Showing papers in "Reports on Progress in Physics in 1977"

Intermetallic compounds of rare-earth and 3d transition metals

Abstract: A survey is given of the physical properties, composition and crystal structure of intermetallic compounds formed between rare-earth elements and 3d transition elements. Apart from binary compounds the results of pseudobinary series are also considered. The magnetic properties determined by the exchange interactions involving 4f as well as 3d electrons, are discussed together with experimental results available on magnetovolume effects and various resonance techniques such as NMR and the Mossbauer effect.

Atoms in high magnetic fields (white dwarfs)

Abstract: High magnetic fields have been of interest for some time in solid-state physics. Recent spectroscopic experiments on highly excited barium atoms and the discovery of very large magnetic fields in white dwarf stars and neutron stars have stimulated renewed interest in the spectroscopy of free atoms in high magnetic fields. The various spectroscopic regimes ranging from the low-field Zeeman effect to the high-field Landau regime are reviewed. Calculations of the energy levels of hydrogen, helium and their isoelectronic ions in fields of any strength are surveyed. Observations of the quadratic Zeeman effect in the alkalis and high-field effects in the neutral barium spectrum are described. Finally, recent discoveries of continuum polarization in white dwarf stars, its interpretation in terms of the presence of high magnetic fields, the confirmation of such fields spectroscopically, and some remaining mysteries in these remarkable stellar spectra are reviewed.

Beam-foil spectroscopy

Abstract: The measurement techniques of beam-foil spectroscopy are reviewed. These comprise the study of wavelengths of heavy ions with low and high charge states at visible, UV and X-ray wavelengths from normal excited states, multiply excited states and high-n Rydberg levels; the measurements of mean lifetimes above 10-12 s and metastable states up to 100 ns; high-resolution Auger electron measurements; and the application of atomic alignment and orientation by both perpendicular and tilted foils to measurements of atomic structures and analysis of the beam-foil excitation process. A theoretical description of the emission of light from anisotropically excited systems is included. Associated techniques of time-resolved spectroscopy are compared.

Doppler-free multiphotonic spectroscopy

Abstract: The review describes atomic transitions where two or more photons are simultaneously absorbed by an atom or a molecule (without any intermediate resonant level). The authors show theoretically and experimentally the possibilities of these transitions for numerous high-resolution spectroscopic studies. The Doppler broadening is suppressed in two-photon transitions if the atoms are irradiated by two laser waves travelling in opposite directions, because the Doppler shift in one wave cancels that in the other. A generalization is made for multiphotonic transitions. A typical experimental set-up with a CW dye laser is described in detail, but the possibilities of other light sources are also discussed.

Quantum-mechanical perturbation theory

Abstract: Modern computers have made possible the evaluation of higher order terms of perturbation series. Apart from the empirical work involving numerical calculations, much formal mathematical work has been done on the convergence properties of perturbation series and of Pade approximants which represent them. Some of the modern work in perturbation theory is reviewed in the context of traditional results from the theory of real and complex variables. The two major versions of time-independent perturbation theory, the Rayleigh-Schrodinger (RS) and Brillouin-Wigner (BW) theories, are compared and contrasted, and the alternative techniques for evaluating the terms in the energy series are discussed. The sum-over-states method, the differential equation method and the variational principle method are treated, with emphasis on their inter-relations.

The measurement of lifetimes in atoms, molecules and ions

Abstract: The field of the direct measurement of atomic and molecular lifetimes has been characterized in the past few years by the development of several new and powerful techniques and by an improving ability to control and limit the measurement inaccuracies of older techniques. The establishment of a wide range of accurate values for atomic and molecular lifetimes is now possible. This review deals with the measurement of lifetimes of atoms and molecules in the gas phase, in the range 10-10-10-5 s, and concentrates on the more accurate and promising techniques, which are grouped together according to whether their most prominent feature is the means of excitation or the interference between excited levels or the coincident detection of two decay events. Possible sources of systematic error are discussed at length and the theoretical background appropriate to the measurements is given when necessary. The techniques and their areas of applicability are compared and the likely future development of them is discussed.

Invariants and stability in classical mechanics

Abstract: In the last 20 years there have been discoveries in classical mechanics that have made an impact on several other branches of physics. The important result from which these developments stem has become known as the KAM theorem. This relates to a situation in which a simple integrable dynamical system is subjected to perturbations. Whereas before perturbation the motion could be described as regular, after perturbation the phase space is sharply divided into regions of regular and highly irregular motion. Computer calculations have made it possible to appreciate both the complexity and the detailed structure of the situation. Examples of this behaviour have been found in plasma physics, statistical mechanics and astronomy. A qualitative review is given of the KAM theorem, the work that led up to it and its application to various branches of physics.

