MEETINGS AND CONFERENCES

FOURTH MEETING ON LUMINESCENCE

(Molecular Luminescence and Luminescence Analysis)

From June 20 to 25, 1955, the Fourth Meeting on Luminescence was held in Minsk, convened by the Academy of Sciences of the BSSR, the P. N. Lebedev Physical Institute of the Academy of Sciences of the USSR, and the S. I. Vavilov State Optical Institute. Unlike the preceding meetings, the subject matter of the present meeting was limited to questions of molecular luminescence. This was due to the significant increase in the number of works on molecular luminescence, which determine its scientific and applied importance. The study of molecular luminescence is one of the principal methods for investigating the structure of individual molecules and of substances in the condensed phase, the influence of the surrounding medium on the properties of molecules, the energetics and kinetics of chemical and physicochemical transformations, energy migration, and so forth. The most important among the applications of molecular luminescence are diverse analytical applications based on the exceptionally high sensitivity of the method.

More than 200 delegates took part in the work of the meeting, representing various fields of science and technology—physicists, chemists, biologists, geologists, physicians, and others. The meeting held in Minsk—a new center for luminescence research—brought together representatives of about 50 organizations located in 14 cities of the Union. At the ten sessions of the meeting, more than 50 reports were presented on various questions of molecular luminescence and its applications. Six reports of a survey character opened sessions devoted, as a rule, to a particular range of questions.

In the brief introductory address by the chairman of the Organizing Committee for convening the meeting, V. L. Levshin, the scientific and practical importance of work on the study of molecular luminescence was emphasized, and a short survey was given of the main directions of these investigations in recent years. At the first session a number of reports devoted to intermolecular migration of excitation energy were heard, this phenomenon determining many phenomena in physics, chemistry, and biology. As is known, in the classical works of S. I. Vavilov it was shown that luminescence can serve as one of the principal methods for studying processes of intermolecular energy migration.

The session opened with a major report by A. N. Terenin and V. L. Ermolaev, “Intermolecular Transfer of Excitation Energy Manifested in Sensitized Luminescence.” The authors began their report with an analysis of works on sensitized photoluminescence in liquid solutions and molecular crystals. In these works over the—

in recent years, on the basis of subtle experimental and theoretical studies by M. D. Galanin, A. S. Davydov, Förster, Schmillen, Bay and co-workers, N. D. Zhevandrov, and others, ideas have been developed that describe the process of energy migration in many details. Next, works were considered on the luminescence of liquid solutions under the action of hard radiation, whose authors connect the phenomenon with various variants of energy migration from solvent molecules to luminescence centers (Kallmann and Furst, Birks, and others). The speakers, however, tend to the view that in the case under consideration what occurs is rather luminescence of the chemiluminescence type, connected with the use, for exciting the luminescence, of the energy of recombination of ions, electrons, and radicals formed in the solution under the action of hard radiation.

After considering works on energy migration in heterogeneous systems, the authors turned to processes connected with the optical properties of molecular complexes, whose absorption and fluorescence spectra had been studied by Reid, Bier, Ketelaar and co-workers. The authors’ own studies were also considered, studies that revealed intermolecular energy transfer with excitation of the triplet level—sensitized phosphorescence in frozen solutions of organic compounds.

The report concluded with a detailed analysis of a number of works on the migration of excitation energy and sensitized fluorescence in biological objects (the works of Frenkel and Ung, Krasnovsky, Bannister, and others). The speakers showed that the study of sensitized luminescence is one of the principal methods for investigating intermolecular energy migration, which plays an important role in many physicochemical and biological processes.

T. P. Belikova, M. D. Galanin, and Z. A. Chizhikova, in their report “Transfer of excitation energy from solvent to dissolved luminophore in liquid and solid solutions,” reported on their investigations of the luminescence of solutions of terphenyl in xylene and anthracene in toluene when excited in the region of absorption of the solvent. Analysis of the results, carried out from the standpoint of the theory of resonance energy transfer, showed that the probabilities of such a process, at concentrations of the dissolved substance of the order of \(10^{-3}\) g/ml, reach values of 0.5–1, i.e., exceed the theoretical values by more than an order of magnitude. The discrepancy was explained by the significant role played in liquid solutions by transfer of energy through collisions, with the authors proposing that the effectiveness of collisions be determined by the probability of resonance transfer. The discrepancies between the measured and calculated values of the transfer probability for solid solutions of naphthalene in anthracene were explained by the authors as the migration of excitation energy through the crystal lattice, analogous in the sense considered to the diffusion of excited molecules in a liquid solution.

The report by V. L. Ermolaev was devoted to the study of sensitized phosphorescence of aromatic compounds—a phenomenon discovered in 1952 by A. N. Terenin and the speaker, and consisting in the transfer, in frozen solutions, of the electronic excitation energy from molecules located at the triplet level to unexcited molecules, with their transfer to the phosphorescent (triplet) level. The author measured the relative yield of luminescence of the energy acceptor upon excitation in the donor absorption band and found a reduction in the lifetime of the donor after introduction of the acceptor. Analysis of these and other facts established by the author shows that the phenomenon indeed consists in nonradiative energy transfer between the triplet levels of the donor and acceptor, analogous to transfer between fluorescent levels, which determines the phenomenon of sensitized

fluorescence. In sensitized phosphorescence the specifically quantum aspect of the phenomenon is clearly manifested: during the transfer, the total spin of the acceptor–donor system remains unchanged.

