV}$) to the conduction band. The return of these attached electrons to the conduction band occurs at the expense of the energy of thermal motion. Recombination of the electrons that have undergone localization with excited centers leads to the appearance of phosphorescence.

For an explanation of the phenomenon of thermoluminescence and of the long-term preservation of absorbed energy, it is necessary to admit the existence of especially deep trapping levels.

The energetics of the process of phosphorescence fits entirely within the framework of modern conceptions of the energy levels of solids. However, the simple scheme of the process of phosphorescence, based on the idea of bands of allowed electron-energy values in the crystal and of discrete local energy levels corresponding to the activator, is insufficient to explain a number of features of the laws of phosphorescence decay, as was shown especially convincingly by V. V. Antonov-Romanovskii (FIAN) in his report on the mechanism of phosphor luminescence. The speaker showed that the complex kinetics of the recombination process can be explained only on the basis of a detailed consideration of the behavior of the electron in the conduction band. The law of recombination may be either monomolecular, if the electron moves away to not very great distances and recombines with its ionized center, or bimolecular, if the concentration of ionized centers is sufficiently large (intense excitation), and the electron can recombine with any center. The action of heating or of “red” light, which accelerates the process of afterglow and changes the monomolecular law of decay into a bimolecular one, reduces to a “mixing” of electrons, as a result of which the distinction between “one’s own” and “foreign” centers disappears.

D. I. Blokhintsev (FIAN), in a report on the present state of the theory of phosphorescence, gave an outline of the theory of semiconductors, on which the modern band conceptions of the process of phosphorescence are based, and which had been developed to a considerable extent by the speaker. The band theory of semiconductors, as the speaker showed, makes it possible to give a sufficiently complete qualitative picture of the phenomenon of phosphorescence in solids. The quantitative conclusions of the theory also lead to satisfactory agreement with experiment. However, a number of circumstances (conversion of optical energy into heat, the Stokes shift in absorption in active centers, etc.) cannot be explained on the basis of band theory alone. The theoretical scheme admits two forms of decay laws: a bimolecular one, \(1/t^2\), and a monomolecular one, \(e^{-qt}\). The more complex laws observed in experiments are apparently the result of the action of secondary phenomena.

The reports devoted to the physics of phosphorescence showed that the theory of phosphorescence, which has grown up on the basis of modern ideas about the structure of the solid body, introduces corrections into these ideas. Thus, deviations of the laws of decay from the basic forms make it necessary to take into account diffusion of electrons, the formation of volume charges, the nonuniformity of the distribution of centers and electrons over the crystal, etc.; that is, to create a more complete picture of the processes taking place in the crystalline state.

In the work of V. M. Kudryavtseva (Sib. FTI) on the transfer of energy in the crystal lattice in luminescence processes, the existence of such transfer was proved experimentally.

Great interest was aroused by the report of F. D. Klement (LGU) on the so-called sublimate-phosphors investigated by him. The speaker activated alkali-halide crystals by subliming thallium halide salts onto their surface. In some cases (crystals with a large lattice constant) activation occurred already at room temperatures; in other cases the crystal had to be slightly heated, and the activation temperature was the higher the smaller the lattice constant. These experiments clearly show that, in order to obtain a phosphor—for activation of the crystal—it is necessary for the activating atom or ion to be introduced into the crystal lattice.

Special cases of excitation of crystalline phosphors—the phenomena of radioluminescence and cathodoluminescence, which are of exceptional technical significance—were the subject of separate

reports by S. A. Fridman (FIAN) and A. V. Moskvin (VEI). All those who took part in the discussion of these reports noted the insufficient volume of work being carried out in the field of these phenomena, which are of great theoretical and primary practical interest.

1. A large survey report by M. A. Konstantinova-Shelezinger (FIAN) was devoted to questions of luminescent analysis. Luminescent analysis is entering laboratory practice more and more widely, finding ever new applications. On the basis of his own extensive experience and the experience of other investigators, the speaker systematized the problems solved with the aid of luminescent analysis, and, using a number of examples, showed the applicability and advantages of luminescent methods in solving the most varied analytical questions.

