Nato ASI Subseries B: – serie

The 1988 Summer School on New Aspects of Nuclear Dynamics took place in the style that by now has become a tradition: a series of lectures by well known scientists on modem topics of nuclear physics, where special emphasis is placed on the didactic aspects of the lectures. In the past few years, we have witnessed a rapid evolution of the field of nuclear physics towards novel directions of research. This development is accompanied by the construction of some of the largest experimental facilities ever built for nuclear research. The subjects covered by the Summer School focussed on two main issues currently under active investigation and which will be pursued with the new facilities: the transition from nucleonic to quark degrees of freedom in the decription of nuclear reactions, and the behavior of nuclear matter as one approaches extreme densities and temperatures. These topics in many respects go beyond traditional nuclear physics and the speakers therefore also included high energy physicists. From the response of the participants it was clear that the program of the school filled a gap in the curriculum of many students. We wish to thank all the speakers for their well organized lectures, which were nicely geared to the level of the school, and for spending extra time on problem sessions and extensive discussions. The organization of this Summer School was made possible by substantial support from the Science Committee of the North Atlantic Treaty Organization.

This collection of papers is the result of a symposium sponsored by NATO's Defense Research Group Panel VIII in the Spring of 1985. The symposium came into being when it became obvious to the NATO countries that research, development and utilization of advanced technologies for training was the best means of increasing both training effectiveness and efficiency. This symposium was the second in a series of three devoted to training. The series was structured to cover all aspects of training. The first series addressed the value of training, the second one dealt with the application of training technologies and the third and last of the series focused on academic issues concerned with the effect of prior learning on subsequent learning. The fact that a major American publisher has determined that computer based instruction is the technology of greatest interest to the NATO community is not surprising. Advances in microprocessor technology have revolutionized both how and where we train. During this symposium there were a limited number of carefully chosen exhibits to demonstrate the various applications of computer based training techniques. In the following papers you will find both a practical and scientific basis for the way current and future training and training systems should be designed, applied and utilized. We know that training must be done faster and more effectively.

This volume contains the lectures and communications presented at the NATO Advanced Research Workshop on "Atomic and Molecular Processes with Short Intense Laser Pulses" (NATO ARW 848/86). The workshop was held at Bishop's University, Lennoxville, Que, Canada, July 19-24, 1987, under the directorship of Prof. A.D. Bandrauk, Universite de Sherbrooke. A scientific committee made up of Dr. P. Corkum (Laser Physics, ~ational Research Council of Canada), Dr. P. Hackett (Laser Chemistry, National Research Council of Canada), Prof. S.C. Wallace (Dept. of Chemistry and Physics, University of Toronto), and Prof. F.H.M. Faisal (FakultHt fUr Physik, UniversitHt Bellefeld) was called upon to invite and organize eminent lectures in the fields of i) Coherence Phenomena in Atomic and Molecular Photoprocesses. ii) High Intensity Atomic and Molecular Phenomena. iii) Laser Chemistry The aim of the workshop was to bring together chemists and physicists in order to~iscuss and analyze the progress made in the use of short in- tense laser pulses in understanding coherence phenomena and high intensity, nonlin~adiative effects in atomic and molecular systems.

The NATO Advanced Study Institute on "Interfaces, Quantum Wells and Superlattices" was held from August 16th to 29th, 1987, in Banff, Alberta, Canada. This volume contains most of the lectures that were given at the Institute. A few of the lectures had already been presented at an earlier meeting and appear instead in the proceedings of the NATO Advanced Study Institute on "Physics and Applications of Quantum Wells and Super lattices" held in Erice from April 21st to May 1st earlier in the year and published by Plenum Press. The study of semiconductor interfaces, quantum wells and super­ lattices has come to represent a substantial proportion of all work in condensed matter physics. In a sense the growth of interest in this area, which began to accelerate about 10 years ago and seems to be continuing, has been driven by technological developments. While the older generation of semiconductor devices was based on adjacent semiconductors with different properties (e. g. different doping levels) separated by interfaces, modern semiconductor devices tend to be based more and more on properties of the interfaces themselves. This has led, as an example, to the field of band-structure engineering. Improved understanding of the fundamental physics of these systems has aided technological developments and, in turn, technological developments have made available systems which exhibit novel and fascinating phYSical properties, such as the integer and fractional quantum Hall effects.