Theoretical study of one aspect of the process of energy migration was contained in the report by A. S. Selivanenko, “The Exciton State of an Imperfect Molecular Crystal.” The speaker considered a “nonlocalized” exciton for a semi-infinite molecular crystal, treating the boundary of the crystal as a local perturbation of the lattice. Solving Schrödinger’s equation leads to exciton waves whose wave vector is parallel to the boundary of the crystal. This state is called a “surface exciton.” The appearance of lines corresponding to the levels of a surface exciton should be expected in powders or in crystals with a developed block structure.

In the next theoretical report, A. F. Lubchenko attempted to apply the quantum electrodynamics apparatus to calculating the average radiative lifetime of impurity centers and localized excitations. The author obtained expressions for the dependence of the average radiative lifetime on temperature, which are in satisfactory qualitative agreement with experimental data.

The reports that were delivered showed that in recent years studies of processes of excitation-energy migration, begun as early as by S. I. Vavilov, have led to clarification of the general picture of the phenomenon and its details. In this connection there arose a need for thorough study of other phenomena observed in the interaction of luminescent molecules in solutions. Therefore the next circle of questions considered at the meeting was connected with elucidating the role of molecular associations in luminescence phenomena.

V. L. Levshin began his report “The Influence of Association and Other Physicochemical Factors on the Luminescence and Absorption of Complex Molecules in Solutions” with a review of the principal experimental data relating to the influence of concentration on the optical properties of solutions, and showed the great diversity of concentration effects, indicating differences in intermolecular interactions in solutions. The totality of the complex concentration dependences of the optical properties of solutions cannot be completely described within the framework of the migration theory, which takes into account only the inductive interaction of molecules. It is necessary to take into account various phenomena that occur in real systems. In this connection the speaker proposed introducing into consideration the influence of physicochemical factors acting in concentrated solutions, for example the association of molecules, tautomeric transformations, changes in the degree of ionization, as well as changes in the direct action of the solvent field on emission centers. The report then gave characteristic signs of various types of concentration effects, making it possible to differentiate the interactions experienced by dissolved molecules. On this basis, the concentration dependences of several groups of luminescent compounds were considered. In conclusion, considering the prerequisites for constructing a general theory of the phenomena under discussion, the speaker pointed to the possibility of explaining the discrepancies in the course of changes in the lifetime and in the luminescence yield of solutions by taking into account inactive absorption by associates and energy migration from emission centers to them.

Concentration phenomena in solutions of cyanine dyes were studied by L. D. Derkacheva from the absorption and luminescence spectra. It was thereby established that association of the molecules under consideration can occur not only in water but also in other solvents characterized by high values of the dielectric constant.

Association, caused by dispersion forces, ends with the formation of dimers, which in aqueous solutions serve as elements of threadlike macromolecules. In the work, the absorption spectra of dimeric molecules were determined and, from the temperature dependences of the spectra, the heat of dimerization was found. The oscillator strength corresponding to the narrow band associated with the macromolecules was determined to be 1.03 when calculated per molecule of pseudocyanine entering into the chain. By A. S. Davydov’s method, the work calculated the directions of polarization of absorption for a chain regarded as a linear crystal. Good agreement with experimental data was obtained.

L. V. Levshin reported the results of an investigation of the luminescence and absorption of concentrated solutions of 3,6-diaminoacridine. After studying in detail the dependence of the absorption and luminescence spectra on the concentration of the substance under investigation and on temperature, the author established certain contradictions between the experimental results and the assumption that associates exist in the solution. The observed changes in the spectra (the weakening of one band and the strengthening of another) are connected by the author with the transition of the substance under study from one form to another when the concentration of hydrogen ions in the solution changes, which in turn depends on the concentration of the substance being studied, a strong base. The results of the reported work illustrate the importance of taking complicating circumstances into account when investigating concentration phenomena.

V. L. Levshin and E. G. Baranova, in their study of the nature of concentration phenomena in solutions of rhodamines, established that the concentration dependences of the optical properties of solutions of these substances are not connected with changes in the concentration of hydrogen ions in the solution accompanying changes in the concentration of the dyes. These dependences, as the authors showed, are caused by association phenomena. Analysis of the spectra leads, moreover, to the conclusion that association of the molecules takes place through interaction of $\pi$-electrons. The authors carried out quantitative measurements of the concentration and temperature dependences of the absorption spectra of the solutions, and on the basis of the experimental results determined the degree of association of rhodamine molecules in the solutions. The data obtained are in good agreement with independent determinations made on the basis of the authors’ experiments on the quenching of fluorescence as a consequence of the decomposition of nonfluorescing complexes when the temperature of the solutions is raised.