Representatives of various organizations who spoke in the discussion reported on the successful application of luminescent analysis in the vitamin industry, in oil prospecting, in mineralogy, etc. Of interest was the presentation by Prof. Komovskii (Gipredmet), who had developed a luminescent method for determining wolfram in ores. By means of a number of physicochemical procedures, brightly luminescing calcium tungstate is formed in the ore under study under the action of cathode rays. The speakers described apparatus they had developed for luminescent analysis and emphasized the necessity of creating standardized typical apparatus. In a resolution of the conference it was decided to publish a special bulletin on questions of luminescent analysis.

1. Of particular interest, naturally, was the session devoted to new luminescent light sources. The advantages of luminescent lamps in comparison with ordinary incandescent lamps—their economy and high spectral qualities—are unquestionable. American industry is producing luminescent lamps by the tens of millions. In our country, the development of the technology for the production of these lamps was greatly slowed by the war, and the great merit of the Electrolamp Plant is that, despite difficulties, work on certain questions of the technology did not cease; and at the present time, as shown by the reports of the chief engineer of Plant 632, R. A. Nilsen, V. A. Fabrikant (VEI), and V. P. Sasorov (Plant 632), every possibility exists for expanding the mass production of luminescent lamps. The production technology has been mastered by the Electrolamp Plant, as is evidenced by the first successful experience of lighting large rooms with luminescent lamps—the hall of FIAN, in which the sessions of the conference took place and which was illuminated by luminescent lamps manufactured by the plant. Measurements of the illumination in the hall showed that, for the same consumed power, luminescent lamps create an illumination three times greater than the parallel existing system of illumination by incandescent lamps. The principal task, as the conference stated, is the expansion of the mass production of luminescent lamps. At the same time, much research work remains to be done on improving the lamps—possible further increase of efficiency, complete elimination of flicker, adaptation of the lamps to low temperatures, reduction of dimensions, rationalization of the electrical part, etc.

2. The varied applications of luminous compositions, especially their lighting-engineering application, were considered in the major reports by Z. M. Gorev (VEI) and A. N. Sevchenko (GOI). As a result of the work carried out in these institutes, extensive experimental material has been collected concerning the lighting-engineering characteristics of luminous compositions produced by domestic industry. A. N. Sevchenko also spoke about the work carried out at GOI on the application of luminescence in light masking and about the luminescent illuminators developed at this institute.

Interesting and varied were the communications from organizations producing and using luminous compositions. Here chemists who are engaged in

synthesis of phosphors, and architects who mock up lighting in their designs with the aid of phosphors; representatives of military institutions and artists speaking of a new genre of luminescent painting. The increased demand for various compositions led the conference to adopt a resolution on the necessity of establishing a large plant for the production of phosphors.

The various applications of phosphors were demonstrated at an exhibition opened during the conference. Here were collected interesting collections of phosphors manufactured by various organizations, recently developed luminous plastics, fabrics and oilcloths, and all kinds of products made from them. Samples of luminous instrument lighting for aircraft cabins, etc., were demonstrated. Paintings executed with luminous paints were extremely effective.

At the center of attention of the visitors to the exhibition was the luminescent and ultraviolet microscope made at the State Optical Institute, developed by E. M. Brumberg (GOI). The concluding session of the conference was devoted to a report on microscopy in ultraviolet rays. The study of micropreparations in ultraviolet light opens up, as E. M. Brumberg showed, new possibilities for biology, medicine, mineralogy, and many other disciplines.

The conference showed that in the Soviet Union a great and necessary effort is being carried out to study the phenomena of luminescence and to apply it in the most diverse branches of the national economy. It may be said that in the field of studying the physics of luminescence phenomena, both molecular and the luminescence of crystalline bodies, Soviet researchers occupy a leading place. At the same time, it is now quite clear that luminescence has ceased to be a “pure” science of interest only to a narrow circle of persons—the applications of luminescence are varied and important. The conference will undoubtedly serve as an impetus to the organization of new research in the field of luminescence and to the most rapid and broad introduction of the achievements of luminescence into life*).

P. Feofilov.

) A detailed report on the conference on questions of luminescence was published in Izvestiya Akademii nauk SSSR. Seriya fizicheskaya*, vol. IX, Nos. 4–5, 1945, pp. 277–576.