The 1981 Cargese Summer Institute on Fundamental Interactions was organized by the Universite Pierre et Marie Curie, Paris (M. LEVY and J-L. BASDEVANT), CERN (M. JACOB), the Universite Catholique de Louvain (D. SPEISER and J. WEYERS), and the Kotholieke Universiteit te Leuven (R. GASTMANS), which, since 1975 have joined their efforts and worked in common. It was the 24th Summer Institute held at Cargese and the 8th one organized by the two institutes of theoretical physics at Leuven and Louvain-Ia-Neuve. The 1985 school was centered around two main themes : the standard model of the fundamental interactions (and beyond) and astrophysics. The remarkable advances in the theoretical understanding and experimental confirmation of the standard model were reviewed in several lectures where the reader will find a thorough analysis of recent experiments as well as a detailed comparaison of the standard model with experiment. On a more theoretical side, supersymmetry, supergravity and strings were discussed as well. The second theme concerns astrophysics where the school was quite successful in bridging the gap between this fascinating subject and more conventional particle physics. We owe many thanks to all those who have made this Summer Institute possible ! Thanks are due to the Scientific Committee of NATO and its President and to the "Region Corse" for a generous grant. .. We wish to thank Miss M-F. HANSELER, Mrs ALRIFRAI, Mr and Mrs ARIANO, and Mr BERNIA and all others from Paris, Leuven, Louvain-la-Neuve and especially Cargese for their collaboration.

A NATO Advanced Research Workshop on Strongly Coupled Plasma Physics was held on the Santa Cruz Campus of the University of California, from August 4 through August 9, 1986. It was attended by 80 participants from 13 countries, 45 of whom were invited speakers. The present volume contains the texts of the invited talks and many of the contributed papers. The relative length of each text is roughly proportional to the length of the workshop presentation. The aim of the workshop was to bring together leading researchers from a number of related disciplines in which strong Coulomb interactions play a dominant role. Compared to the 1977 meeting in Orleans-la-Source, France and the 1982 meeting in Les-Houches, France, it is apparent that the field of strongly coupled plasmas has expanded greatly and has become a very significant field of physics with a wide range of applications. This workshop had a far greater participation of experimental researchers than did the previous two, and some confrontations of real experiments with theoretical calculations occurred. In the two earlier meetings the theoretical presentations were dominated by numerical simulations of static and dynamic properties of various strongly coupled plasmas. The dearth of experiments in the 1970's is now replaced by some very good experimental efforts.

The 1986 Advanced Study Institute on "The Physics of the two-Dimen­ sional Electron Gas" took place at the Conference Centre liTer Helme", close to Oostende (Belgium), from June 2 till 16, 1986. We were motivated to organize this Advanced Study Institute in view of the recent experimental and theoretical progress in the study of the two-dimensional electron gas. An additional motivation was our own theore­ tical interest in cyclotron resonance in two-dimensional electron systems at our institute. It is my pleasure to thank several instances and people who made this Advanced Study Institute possible. First of all, the sponsor of the Advanced Study Institute, the NATO Scientific Committee. Furthermore, the co­ sponsors: Agfa Gevaert, Bell Telephone Mfg. Co. N.V., Burroughs Belgium. Control Data. Digital Equipment Corporation, Esso Belgium. European Research Office (USA). Kredietbank. National Science Foundation (USA). Special thanks are due to the members of the Program Committee and the members of the Organizing Committee. I would also like to thank Mrs. H. Evans for typing assistance.