The influence of association of the dye on the luminescence of frozen alcoholic solutions of acridine orange was investigated in the work of L. A. Kuznetsova and B. Ya. Sveshnikov. The authors established that, at low dye concentrations, the phosphorescence spectra and the spectra of total emission possess an identical vibrational structure. Thus the lower state, contrary to Jablonski’s assertions, is common to the transitions that determine fluorescence and phosphorescence. A strong change in the fluorescence spectra in the region of high concentrations led the authors to conclude that fluorescence is present in the dimers and tetramers of the dye. Investigation of the duration of emission on a phase fluorometer showed that association causes a considerable increase in the lifetime of the fluorescent state. The lifetime of fluorescence, as well as the phosphorescence spectrum, does not depend on the concentration of the dye. The increase, discovered by the authors, in the relative yield of phosphorescence with increasing concentration made it possible to conclude that the probability of transition from the fluorescent state to the metastable state increases as association of the dye proceeds.

Closely adjoining the series of reports devoted to the study of the influence of association of complex molecules on their luminescent properties was the report

A. L. Arvan, who studied, by absorption spectra, the influence of organic impurities on the aggregation of dyes in aqueous solutions. In the reported work it was established that the presence of a number of organic impurities (alcohols, ketones, pyridine, quinoline, etc.) in an aqueous solution of an aggregated dye reduces the degree of aggregation. In the homologous series of alcohols, activity with respect to dye deaggregation increases with increasing ordinal number of the alcohol. The activity of isoalcohols proves to be somewhat lower than the activity of the corresponding normal alcohols. The difference in activity is connected in the work with the differing ability of alcohols to form intermolecular bonds. The deaggregating action of organic impurities was explained with the aid of the assumption of the formation in solutions of dye—organic solvent—water complexes, which hinder dye aggregation.

The papers read at the third session, devoted to the experimental and theoretical investigation of luminescence and absorption spectra, showed that, over the years that have elapsed since the time of the last conference on luminescence, substantial progress has been achieved.

The survey report by B. S. Neporent, “Regularities in the Fluorescence and Absorption Spectra and the Structure of the Levels of Complex Molecules,” contained in its first part a consideration of the results of investigations by the author and his coworkers. In these investigations it was shown that the subdivision of molecules into those possessing modulated spectra and spectra of damping is characteristic of various classes of aromatic compounds, and that the transition from one kind of spectrum to another occurs as the action of substituent groups on the aromatic residue of the molecule is strengthened. Substances possessing spectra of damping are characterized by the absence of long-duration emission and by a prolongation of the antistokes boundary of the emission; fluorescence is distant from the region of the fluorescence spectrum. In this case the frequency corresponding to the distance between electronic levels upon absorption of light is determined by the position of the maximum of the absorption spectrum. Analyzing the results obtained, the author comes to the conclusion that, in the case of spectra of damping, the position of all electronic levels of a molecule depends on its energy state and, in particular, the distance between electronic levels in fluorescence is determined by the position of the maximum of the fluorescence band. Upon excitation the electronic system of the molecule undergoes substantial changes, and the width of the spectra is determined by the rate at which, after absorption of a quantum, changes occur in the structural parameters of the molecule connected with the state of its electronic shell. In the report examples were considered confirming the point of view presented. In the classical analogy, the corresponding electronic system must be described as a nonlinear oscillator. In the author’s opinion, the mechanism considered for the formation of continuous spectra should be very widespread among complex organic substances.

B. I. Stepanov reported the results of the calculation of the contour of the absorption and emission bands of complex molecules. Proceeding, like the preceding investigators, from a one-dimensional configurational model based on the strong interaction of the vibrational degrees of freedom of complex molecules, the author took account of the distribution of molecules according to the values of the vibrational energy reserve along the configurational coordinate, using in this connection the distribution function that he had obtained earlier for “gray matter.” In addition, the author assumed that, in the process of the electronic transition, the positions of the nuclei of the molecule change. The expressions obtained as a result contain fewer constants than those proposed by other investigators.

The author’s assumption concerning a change in the positions of the nuclei in the process of an electronic transition evoked a lively discussion.

Spectra and yields of anti-Stokes and Stokes fluorescence of vapors of aromatic compounds upon excitation in a broad spectral region at various temperatures and vapor pressures were studied by B. S. Neporent and N. A. Borisevich. The authors established that in individual cases the absolute values of the fluorescence yield of vapors exceed those for solutions, and showed that the emission spectrum of vapors, which, as is known, includes the descending branch not only in the anti-Stokes but also in the Stokes region, serves as a characteristic of the molecule under study. The authors’ investigations showed that the decrease in fluorescence yield in the anti-Stokes region is connected with intramolecular processes occurring directly upon excitation of the molecule and is due to quenching of the first kind. Responsible for the excitation of anti-Stokes fluorescence are molecules possessing an excess of vibrational energy; in the experiment, molecules with a small reserve of it. Careful measurements by the authors showed that every change in yield is accompanied by slight but quite definite changes in the shape of the fluorescence spectra.

In the work of A. S. Cherkasov, the absorption and fluorescence spectra were studied, as were the quantum yields of fluorescence of alcoholic solutions of a series of alkyl- and aryl-substituted anthracenes, and also of nitro-, amino-, and acylamino-derivatives. Having studied in detail the influence of the nature and position of various substituents on the character and position of the spectra and on the luminescence yield, the speaker established a number of essential empirical regularities governing the spectra and the luminescence yield of complex molecules. In the discussion the fruitfulness demonstrated in the work was noted: the combination of chemical preparative work with investigation of physical characteristics for solving one of the fundamental problems of molecular luminescence—the question of the connection between the structure of molecules and their luminescent characteristics.