Three-Body Systems in Nuclear Physics

Abstract: Recent developments in the three-body problem in nuclear physics are reviewed. The general properties of three-particle states are discussed; particular reference is made to the theory of final-state interactions and to the Efimov effect. The properties and interactions of three-nucleon systems are considered. The main emphasis is on the relevance of these properties to the nature of two- and three-nucleon forces to the role of meson exchange currents in electromagnetic and weak processes and to the reaction mechanisms of pions with nuclei.

Interaction of laser radiation with free atoms

Abstract: The review describes and interprets the basic phenomena recently observed in the interaction between free atoms and laser radiation. Some of these have been known for a long time in magnetic resonance but in the optical range, the situation is strongly modified because of spontaneous emission and the Doppler effect. Therefore, particular attention is paid to resonance fluorescence and Doppler-free spectroscopy but without dwelling on applications. Transient effects are also discussed, but all the phenomena which characterize, not specifically the interaction between atoms and fields, but the free evolution of atoms after coherent excitation are ignored. Pulse propagation experiments are described in detail and multiphoton processes are considered, with special emphasis on Doppler-free multiphoton excitation and resonant multiphoton ionization.

Waves and microinstabilities in plasmas-linear effects

Abstract: The review is concerned with wave propagation in plasmas where collisions may be neglected. The relevant kinetic theory is described by the Vlasov equation. Waves in the absence of a magnetic field are reviewed including Landau damping of electron waves and ion acoustic waves. The complications due to purely ballistic effects are described and the latter case includes the study of mixtures of ions. Thermal motion has little effect on electromagnetic waves with the exception of the anomalous skin effect. Beam-plasma interactions are reviewed and general concepts of stability are discussed. The conversion of wave energy from one kind of wave and linear coupling mechanisms are described. The importance of resonance cones is also noted.

The Lamb shift in hydrogenic ions

Abstract: Recent experimental progress and probable future directions in the study of the Lamb shift in high Z hydrogenic ions are reviewed. Advances in techniques and instrumentation have allowed an extension of Lamb shift studies to hydrogenic ions of increasingly higher nuclear charge (Z) using heavy-ion accelerators. New calculational techniques have been applied to high-Z systems. This has stimulated wide interest because the large enhancement that occurs in the Lamb shift due to an approximate Z4 dependence allows sensitive studies of QED interactions in the high-field regime and suggests the possibility of observing new and unexpected phenomena. The major experimental methods are the Stark quenching of the metastable 2s1/2 state in accurately known applied fields and the laser resonance absorption technique. The accuracy of experiments and calculations is considered.

Quantization of non-linear systems

Abstract: The developments that have taken place over the last five years in semi-classical quantization methods are reviewed. They generalize the WKB method to many degrees of freedom. These techniques, most of which make extensive use of Feynman path integrals, can be extended naturally to quantum field theory, where they have found their most spectacular applications. The basic mathematics are explained by treating, in the path integral formalism, the simple case of the quantization rule of bound states for the one-dimensional Schrodinger equation. The extension to many degrees of freedom is discussed, in particular, how much quantum-mechanical information can be retrieved from given amounts of classical information. The functional integration method is applied to the quantization of classical static or time-dependent solutions of interacting field theories, with particular emphasis on particle-like and bound-state solutions.

Microinstabilities in plasmas-non-linear effects

Abstract: The review attempts to bring together and present the wide range of non-linear kinetic phenomena which occur in a plasma. The extent to which the theory has been borne out by experiment is indicated in detail and in several instances it is convenient to catalogue the results. While the main thrust of laboratory plasma physics has been associated with magnetic containment fusion, there has been the recent emergence of laser heating as an alternative path to fusion, and the refinement of measurements in the ionosphere and magnetosphere means that there has been a parallel development of interest in non-linear processes. The processes considered range from resonant three-wave interactions in homogeneous and inhomogeneous plasmas to situations in which the plasma parameters themselves vary as a result of interactions under consideration.

The shape of nuclei

Abstract: For the class of nuclei which are 'strongly deformed' it is possible to introduce the idea of an empirically measurable static nuclear shape. The limitations of this concept as applied to nuclei (fundamentally quantum-mechanical objects) are discussed. These are basically the limitations of the rotational model which must be introduced in order to define and measure nuclear shape. A unified discussion of the ways in which the shape has been parametrized is given with emphasis on the fact that different parametrizations correspond to different nuclear structures. Accounts of the various theoretical procedures for calculating nuclear shapes and of the interaction between nuclear shapes and nuclear spectroscopy are given. A coherent account of a large subset of nuclei (strongly deformed nuclei) can be given by means of a model in which the concept of nuclear shape plays a central role.