An Institute like ours cannot help but lend credence to the notion of the late Derek J. de Solla Price of Yale University that "the scientific revolution was largely the improvement, invention and use of a series of instruments . . . . that expanded the reach of science in innumerable directions". Most of science today and in years gone by depends on the experimental observation of struc­ ture on the small scale with microscopes, and on the large scale with telescopes. The first instruments to expand the observational range of the human eye were simple optical systems, designed in the case of microscopes and telescopes to magnify the image. The big breakthrough in the 17th century was not when Galileo first turned his telescope to the heavens, but when improvements in lens-grinding techniques allowed eyeglass makers to make the first telescope. Early microscopy revealed new and previously unsuspected microstruc­ tures in biological and non-biological materials and thus helped to enlarge on the understanding of the relationship between structure and properties. The natural inclination of all microscopists, the desire to observe ever smaller structures, was satisfied by the construction of higher quality optical systems which reduced the aberrations limiting the usable magnification. The modem optical microscope is the result of this evolution in design and construction, and it can be operated easily to achieve close to the theoretical resolution.

Phonons are always present in the solid state even at an absolute temperature of 0 K where zero point vibrations still abound. Moreover, phonons interact with all other excitations of the solid state and, thereby, influence most of its properties. Historically experimental information on phonon transport came from measurements of thermal conductivity. Over the past two decades much more, and much more detailed, information on phonon transport and on many of the inherent phonon interaction processes have come to light from experiments which use nonequilibrium phonons to study their dynamics. The resultant research field has most recently blossomed with the development of ever more sophisticated experimental and theoretical methods which can be applied to it. In fact, the field is moving so rapidly that new members of the research community have difficulties in keeping up to date. This NATO Advanced Study Institute (ASI) was organized with the objective of overcoming the information barrier between those expert in the field and those who are new to it. Thus it was decided to (i) organize a set of tutorially based lectures covering most of the important facets in the field, and (ii) to produce an Institute proceedings which would serve both as the first general textbook, as well as a valuable reference book, for this field of knowledge.

This book is concerned with recent experimental and theoretical work dealing with phenomena created by the transient dipoles and polarizabilities produced by intermolecular interactions. The for­ mer produce absorption from the microwave to the optical regions of the spectrum and the latter produce Rayleigh and Raman scattering; such absorption and scattering would be absent without collisions. Static properties, such as dielectric constant, refractive index, and Kerr effect, also exhibit the effects of induced dipoles and polarizabilities. The first observation of an infrared absorption spectrum pro­ duced by the collisions of molecules which ordinarily do not have an allowed dipole transition was reported in 1949 (Crawford, Welsh, and Locke). The first observation of depolarized Rayleigh spectra due to collisions in atomic gases appeared in 1968 (McTague and Birnbaum). However, it was not until 1977 that the first conference dealing with collision-induced phenomena was organized by J. D. Poll at the University of Guelph. This conference was mainly concerned with studies of collision-induced absorption in gases. Light scat­ tering received more attention at the second meeting of the colli­ sion-induced community in 1978, at the E. Fermi Summer School on "Intermolecular Spectroscopy and Dynamical Properties of Dense Sys­ tems," organized by J. Van Kranendonk. However, the emphasis was still on collision-induced absorption in compressed gases, although some work on liquids, solid H , and related subjects such as ro­ 2 tational relaxation was included. The third induced phenomena con­ ference, organized by F.