E. V. Shpol’skii and L. A. Klimova reported the results of a study of the influence of solvents on the luminescence spectra of aromatic hydrocarbons at low temperatures. The authors studied in detail the fine structure, which they had previously discovered, of the fluorescence spectra of frozen solutions of coronene, 3,4-benzpyrene, and other compounds with condensed nuclei in normal aliphatic hydrocarbons. In the work a detailed study was also made of the character of the splitting of bands in phosphorescence spectra, which differs from that observed in fluorescence spectra. The magnitude and character of the splitting of bands in fluorescence and phosphorescence spectra differ strongly for solvents that differ little in their basic physical properties (hexane—heptane—octane—nonane). Analysis of the data obtained led the authors to the conclusion that the splitting of bands in the spectra is connected with the superposition of low frequencies of the solvent lattice on the levels of the molecules under study. This explanation is in agreement with the ratio of the sizes of the molecules under study and the elementary cells of the solvent crystals, which cause the appearance of fine structure in the spectra (hexane—nonane).

In the report by T. N. Godneva, “On the Nature of the Chlorophyll-Protein-Lipid Complex,” questions of optical studies of the properties of chlorophyll, which are of great scientific interest, were considered. As a result of an analysis of extensive literature material, the author proposed certain changes to the scheme of the structure of the complex under consideration that has recently been accepted by other investigators. Further, the author pointed out the importance of studying the spectra of chlorophyll for resolving the question of its states in the living leaf. In this connection, the work of T. N. Godnev, R. V. Efremova, and L. A. Kravtsov, “The Influence of Certain Acidic and Basic Substances on the Absorption Spectra of Chlorophyll and Chlorophyllides,” was presented. Having studied the action of amino acids, organic ...

of acids and amines on the absorption spectra of chlorophyllide, the authors established that the addition of glutamic and aspartic acids causes decomposition of chlorophyllide; fatty acids cause changes in the spectra analogous to those observed by Krasnovsky and also by Livingston in chlorophyll; and, finally, amines lead to changes opposite to those observed in acids. The authors attribute the spectral changes to the formation of unstable molecular complexes of chlorophyll with amino or carboxyl groups, possibly through additional valences of magnesium.

G. A. Tishchenko and L. P. Feofilov, in their report, described the appearance of anomalous luminescence of color centers in crystals of fluorite and of certain fluorides of alkali metals. By studying the shape and relative positions of the absorption and luminescence bands, the authors established that the observed regularities are in agreement with the predictions of the theory of impurity absorption developed by S. I. Pekar. A study of the nature of the elementary radiators of anomalous coloring in fluorite crystals by the method of polarization diagrams made it possible to establish the electric-dipole character of the luminescence, consistent with data on the duration of the luminescence \((\tau \sim 10^{-9}\ \text{sec})\) and the oscillator strength \((f \sim 1)\). The authors suggested that the luminescence of complex color centers is not an exclusive property of the crystals studied and indicated the spectral region (near infrared) in which one should seek the luminescence of complex color centers in crystals of other alkali-halide salts.

Also devoted to the study of the properties of color centers in ionic crystals was the paper presented by A. A. Shatalov, in which the quantum yield of the photochemical transformation of \(F\)-centers into centers of the colorless potassium chloride crystals studied by the author—possessing longer-wavelength absorption and named by the author \(X\)-centers—was investigated. The quantum yield of the photochemical transformation depends strongly on temperature: at room temperature it is only 0.0016, reaches a maximum (0.032) at \(250^\circ\text{C}\), and falls sharply to \(\sim 0.001\) at \(300^\circ\text{C}\). Alongside the formation of \(X\)-centers there is the reverse process—the thermal restoration of \(F\)-centers.

The session devoted to studies of the quantum yield of luminescence opened with a survey report by B. I. Stepanov on the quantum yield of fluorescence of complex molecules. Beginning with an examination of Vavilov’s law, formulated for condensed systems, the speaker proceeded to present the results of work by Soviet authors who extended this law to the region of rarefied vapors and analyzed deviations from it in systems of this kind. Further in the report were considered the results of experimental investigations of the anti-Stokes branch of the fluorescence yield curve of vapors and solutions, as well as certain theories subjected to criticism, containing a phenomenological justification of the fall in fluorescence yield in the anti-Stokes region. The speaker presented the results of theoretical studies carried out by him jointly with V. V. Antonov-Romanovsky, M. V. Fok, and Khapalyuk, in which, for a model system characterized by three levels, new conclusions were obtained on the possibility of anti-Stokes fluorescence with quantum yields greater than unity. At the end of the report there was a consideration of the anti-Stokes fluorescence of complex molecules, based on concepts of inactive absorption, formulated in the aforementioned experimental studies of this phenomenon in solutions and vapors, and on the author’s ideas concerning the optical properties of a “gray substance.”

