The study of the vibrations of polyatomic molecules has recently turned into one of the most widespread and powerful methods of studying molecular structure. These vibrations ap pear directly in the infrared absorption spectra and Raman spectra of gases, liquids, and solids. A measurement of the number of bands in addition to their positions (frequencies or wavelengths) offers the possibility of obtaining a great deal of important information regarding the geometric and mechanical properties of the molecules, the types of chemical bonds, and so forth. It is now quite difficult to list the vast number of specific problems solved by measuring vibrational fre quencies. As a result of the successful development of research methods and the widespread applica tion of vibrational spectra in analyzing the structures of molecules and the constitution of ma terials, it now becomes necessary to develop the theory of molecular vibrations further. Existing theory, of course, is based on the assumption of the harmonicity of molecular vi brations, which, strictly speaking, is not justified experimentally. The anharmonicity of the molecular vibrations has therefore to be taken into account by introducing appropriate approxi mations. Thus, in carrying out calculations on the vibrations of polyatomic molecules, one uses the force constants calculated from the observed frequency values. However, as a result of the anharmonicity of the vibrations, the values of the observed frequencies differ from the harmonic values, and the force constants used therefore differ from the true ones, i. e.

In this paper we investigated the dynamics of the processes occurring in a Q-switched laser. This work was stimulated by the lack of data on the spatial and temporal development of generation, despite the obvious importance of such data in the use of giant light pulses in in vestigations of the nonlinear interaction of radiation and matter. From a systematic con sideration of a relatively simple model of a Q-switched laser we analytically investigated two main phases of development of the giant pulse - the phase of linear development of generation, which begins with amplification of the spontaneous emission in the modes, and the phase of nonlinear transverse development, during which the giant light pulse proper is emitted. In ad dition, fo r a thorough inve stigation of the picture of development of the pulse as a whole the equations were numerically integrated. ' Subsequent experiments [26, 27] confirmed the occurrence of transverse development of the giant pulse, while recent experiments on nonlinear amplification [28] have shown the sig nificance of this effect in the propagation of the giant pulse in a nonlinear medium. A know ledge of the transverse development of the giant pulse would appear to be essential for the exact determination of the true strength of the light field in experiments on multi photon pro cesses [29]. The developed theory also leads to recommendations for the design of lasers to generate giant light pulses of minimum length and minimum divergence of emission.

This collection of articles contains a systematic outline of original experimental and theoretical research on photoproduction of neutral pions at protons and at a strongly bound system of a few nucleons, i.e., the helium nucleus. Spark chambers and their use as spectrometers for photons and electrons are described in detail. The articles of the collection include information on a novel method of determining the efficiency of recording apparatus by generating monochromatic photons. The articles de- scribe original theoretical research on the optical anisotropy of nuclei. Problems encountered in experimental studies of operating the synchrotron as a storage-type accelerator of electrons and positrons receive particular attention. The results of this research work are listed, and the problems of oppositely directed electron-positron beams in the 250-MeV synchrotron are considered. The articles should be of interest to physicists, including research workers, teachers, engineers, graduate students, and students in advanced undergraduate courses. v CONTENTS Photoproduction of Neutral Pions at Nucleons and Nuclei B. B. Govorkov, S. P. Denisov, and E. V. Minarik 0 Photoproduction of 1r Mesons at Helium and at Photon Energies of 71 160-240 MeV ...

The principal results of the work are as follows: 1. A simplified technique was devised for obtaining Raman spectra of powders. 2. The Raman and infrared absorption spectra of the following oxides were investigated: AsP3' SbP3' As 0 * SbP5. Te02. Ge02 (two forms) in the crystalline state, and ASP3' Ge02' Te0 in the glassy state. 2 5 2 3. The vibrational spectra of the crystalline and glassy forms of the oxides ASP3' Ge02' Te02 are simi- lar in many main respects, indicating a similarity of their structural units which determine the vibrational spec- tra of these substances. 4. By a study of the vibrational spectrum it was shown that arsenious anhydride has a molecular structure and consists of As0 molecules having symmetry of the point group Td' The force constants and vibration fre- 4 6 quencies of the As 0 molecule were calculated. and the vibrational spectrum of arsenious anhydride was re- 4 6 liably interpreted with the aid of these results. 5. A similar result was obtained for antimonous anhydride. consisting of Sb0 molecule.s with symmetry 4 6 ofthe point group Td. The force constants for this molecule were found and the vibrational spectrum was calculated.

It is well known that luminescence is the term used to describe the excess radiation from a body over and above the thermal radiation and persisting for a time which greatly exceeds the period of a light vibration. The first half of this definition, proposed by Wiedemann, distinguishes luminescence from equilibrium thermal radi ation; the second half, introduced by Vavilov, distinguishes luminescence from various forms of scattering and from induced radiation, such as Vavilov-Cherenkov radiation, etc. Distinctions are made between photo-, cathodo-, x-ray-, and other forms ofluminescence, depending on how energy is introduced into the luminescent body. Electroluminescence is the name given to that form of fluorescence in which the radiating body receives energy directly from an electric field. It should be noted that luminescence under the influence of cathode rays is not called electroluminescence, because in this case the necessary energy is not supplied directly from the electric field to the radiating body but by means of extraneous electrons. Electroluminescence of gaseous bodies (radiation from a gas discharge) has been known for a long time and is widely used in luminescent lamps and gas discharge tubes. In 1923 Losev [1J observed radiation from silicon carbide crystals when a voltage was applied to them di rectly.

This volume contains two papers that review certain theoretical problems that have been studied in the Laboratory of Plasma Accelerators and Plasma Physics of the P. N. Lebedev Physics Institute of the Academy of Sciences of the USSR. The review of R. R. Kikvidze and A. A. Rukhadze, "Theory of oscillations and stability of a semiconductor plasma with low carrier density in a strong electric field," is devoted to a solid-state plasma. The main attention is devoted to the fact that in such a plasma electro magnetic waves are effectively generated if there is a negative current-voltage characteristic in the carrier current; this effect can compete in importance with the well-known Gunn effect. In their fundamental review paper "Nonlinear theory of the interaction of waves in a plasma," V. V. Pustovalov and V. P. SHin set forth the fundamentals of the theory of nonlinear interaction of waves in a hot rarefied plasma. Besides a systematic exposition of the pro cedure for deriving the equations that describe the nonlinear interaction of waves in an iso tropic or an anisotropic (magnetized) plasma, they study many concrete examples relating to the interaction of definite types of waves under different conditions."