The 1982 Antwerp Advanced Study Institute on "Physics of Polarons and Excitons in Polar Semiconductors and Ionic Crystals" took place from July 26 till August 5 at the Conference Center Priorij Corsen­ donk, a restored monastery, close to the city of Antwerp. It was the seventh Institute in our series which started in 1971. This Advanced Study Institute, which was held fifty years after Landau introduced the polaron concept, can be considered as the third major international symposium devoted to the physics of pola­ rons. The first such symposium took place in St. Andrews in 1962 under the title "Polarons and Excitons" [I]. The early theoretical developments related to polarons were reviewed in depth at this meeting; the derivation of the polaron hamiltonian by Frohlich, the Frohlich weak coupling theory (and the equivalent weak coupling canonical transformations), the Landau-Pekar and Bogolubov strong coupling theory and the Feynman polaron model formulated with his path integrals. The main emphasis was on the polaron self-energy, effective mass and mobility. From the experimental side the first evidence for polaron effects was provided by the pioneering cyclotron and mobility measurements o~ the silver halides by F. e. Brown and his group. Also the significance of polaron effects for the under­ standing of excitons in ionic crystals was a central topic in St. Andrews. The second Advanced Study Institute concerning polaron physics was organized at the University of Antwerp (R. U. C. A.

The borderline of quantum electrodynamics and quantum optics offer spectacular results and problems concerning the foundations of radiation theory. Perhaps the major new viewpoint that has emerged from recent investigations is that one can now work inside a time-dependent quantum process, whereas up to now all elementary quantum processes were either stationary, or one worked with asymptotic in-and out-states, i.e. an S-matrix approach. In the-rirst part of this volume, the Quantum Electrodynamics, the present status of the main approaches to this most accurate of all physical theories are discussed: the Hamiltonian approach, the Green's function approach with particular emphasis to bound state problems, and the newer, nonperturbative approach. The latest numerical results on radiative corrections, Lamb shifts and anomalous magnetic moments are reviewed with new results for high Z atoms. Also discussed are different theoretical interpretations of the radiative phenomena as due to quantized field vacuum fluctuations or due to self energy. A small group of contributions are devoted to the physics and mathematical description of decaying or unstable states in quantum theory. This remarkable phenomenon of quantum theory still needs complete clarification, it is a time-dependent phenomenon, which can be described also by asymptotic S-matrix methods, but with complex energies.

The Advanced Study Institute on Strongly Coupled Plasmas was held on the campus of the Universite d'Orleans, Orleans-la-Source, France, from July 6th through July 23rd, 1977. 15 invited lecturers and 50 other participants attended the Institute. The present Volume contains the texts of most of the lectures and of some of the numerous seminars presented at the Institute. The topic of strongly coupled coulomb-systems has been an area of vigorous activities over the last few years. Such systems occur in a great variety of physical situations: stellar and planetary interiors, solid and liquid metals, semiconductors, laser compressed plasmas and gas discharges are some of the most important examples. All these systems have the common feature that for one or more of their constituent charged particle liquids the potential energy to kinetic energy ratio is not small, and therefore the application of the traditional plasma perturbation techniques is not feasible. Many ingenious theoretical schemes have been worked out in order to attack both the related equilibrium and nonequilibrium problems, and also various methods have been borrowed from areas where problems not dissimilar to the ones arising in coulomb-systems had already been tackled. At the same time, computer simulations have led to a probably unparalleled accumulation of data on the behavior of an ensemble of classical charged particles. For the first time, the Institute assembled workers from various disciplines who had been involved with diverse aspects of the strongly coupled plasma problem.

A NATO Advanced Studies Institute was held June 12-23, 1978, at the University of Wisconsin in Madison, Wisconsin. It was a topical Institute in theoretical nuclear physics and had the some­ what novel feature of focussing not on a single topic but on two closely allied ones: pion-nucleus and heavy-ion physics. These two fields. both dedicated to the investigation of short-wave­ length properties of nuclei, have many techniques and concepts in cornmon, and essentially become one in the topic of relativistic heavy-ion physics. The purpose of including both in a single Institute was to encourage the practitioners in each of these fields to learn from those in the other; to judge from the liveli­ ness of the questioning which ensued, the purpose was well-served indeed. Because the Institute was viewed as one which served both educational and research ends, the lecturers took particular pains to develop their subjects in a careful, coherent sequence. The result is a compendium of advanced techniques and current results in these two rapidly-expanding fields of nuclear theory which should serve interested physicists as an ideal introduction to the fields. In addition to the support provided by the Scientific Affairs Division of NATO, substantial financial assistance was provided by the U. S. National Science Foundation and the Graduate School of the University of Wisconsin.