N. A. Borisevich and B. S. Neporent studied the influence of foreign gases on the spectra and yield of fluorescence of vapors of aromatic

of compounds upon excitation in a broad spectral region, including the anti-Stokes region. The authors established that, as the frequency of the exciting light is decreased to values below the frequency of the electronic transition, the enhancement of vapor fluorescence caused by a decrease in the reserve of vibrational energy of the excited molecules during collisions changes into a weakening associated with an increase of this reserve. The authors showed that, by the action of foreign gases, one can determine the position of the electronic level of the molecules under study. In the work it was also established that the effect of a solvent on the fluorescent capacity of a dissolved substance is not limited to the rapid bringing of the system to thermal equilibrium, and ways were indicated for studying the specific action of the solvent on luminescent molecules.

In the work of D. Borisov the photoluminescence yield in anthracene and naphthalene single crystals was investigated at \(20^\circ\mathrm{C}\) and \(-180^\circ\mathrm{C}\). The luminescence yield of anthracene crystals proved to be independent of the thickness of the crystal (from \(3\,\mu\) to \(1\,\mathrm{mm}\)), despite the sharp difference in the spectra caused by strong reabsorption. No temperature dependence of the yield was found either. By contrast, the fluorescence yield of naphthalene depended strongly on the temperature, changing from 0.90 (\(-180^\circ\mathrm{C}\)) to 0.42 (\(20^\circ\mathrm{C}\)). The absolute values of the luminescence yield of crystals measured by the author differ noticeably from the values obtained by M. D. Galanin.

The report of N. P. Mets, V. V. Zelinskii, V. P. Kolokov and L. G. Pikulik, who investigated the fluorescence and phosphorescence yields of complex organic molecules, was devoted to the important question for molecular luminescence of the connection between the luminescent characteristics of molecules and their structure. The authors’ main task was to consider the action of various factors (temperature, medium, etc.) on individual transitions in excited molecules possessing different structures. Measurements of the quantum yields of phosphorescence and fluorescence showed that their ratios are extremely strongly dependent on the structure of the molecules; in some cases the probabilities of transition from a labile level to a metastable one may considerably exceed the probabilities of transitions from a labile level to the ground one. In these cases the phosphorescence yield may approach 100%. The authors noted that the quenching action of the solvent on the fluorescence yield, which determines the difference in the fluorescent capacity of molecules in different solvents, depends extremely strongly on the temperature.

Several more reports, in addition to those already mentioned, were devoted to the study of the long-duration emission of organic molecules. In the work of M. S. Fadeeva, “Temperature Quenching of the Phosphorescence of Aromatic Acids,” it was established that an increase in temperature is accompanied by quenching of the long-wave fluorescence band. In accordance with the theory, the change in the initial intensity depends on quenching both at the metastable and at the labile levels. The decrease in the mean duration of emission with increasing temperature can be described by the formula proposed by Patiankin, but in this case the activation energy of quenching does not retain a constant value for each substance in the temperature interval studied and in different solvents. P. A. Teplyakov reported on the results of studying the influence of concentration and solvent on the phosphorescence of aromatic compounds at low temperatures. The report established that solvents affect not only the decay constant and the intensity of emission, but also the character of the process—in individual cases the law of decay is complex and decomposes into two exponentials. For some of the 12 aromatic compounds investigated, concentration quenching of phosphorescence was found.

On an attempt at the experimental detection of the paramagnetism of metastable states of boron phosphors activated with fluorescein, N. A. Lebedev and I. D. Shmerkovich reported. For this purpose the authors applied several sufficiently sensitive methods; however, they did not succeed in detecting the paramagnetism of the excited phosphors predicted by theories that regard the metastable state as a triplet state.

A group of reports at the conference was devoted to the study of questions of polarization of luminescence, as well as anisotropy of the optical properties of crystals.

In P. P. Feofilov’s review report, the author’s works on the polarized luminescence of cubic crystals were considered. The author established that the peculiar regularities observed by him in studies of the polarized luminescence of optically isotropic cubic crystals receive an exhaustive explanation on the assumption that anisotropic luminescing centers are oriented in these crystals along one or another symmetry axis of the crystal. Comparison of the calculated and experimental dependences made it possible to establish, and to reconcile with crystallochemical data, the character of the orientation of color centers in a number of ionic crystals, as well as of europium ions introduced into the crystal lattice of fluorite. The study of photochemical processes occurring in colored crystals under the action of polarized light made it possible to make an unambiguous choice between two possible oscillator models of luminescing color centers. The method of polarization diagrams of luminescence, proposed by S. I. Vavilov for establishing the nature (multipolarity) of elementary emitters, was extended by the speaker to the case of cubic crystals containing oriented luminescing centers. Application of this method to fluorite crystals activated with europium made it possible to detect, in the luminescence spectrum of the latter, lines corresponding to four different types of elementary emitters—electric and magnetic dipoles and rotators. The last kind of radiation—the magnetic rotator—was discovered experimentally for the first time.

Polarization diagrams of luminescence, characterizing the spatial distribution of the polarization of radiation, make it possible not only to establish the nature of elementary emitters but also, as N. D. Zhevandrov showed in his report, to determine the orientation of molecules in a crystal lattice. Having analyzed in detail the data on the orientation of the radiating oscillator in the anthracene molecule and on the optical anisotropy of crystals formed by these molecules, the author showed that the orientation of molecules in anthracene crystals, determined from polarization diagrams, agrees with X-ray structural data.