Inelastic lepton-nucleon scattering

Abstract: The deep inelastic scattering of leptons (electrons, muons, neutrinos) by neutron and proton targets is reviewed. The main conclusion is that the processes involved can be interpreted in terms of a simple but powerful model, according to which leptons are scattered elastically by quasi-free point-like constituents (partons) of the nucleon, which subsequently interact to form the final hadronic state. The magnitudes of the cross sections and the values of certain sum rules identify the partons with quarks of fractional charge (+2e/3 and -e/3) and neutral vector gluons. At finite momentum transfers, there are observed to be small departures from the exact Bjorken scaling of the parton model, and these are discussed in terms of the quark-quark couplings. Anomalies in antineutrino scattering in one experiment indicate the possibility of further quark varieties.

Recent progress in atomic collisions theory

Abstract: A survey of some of the recent advances which have been made in various branches of the theory of atomic scattering processes is given with the general aim of providing an overall view of the subject as it stands at present. The review describes, without undue detail, the methods which have been used to obtain and solve approximately the equations which arise in the scattering of electrons and positrons by atoms and molecules and in collisions between atomic systems including charged as well as neutral species.

Experimental studies of plasma confinement in toroidal systems

Abstract: To produce a net gain of energy during the fusion reactions of light nuclei (e.g. H,D,T,He,Li) in a plasma the Lawson criteria concerning temperature, particle number density and confinement time, must be met. The theoretical and experimental aspects of the closed-line magnetic-field approach to the plasma isolation and confinement problem is reviewed. The topology of such systems is toroidal. Plasma energy loss processes are discussed. Stellarators, toroidal punches, internal-ring devices (multipoles and levitrons) and, in particular, Tokamaks are considered.

Fermi surface studies of pure crystalline materials

Abstract: A review is presented of Fermi surface studies in pure materials, and of the lines along which the Fermi surfaces can be represented and understood. The application of the de Haas-van Alphen effect and the high-field magnetoresistance to experimental Fermi surface determinations is discussed and specimen requirements assessed. The representation by pseudopotentials of the band structures of metals without d bands is described and models which account for the band structures of d-band metals are presented, particularly with the aim of bringing out the features common to each metal of a class. Modern methods of parametrization for Fermi surfaces are discussed.

Heavy-ion transfer reactions

Abstract: This review is a non-technical outline of our understanding of the mechanisms of heavy-ion transfer reactions, of what has been learned about the nucleus from the use of these reactions, and of the problems presently being investigated. The subject matter is chiefly restricted to reactions in which transitions to individual levels can be resolved and to reactions induced by projectiles heavier than boron and lighter than calcium at energies up to a few MeV per nucleon. The considerations involved in the design of experiments are summarised and the theoretical framework within which the reactions are usually analysed is described. The nuclear structure information which can be obtained from heavy-ion transfer reactions is illustrated by examples culled from single- and multi-nucleon transfer reaction experiments.

The growth of insulating crystals

Abstract: The growth of insulating crystals, i.e. crystals primarily of interest for their dielectric, magnetic, mechanical and optical properties, is reviewed. After a brief survey of the uses of crystals and the early work on their growth, attention is turned to the mechanisms of crystal growth. The thermodynamic, kinetic and mass transport processes are discussed in terms of their effects on growth rate, impurity incorporation and interface stability. The effects of heat transport and solute-induced strain on crystal perfection are described. In the section on practical techniques attention is concentrated on the widely used methods, i.e. those originated by Verneuil, Czochralski, Kyropoulos and Bridgman, the techniques of growth from solutions at high and low temperatures and hydrothermal growth. The problems of method selection and optimization are reviewed.

The organization of human vision for pattern detection and recognition

Abstract: The anatomical, electrophysiological and psychophysical data relating to the visual detection and recognition of spatially structured light stimuli are reviewed and a brief survey of the diverse experimental methods by which such data are obtained is given. Experimentally determined acuity and transfer functions are defined and their application to the prediction of visual threshold detection of stimulus patterns is examined. Three models of visual pattern recognition representing three different levels of abstraction of visual function are considered. The development of the structural and functional organization of the visual system in cat and primate is reviewed.

Colour reproduction by photography

Abstract: The subjective and objective terms necessary for discussing colour are explained, and the various possible aims for colour reproduction in pictures are discussed. The tone reproductions required for reflection prints and for displayed transparencies are described and their differences explained. The methods used for forming the cyan, magenta and yellow dye images in colour photography are described; the unwanted retinal stimulations and the unwanted absorptions of the dyes are shown to reduce the reproducible colour gamut. Factors affecting the granularity and sharpness of colour photographs are discussed, and the methods adopted for the mass-printing of amateurs' negatives are described. Systems of colour photography in which the picture is processed in the camera are briefly described.