This book has its or1g1n in a NATO Summer School organized from June 25 to July 7 1979, in Menton, France. The purpose of this School was a comparative study of the various aspects of vibra­ tional spectroscopy in molecular liquids and solids. This field has been rapidly expanding in the last decade; unfortunately, its development took place independently for liquids and for solids. In these circumstances, the comparison of the basic concepts and techniques used in these two branches of physics appeared as a necessity. The lectures given at the Menton Advanced Study Institute, as well as the exceptionally fruitful and lively discussions which followed them confirmed this point of view. The need of putting together these lectures, in the form of a monograph, clearly appeared during the ASI and the lecturers accepted to write down the material they presented at the Institute, improved thanks to the remarks of the participants. It is the result of this collective work which appears in the familiar Plenum Series.

That man is a social being is almost axiomatic. Our interpersonal relation­ ships can be sources of the most rewarding or the most painful of human experiences. To a large measure our accomplishments in life depend on the facility with which we interact with others-our social skill. The acquisition of social skills is, of course, a natural part of the overall socialization process. However, in many instances it becomes necessary or desirable to develop further an individual's social facilities. Such skill development is the topic of this book. Two major goals were kept in mind in the writing of this book. The first was to provide a conceptual framework within which to view social skills. Such a framework allows one to understand why it is important to develop social skills, and the effects that such skill development should have. If the reader has a thorough understanding of the concept of social skills and their development, it becomes possible to make appropriate innovations and adaptions to his or her own circumstances. Without such a framework, social-skills training becomes little more than a collection of disjointed techniques. Also, without a conceptual understanding, pro­ cedural innovations are difficult to incorporate into training.

Some of the earliest civilizations regarded the universe as organized around four principles, the four "elements" earth. water, air, and fire. Fire, which was the source of light and as such possessed an immaterial quality related to the spiritual world. was clearly the most impressive of these elements, although its quanti­ tative importance could not have been properly discerned. M- ern science has changed the names, but macroscopic matter is still divided into four states. The solid, liquid, and gaseous states are ordinary states, but the fourth state of matter, the plasma state, has retained a somewhat extraordinary character. It is now recognized that most of the matter of the universe is in the ionized state. but on the earth. the plasma state is still the exception. Hence the importance and also the difficulty of investigations dealing with ionized matter, which have been greatly furthered by the development of thermonuclear fusion research. The study of matter in the ionized state comprises a large diversity of problems belonging to many different branches of phys­ ics. A number of them relate to the microscopic properties of plasmas and concern the structure and the collisional behavior of atomic constituents. Although they are clearly of basic importance, their relevance to thermonuclear research was at first overlooked, at a time when most of the effort was concentrated on designing fusion devices and understanding macroscopic phenomena, mostly of an electromagnetic nature. At present.

This volume contains the proceedings of a two-week NATO A.S.I. on Integrated Optics: Physics and Applications, held from August 17 to August 30, 1981 in Erice, Italy. This is the 8th annual Course of the "International School of Quantum Electronics" presented under the auspices of the "E. Majorana" Centre for Scientific Culture. The subject was chosen in order to satisfy the demand for a course on integrated optics which is relevant to the expanding use of fiber optics for communication and signal processing. Integrated Optics, encompassing all of the optical waveguide circuits which are the optical analog of integrated circuits, is finding its way into a variety of applications involving communi­ cations, high speed signal-processing, and sensors of many kinds. However, because the technology is still changing very rapidly, the development of these exciting applications relies heavily upon the physics of the integrated optical circuits themselves and the pro­ cessing techniques used to fabricate them. This NATO A.S.I. provided not only a thorough tutorial treatment of the field, but also through panel discussions and additional lectures treated topics at the forefront of present work. Therefore the character of the Course was a blend of current research and tutorial reviews. "The Physics and Applications of Integrated Optics" could hardly be a more appropriate title to be chosen for this volume. Many of the worlds' acknowledged leaders in the field have been brought together to review and speculate on the accomplishments of integrated optics.