Of considerable interest to opticians, in particular to researchers of crystal luminescence, is the work reported by F. I. Fedorov on the theory of determining the optical parameters of absorbing crystals. The author developed a general invariant theory of the optical properties of absorbing crystals, which makes it possible, in particular, to determine the optical parameters of absorbing uniaxial crystals from the properties of the light reflected by their surface.

N. D. Zhevandrov and A. F. Serkin investigated the polarized spectra of certain naphthalimides and polyene. Analysis of these spectra enabled the author to establish the orientation of the oscillators by means of which individual bands in the absorption spectrum can be described, relative to the geometrical axes of the molecule. Thus, in the spectra of the molecules studied, the oscillators corresponding to the 1st and 2nd bands

absorptions, in contrast to what is observed in the majority of molecules, are parallel to one another.

A survey report by M. D. Galanin was devoted to the topical and theoretically important question of the luminescence of organic substances under the action of particles and hard radiation. The speaker presented the main experimental results and outlined the range of problems connected with the mechanism of energy conversion leading to the excitation of molecules—centers of luminescence. The report examined substances used for scintillation counters, the spectra of their emission, and the luminescence yield. It was indicated, in particular, that in the case of $\gamma$-excitation of certain crystals the ratio of the energy yield of emission to the quantum yield of photoluminescence is constant and, consequently, the mechanism of excitation of different crystals is the same. Further, the report considered the dependence of the emission yield on the type and energy of the exciting particles, noted the greater duration of emission excited by particles as compared with photoluminescence, and discussed possible observed discrepancies. Having critically examined Birks’s theory of excitation, the speaker pointed out the need to take into account energy-transfer processes from the achieved high excited electronic states, as well as “distant collisions,” leading to excitation of the lower excited levels of the $\pi$-shell of fluorescent molecules. Such a treatment was carried out by the author of the survey. In the conclusion of the report, the conditions for excitation of solutions by hard radiation were considered, and the importance of detailed investigation of the processes of energy transfer from the solvent to the dissolved substance was emphasized.

A report by A. N. Sevchenko was devoted to the photoluminescence of solutions of uranyl compounds and uranium glasses. The speaker found that all frozen organic solutions of acetate, nitrate, chloride, and sulfate of uranyl exhibit bright luminescence having a characteristic band spectrum, which, however, does not display the fine structure observed in the spectra of crystals of uranyl compounds. The strong dependence of the luminescent characteristics on the nature of the solvent led the author to the conclusion that there is strong interaction between the uranyl ion and the surrounding solvent molecules. Investigation of the yield, lifetime, and polarization of the luminescence of uranium glasses of various compositions also indicates a strong interaction of the luminescing ion with the surrounding medium.

In the work of M. U. Belogo and D. A. Shishlovsky, “Absorption and Luminescence of Fluoride Solutions of Lead and Thallium,” the absorption and luminescence spectra of mixed salt solutions of $\mathrm{Pb(ClO_4)_2 + KF}$ and $\mathrm{Tl(ClO_4)_2 + KF}$ were studied. As the concentration of $\mathrm{F^-}$ is increased, beginning with $4\,m/\ell$, the absorption of the solution begins to increase rapidly, and a violet luminescence appears, representing the edge of a structureless band with a maximum near $376\,m\mu$. Analysis of the concentration dependences led the authors to conclude that the emission belongs to $\mathrm{PbF^+}$ ions. When $\mathrm{F^-}$ ions are added to solutions containing $\mathrm{Tl^+}$ ions, a weakening of the fluorescence is observed, following the laws of quenching of the second kind.

M. U. Belyi and K. F. Gudymenko, in their work “Luminescence and Absorption of Solutions of Tin Salts,” established that the absorption spectra of solutions of divalent tin salts in various acids contain three maxima, absent in the spectra of tetravalent tin. Fluorescence is observed only in solutions of divalent tin, and with a gradual disappearance of the emission it is associated with oxidation of tin ions. In aqueous solutions the emission disappears as a result of hydrolysis. Interpreting the results, the authors pointed to the similarity of the outer electron shells of the luminescing cations $\mathrm{Tl^+}$, $\mathrm{Pb^{++}}$, and $\mathrm{Sn^+}$ (2 $s$-electrons in the outer electron shell).

The fruitfulness of applying new methods to the study of luminescence was demonstrated in the report by N. A. Tolstoy, A. K. Trofimov, A. M. Tkachuk, and N. N. Tkachuk, “The Kinetics of Luminescence of Platinum-Cyanide Compounds.” With the aid of a “ultrastrobometer” developed by N. A. Tolstoy—a device that permits study of luminescence relaxation processes in the interval \(10^{-1}\)–\(10^{-7}\) sec—the authors were able for the first time to carry out direct measurements of the lifetime of the excited state of platinum-cyanide salts. They synthesized and investigated about 30 different salts. The afterglow duration proved to be \(10^{-6}\)–\(10^{-7}\) sec. The absence of a dependence of the kinetics on the intensity of the exciting light and the exponential character of the laws of rise and decay of the luminescence allowed the authors to assign the emission of platinum-cyanide salts to monomolecular luminescence. By their work the authors succeeded in filling a gap in the study of relaxation processes (\(10^{-5}\)–\(10^{-7}\) sec), of which S. I. Vavilov had spoken as early as the 2nd Conference on Luminescence in 1948.

In the work of B. D. Ryzhikov, “The Chemiluminescence of Lucigenin,” primary attention was devoted to the question of the nature of the luminescence accompanying the oxidation of lucigenin by hydrogen peroxide in an alkaline medium. The author succeeded in showing that the green glow, which until now had been regarded as chemiluminescent, is secondary and is the reabsorption photoluminescence of lucigenin and, possibly, of its nearest decomposition products, excited by blue luminescence of a chemiluminescent nature, emitted, in the author’s opinion, at the final stages of the oxidation reaction. Direct measurements of the intensity of the blue glow without taking reabsorption into account, which presents known difficulties, cannot serve for kinetic studies.

M. M. Pavlyuchenko and I. N. Ermolenko, in their report “Spectral Investigations of the Products and Mechanism of the Reaction of Oxidation of Cellulose by Nitrogen Dioxide,” showed that the intermediate process of the reaction is cellulose nitrite. As a result of the reaction in an acidic product, carbonyl groups appear. By studying absorption spectra in the ultraviolet and infrared regions, the authors succeeded in establishing the temperature dependences of the rate and the kinetics of the oxidation reaction, which is essential for understanding its mechanism.

A report on dark reactions of dyes was given by A. T. Vartanyan. By restoring the solid salts of dyes with hydrogen vapor, the author obtained leuco- or (possibly also) carbinol bases, which are readily oxidized by atmospheric oxygen in the presence of moisture, giving the original dyes. The speaker also observed the formation of a colorless compound during the sublimation of rhodamine B and rhodamine G in vacuum, apparently owing to the hydrogen of the carboxyl groups entering into the dye molecule.

A thorough critical review of recent work on electroluminescence, in particular on the electroluminescence of organic molecules (the work of Bernanose), was given by L. A. Tumerman. The report on these new, distinctive phenomena, which have not yet received an exhaustive explanation, was heard with considerable interest.

Special sessions were devoted to reports on new methods and instruments for the study of luminescence.

Great interest among the participants of the Conference was aroused by reports on a new phase fluorometer, built by A. M. Bonch-Bruevich and his collaborators. In a survey report devoted to the question of measuring fluorescence duration by means of a fluorometer, A. M. Bonch-Bruevich examined in detail the existing designs of phase fluorometers and systematically analyzed the possible

sources of error that are the cause of discrepancies in the values of lifetimes obtained by different authors. The analysis carried out enabled A. M. Bonch-Bruevich, V. A. Molchanov, and V. I. Shirokov to construct a new phase fluorometer distinguished from those described in the literature by a higher resolving power ($\sim 2 \cdot 10^{-11}$ sec, i.e., almost an order of magnitude higher than that of the previously described fluorometers) and by a comparatively small decrease in the accuracy of measurements in passing to weak luminescence. For modulation of the exciting light, an automatically dynamic disk diffraction modulator with selection of lateral diffraction maxima, operating at a frequency of 12 MHz, is used. In connection with the amplitude-phase distortions established by the authors in the electronic part of the fluorometer, a system of automatic regulation of the electrical signal has been introduced. The measurements are made with the aid of stepped phase shifters and an optical line 30 cm long. As a reference device, in addition to the oscillographic tube, an inertial phase indicator is used, making it possible to increase considerably the accuracy of the measurements.

With the aid of the new fluorometer the authors carried out a number of measurements; in particular, they determined for the first time the luminescence lifetime of color centers in ionic crystals ($\tau \sim 10^{-9}$ sec) and the luminescence of tetraphenylbutadiene in xylene ($\tau = 8 \cdot 10^{-10}$ sec).

B. S. Neporent and V. P. Klochkov described the high-luminosity spectrophotometric apparatus they had built for the investigation of luminescence. The instrument is constructed on the basis of a monochromator, very high in luminous flux, with a diffraction grating. The monochromator is provided with a light-beam illuminator with a relative aperture of 1:1. As radiation receivers, photomultipliers with antimony-cesium and oxide-cesium cathodes are used. An amplifying device with complete line power supply ensures registration of spectra by means of a self-recording potentiometer. The instrument is calibrated for spectral sensitivity from 260 to 1100 mμ. The paper presented examples of spectrum recordings demonstrating the high sensitivity and stability of operation of the apparatus.

An interesting method for stabilizing the radiation of gas-discharge light sources, usually used for exciting luminescence, was proposed in the paper by V. I. Shirokov. A characteristic feature of the method is that stabilization is carried out not by the voltage of the power source, but, by means of optical feedback, by the magnitude of the luminous flux—for example, the luminous flux passing through the entrance slit of a monochromatizing device. Investigation of the constructed stabilizer showed that, when the supply voltage changes by 10%, the magnitude of the stabilized luminous flux of an SVDSh-250 lamp changes by only 0.3%.