The present volume contains contributions presented at the NATO Advanced Study Institute on Molecular Ions held on the island of Kos, Greece, from September 30 to October 10, 1980. The meeting was attended by some 60 participants from 15 different countries. It was the first meeting devoted exclusively to the topic of molecular ions. Its vitality derived from bringing together experts and students from a wide variety of disciplines, whose studies bear upon the structure of molecular ions. The aim of the meeting was to assemble these scientists, representing many countries in Europe and North America, to discuss the advances and capabilities of the various experimental and theoretical approaches and to point out un­ solved problems and directions for future research. The format, in­ volving lecturers and students, served as a tutorial. Molecular ions play an important role in very diverse fields of nature such as reactions in the ionosphere, the processes of forma­ tion of molecules in dense interstellar clouds, and the magnetohydro­ dynamics of plasmas used for energy generation. Our understanding of the properties of molecular ions, their electronic and geometric structures, has been developing from a variety of sources, as far removed as tickling ions with radiofre­ quency radiation and smashing them apart at relativistic energies. Various laser techniques are described, and the queen of structural determination, spectroscopy, is well represented. On the instrumen­ tal side, older techniques have been perfected and new methods have evolved.

The need for long-term energy sources, in particular for our highly technological society, has become increasingly apparent during the last decade. One of these sources, of tremendous poten­ tial importance, is controlled thermonuclear fusion. The goal of controlled thermonuclear fusion research is to produce a high-temperature, completely ionized plasma in which the nuclei of two hydrogen isotopes, deuterium and tritium, undergo enough fusion reactions so that the nuclear energy released by these fusion reactions can be transformed into heat and electricity with an overall gain in energy. This requires average kinetic energies for the nuclei of the order of 10 keV, corresponding to temperatures of about 100 million degrees. Moreover, the plasma must remain confined for a certain time interval, during which sufficient energy must be produced to heat the plasma, overcome the energy losses and supply heat to the power station. At present, two main approaches are being investigated to achieve these objectives: magnetic confinement and inertial con­ finement. In magnetic confinement research, a low-density plasma is heated by electric currents, assisted by additional heating methods such as radio-frequency heating or neutral beam injection, and the confinement is achieved by using various magnetic field configurations. Examples of these are the plasmas produced in stellarator and tokamak devices.

The fundamental nature of human time experience has concerned artists, poets, philosophers, and scientists throughout the ages. Any consideration of human action requires awareness of its temporal aspects. However, simply to view time in the same units and dimensions as the physicist employs in describing events robs personal time of its "lived" quality. The use of physical time concepts in the description of human events is often artificial and misleading. It fails to account for the facts that human time estimates rarely match clock and calendar time; that societies and individuals demonstrate vast differences in their constructions and uses of time; and that temporal perceptions and attitudes change within an individual both during a single day and throughout his life span. The present volume does not view time as something that is sensed in the same way that one would sense or perceive spatial or sensory stimuli. Rather, it views time as a complex set of personally experienced cognitive constructs used by individuals and cultures to account for the order, the duration, and the organization of events. The authors in this book take a strong departure from earlier psychophysical studies of a "time sense" and address themselves to the uses and elaborations of time concepts in personal and social functioning.