In the paper by B. I. Vainberg, F. M. Pekerman, and V. P. Danilov, a luminescent lamp for luminescent analysis was described; it is a low-pressure mercury lamp whose bulb is made of “black” uviol glass and coated on the inside with a phosphor having a maximum of radiation at about 350 mμ, i.e., in the region usually used in practical luminescent analysis. As the speakers and those taking part in the discussion noted, the proposed lamp is a source of luminescence excitation that in a number of cases is more convenient than the commonly used PRK lamps with separate light filters.

The variety of topics of the papers on luminescent analysis read at the Conference testifies to the very broad possibilities of this method and to its penetration into the most diverse fields of science and technology.

MEETINGS AND CONFERENCES

The report by A. V. Karyakin, D. N. Lazareva, M. I. Goland and G. A. Barinova on the use of the luminescent method for determining the viability of seeds and the quality of root crops aroused great interest. The authors succeeded in establishing a distinct difference in the luminescent properties of embryos of viable and nonviable seeds of oats, wheat, barley and other cereals, and also in developing a method for determining the viability of crucifer seeds by isolating fluorescent substances during the swelling of seeds. The method developed by the authors makes it possible to determine the various stages of potato disease caused by phytophthora and damage to it by frost more quickly and accurately than was achieved by previous methods.

Z. E. Kanevskaya, Ovsyannikov, Bel’skaya and Kozelkova described the application of the luminescent method for determining seed viability in the Central Seed Laboratory of the Ministry of Agriculture. The authors developed methods for studying corn and oat seeds.

The results of applying a qualitative-quantitative fluorescent method for studying adrenalin-like substances in the blood in various diseases were the subject of a report by R. B. Zel’manova, A. A. Avakyan and L. M. Popova, and a report was devoted to the luminescent method of histochemical study of the cells of the anterior horns of the spinal cord in normal conditions and in poliomyelitis.

G. N. Kosheleva spoke about work she had carried out on selecting an assortment of fluorescent acid-base indicators. On the basis of a review of the characteristics of more than 100 indicators, the author proposed a rational set of 14 compounds possessing blue-green luminescence and covering a wide interval of pH transition.

The report by D. N. Lazareva and D. P. Erastova was devoted to the application of luminescence in restoration techniques for recovering texts that had lost visibility. The method proposed by the authors for photographing documents in the near infrared region is based on the ability of many varieties of aniline inks to luminesce in this part of the spectrum.

The luminescent method of control in the production process of abrasive papers was successfully applied by N. F. Baranets and R. L. Pevzner, who based the method on the change in the luminescence of the varnish used for applying the abrasive when it is polymerized.

In three reports at the concluding session of the Meeting, questions of the application of luminescence in the study of oil deposits were considered.

In V. N. Florovskaya’s report, “The Luminescence-Bituminological Method and Its Application in Petroleum Geology,” the foundations of the method and its application to the analysis of rocks and to the study of geological regularities connected with the distribution of bituminous substances in rocks were considered. The author is inclined to the view that the luminescence of rocks is connected with resinous components, and not with polycyclic hydrocarbons. In combination with studies of solubility, sorption and other characteristics of bitumens, the method makes it possible to obtain a number of important materials on the classification of deposits determining accumulations of oil, and is of significance for clarifying the origin of oil and for the search for associated mineral resources.

A. A. Il’ina, in her report “Luminescence Spectra of Bitumens,” showed that the luminescence of oils is characterized by a continuous spectrum, the position of whose maximum varies from 410 to 520 mμ depending on the composition of the oils and the concentration of asphaltene fractions in them. In dilute solutions of oils this difference decreases: the maxima are located at about 410–430 mμ. In the study of luminesc—

of bitumens, it was found by the speaker that in some cases the spectra are analogous to the spectra of petroleum oils and conclusions can be drawn about the nature of the bitumens, while in other cases, in addition to the continuous spectrum, bands of polycyclic aromatic hydrocarbons also appear, which, in the author’s opinion, are formed as a result of the catalytic action of the rocks on the organic substances present in them.

Kh. I. Mamedov reported the results of studies of the luminescence spectra of petroleum fractions. In view of the separation, by the luminescent method, of aromatic components in kerosene fractions, the author used monochromatic excitation of luminescence and cooling of the samples under study. It was thus possible to establish that the fluorescence of kerosene in the nearest ultraviolet region is due mainly to naphthalene and its homologues, while the visible fluorescence is associated with the presence of anthracene derivatives.

The results of the Meeting demonstrate the substantial development of the Soviet school of luminescence over the four years that have passed since the 3rd Meeting. During this period, successes have been achieved in the study of the energetics of luminescence centers, the kinetics and mechanism of intra- and intermolecular energy transformations, in the development of new methods of investigation, and in applications of luminescence analysis in agriculture, medicine, and geology. At the same time, in its resolution the Meeting noted the insufficient development of work on luminescence under the action of hard radiations, on luminescence at low temperatures, and on electroluminescence. The effective methods of luminescence analysis that have been developed are still being introduced into the national economy insufficiently widely. The resolution of the Meeting outlined specific paths for the development of work on luminescence in the immediate future.

B. S. Neporent
P. P. Feofilov