For the eighth Advanced Study Institute (ASI) on Techniques and Concepts of High-Energy Physics we returned once again to the Hotel on the Cay on that speck of land in the harbor of Christiansted, St. Croix, U. S. Virgin Islands. This time, the ASI brought together a total of 73 participants, from 21 countries. The primary support for the meeting was provided, as usual, by the Scientific Affairs Division of the North Atlantic Treaty Organization (NATO). The ASI was cosponsored by the U. S. Department of Energy, by the Fermi National Accelerator Laboratory (Fermilab), by the U. S. National Science Foundation, and by the University of Rochester. A special contribution from the Oliver S. and Jennie R. Donaldson Charitable Trust provided an important degree of flexibility, as well as support for worthy students from developing countries. In addition, the International Science Foundation contributed very generously to the support of a participant from Russia. As in the case of the previous ASls, the scientific program was designed for advanced graduate students and recent PhD recipients in experimental particle physics. The present volume of lectures, although, unfortunately, short of three contributions, should complement the material published in the first seven ASIs, and prove to be of value to a wider audience of physicists. It is a pleasure to acknowledge the encouragement and support that I have continued to receive from colleagues and friends in organizing this meeting.

The Advanced Study Institute on "Advances in Superconductivity" was held at the Ettore Majorana Centre for Scientific Culture in Erice, Sicily, during July 3 to July 15, 1982. This Institute was the third course of the International School of Low Tempera­ ture Physics, which was established at the Centre in 1977 with the guidance and inspiration of T. Regge and A. Zichichi. The 1982 Course was centered on a topic which brought together fundamental basic research and the most recent promising technological applications. Accordingly, the participants represented a wide spectrum of industrial and government laboratories, as well as universities from various countries. The program of topics and speakers was developed with the advice of the Organizing Committee, composed of H. Frohlich, T. Regge, B. Stritzker, and L. Testardi. This Institute emphasized recent developments in the science and technology of superconductivity. A historical perspective was provided by H. Frohlich, whose lectures recall the earliest discoveries and theoretical attempts to understand superconductivity. Ironically, his early suggestion of the electron-phonon coupling as a key to superconductivity was met with initial widespread skepticism. Later, the development of field theory methods for solid state physics problems, and the evolution of the BCS theory has led to a seemingly unanimous concensus regarding the e1ectron­ phonon mechanism as the predominant source of superconductivity in known materials. Experimental studies of superconductivity exemplify the strong interplay of science and technology in many ways.

The 1976 Cargese Summer Institute was devoted to the study of certain exciting developments in quantum field theory and critical phenomena. Its genesis occurred in 1974 as an outgrowth of many scientific discussions amongst the undersigned, who decided to form a scientific committee for the organization of the school. On the one hand, various workers in quantum field theory were continuing to make startling progress in different directions. On the other hand, many new problems were arising from these various domains. Thus we feIt that 1976 might be an appropriate occasion both to review recent developments and to encourage interactions between researchers from different backgrounds working on a common set of unsolved problems. An important aspect of the school, as it took place, was the participation of and stimulating interaction between such a broad spectrum of theorists. The central topics of the school were chosen from the areas of solitons, phase transitions, critical behavior, the renormalization group, gauge fields and the analysis of nonrenormalizable field theories. A noteworthy feature of these topics is the interpene­ tration of ideas from quantum field theory and statistical mechanics whose inherent unity is seen in the functional integral formulation of quantum field theory. The actual lectures were partly in the form of tutorials designed to familiarize the participants with re­ cent progress on the main topics of the school. Others were in the form of more specialized seminars reporting on recent research.

Volumes 30 and 31 of this series, dealing with "Many Degrees of Freedom," contain the proceedings of the 1976 International Summer Institute of Theoretical Physics, held at the university of Bielefeld from August 23 to September 4, 1976. This institute was the eighth in a series of summer schools devoted to particle physics and organized by universities and research institutes in the Federal Republic of Germany. Many degrees of freedom and collective phenomena play a critical role in the description and understanding of elementary particles. The lectures in this volume were intended to display how these structures occur in various recent developments of mathematical physics. Lectures ranged from classical nonlinear field theory over classical soliton models, constructive quantum field theory with soliton solutions and gauge models to the recent unified description of renormalization group tech­ niques in probabilistic language and to quantum statisti­ cal dynamics in terms of derivations. The Institute took place at the Center for Inter­ disciplinary Research of the University of Bielefeld. On behalf of all participants, it is a pleasure to thank the officials and the administration of the Center for their cooperation and help before and during the Insti­ tute. Special thanks go to V.C. Fulland, M. Kamper, and A. Kottenkamp for their rapid and competent preparation of the manuscripts.

The importance of real space imaging and spatially-resolved spectroscopy in many of the most significant problems of surface and interface behaviour is almost self evident. To join the expertise of the tradi tional surface scientist with that of the electron microscopist has however been a slow and difficult process. In the past few years remarkable progress has been achieved, including the development of new techniques of scanning transmission and reflection imaging as well as low energy microscopy, all carried out in greatly improved vacuum conditions. Most astonishing of all has been the advent of the scanning tunneling electron microscope providing atomic resolution in a manner readily compatible with most surface science diagnostic procedures. The problem of beam damage, though often serious, is increasingly well understood so that we can assess the reliability and usefulness of the results which can now be obtained in catalysis studies and a wide range of surface science applications. These new developments and many others in more established surface techniques are all described in this book, based on lectures given at a NATO Advanced Study Institute held in Erice, Sicily, at Easter 1987. It is regretted that a few lectures on low energy electron diffraction and channeling effects could not be included. Fifteen lecturers from seven different Countries and 67 students from 23 Countries and a wide variety of backgrounds attended the school.

The interest of describing the ground state properties of a system in terms of one electron density (or its two spin components) is obvious, in particular due to the simple physical significance of this function. Recent experimental progress in diffraction made the measurement of charge and magnetization densities in crystalline solids possible, with an accuracy at least as good as theoretical accuracy. Theoretical developments of the many-body problem have proved the extreme importance of the one electron density function and presently, accurate methods of band structure determination become available. Parallel to the diffraction techniques, other domains of research (inelastic scattering, resonance, molecular spectroscopy) deal with quantities directly related to the one particle density. But the two types of studies do not interfere enough and one should obviously gain more information by interpreting all experiments that are related to the density together. It became necessary to have an International School that reviews the status of the art in the domain of "ELECTRON AND MAGNETIZATION DENSITIES IN MOLECULES AND CRYSTALS". This was made possible through the generous effort of N.A.T.O. 's Scientific Affairs Division, and I would specially thank Dr. T. KESTER, the head of this Division, for his help and competence. An Advanced Study Institute was thus held in ARLES, south France, from the 16th to the 31st of August 1978.

The Advanced Study Institute on 'Elementary Excitations in Solids, Molecules, and Atoms' was held at the University of Antwerp (U.I.A.) from June 18th till June 30th 1973. The In- stitute was sponsored by NATO. Co-sponsors were: Agfa-Gevaert N.V. (Mortsel-Belgium), Bell Telephone Mfg. Co. (Antwerp-Belgium), I.B.M. BelgiumN.V. (Brussels-Belgium), the National Science Founda- tion (Washington D. C. - U. S .A.) and the Uni versi ty of Antwerp (U. I .A.) . A total of 120 lecturers and participants attended the Institute. Over the last few years, substantial progress has been made in the description of the elementary excitations of the elec- tronic and vibrational systems and their interactions. Parallel with this, the experimentalists have obtained outstanding re- sults, partly as a result of availability of coherent light sour- ces from the far infrared through the visible region, and partly because of the availability of synchrotron radiation sources in the soft X-ray region. The results of today will lead to fur- ther progress over the next years. It was the purpose of this NATO Advanced Study Institute to present a state of the art, namely a survey of experiment and theory.