Atoms Iii Molecules i Atome Iii Molekle i

This book contains contributions to the 172. WE-Heraeus-Seminar “Atoms and Molecules in Strong External Fields,” which took place April 7–11 1997 at the Phys- zentrum Bad Honnef (Germany). The designation “strong fields” applies to external static magnetic, and/or electric fields that are sufficiently intense to cause alterations in the atomic or molecular str- ture and dynamics. The specific topics treated are the behavior and properties of atoms in strong static fields, the fundamental aspects and electronic structure of molecules in strong magnetic fields, the dynamics and aspects of chaos in highly excited R- berg atoms in external fields, matter in the atmosphere of astrophysical objects (white dwarfs, neutron stars), and quantum nanostructures in strong magnetic fields. It is obvious that the elaboration of the corresponding properties in these regimes causes the greatest difficulties, and is incomplete even today. Present-day technology has made it possible for many research groups to study the behavior of matter in strong external fields, both experimentally and theore- cally, where the phrase “experimentally” includes the astronomical observations. - derstanding these systems requires the development of modern theories and powerful computational techniques. Interdisciplinary collaborations will be helpful and useful in developing more efficient methods to understand these important systems. Hence the idea was to bring together people from different fields like atomic and molecular physics, theoretical chemistry, astrophysics and all those colleagues interested in aspects of few-body systems in external fields.

Since the early days of modem physics spectroscopic techniques have been employed as a powerful tool to assess existing theoretical models and to uncover novel phenomena that promote the development of new concepts. Conventionally, the system to be probed is prepared in a well-defined state. Upon a controlled perturbation one measures then the spectrum of a single particle (electron, photon, etc.) emitted from the probe. The analysis of this single particle spectrum yields a wealth of important information on the properties of the system, such as optical and magnetic behaviour. Therefore, such analysis is nowadays a standard tool to investigate and characterize a variety of materials. However, it was clear at a very early stage that real physical compounds consist of many coupled particles that may be excited simultaneously in response to an external perturbation. Yet, the simultaneous (coincident) detection of two or more excited species proved to be a serious technical obstacle, in particular for extended electronic systems such as surfaces. In recent years, however, coincidence techniques have progressed so far as to image the multi-particle excitation spectrum in an impressive detail. Correspondingly, many-body theoretical concepts have been put forward to interpret the experimental findings and to direct future experimental research. This book gives a snapshot of the present status of multi-particle coincidence studies both from a theoretical and an experimental point of view. It also includes selected topical review articles that highlight the achievements and the power of coincident techniques.

The recent developement of high power lasers, delivering femtosecond pulses of 20 2 intensities up to 10 W/cm , has led to the discovery of new phenomena in laser interactions with matter. At these enormous laser intensities, atoms, and molecules are exposed to extreme conditions and new phenomena occur, such as the very rapid multi photon ionization of atomic systems, the emission by these systems of very high order harmonics of the exciting laser light, the Coulomb explosion of molecules, and the acceleration of electrons close to the velocity of light. These phenomena generate new behaviour of bulk matter in intense laser fields, with great potential for wide ranging applications which include the study of ultra-fast processes, the development of high-frequency lasers, and the investigation of the properties of plasmas and condensed matter under extreme conditions of temperature and pressure. In particular, the concept of the "fast ignitor" approach to inertial confinement fusion (ICF) has been proposed, which is based on the separation of the compression and the ignition phases in laser-driven ICF. The aim of this course on "Atom, Solids and Plasmas in Super-Intense Laser fields" was to bring together senior researchers and students in atomic and molecular physics, laser physics, condensed matter and plasma physics, in order to review recent developments in high-intensity laser-matter interactions. The course was held at the Ettore Majorana International Centre for Scientific Culture in Erice from July 8 to July 14,2000.

This fourth edition contains a few additional figures. Otherwise only typographical er­ rors have been removed. The final chapter on Fundamentals of the Quantum Theory of Chemical Bonding is continued in an extended way in the textbook Molecular Physics and Elements of Quantum Chemistry by the same authors. This book contains, in particular, a profound presentation of group theory as applied to atoms and molecules. Furthermore, the in­ teraction between atoms and molecules and light is treated in detail. We thank again Springer-Verlag, in particular Dr. H.1. Kblsch and Mr. C.-D. Bachem for their excellent cooperation as always, and Prof. W. D. Brewer for his con­ tinuous support in translating our German text. Stuttgart, February 1994 H. Haken H. C. Wolf Preface to the Third Edition The second edition of this book again enjoyed a very positive reception from both uni­ versity teachers and students. In this edition we have removed all of the typographical errors that came to our attention. In order to keep the book as current as possible, new developments in the direct observation of individual atoms in electromagnetic traps (Paul traps) and of atoms in molecules on solid surfaces using the scanning tunnel microscope have been added to this edition.

Our main aim is to examine whether the atoms and molecules constituting the world around us are distributed in space in a random and disordered fashion, like pebbles on the beach, or in an ordered pattern like the cells of a honeycomb. However, it is often impossible to make such a clear-cut distinction, and it is better not to use "order" and "disorder" as absolute terms but to speak instead of a "degree of order" and a "degree of disorder. " These concepts are fairly new in science. Up to about 20-30 years ago it was still believed (and in fact this belief can still be en­ countered today) that certain states of matter - such as gases, liquids, and amorphous solids - were characterized by a totally disordered distribution of the constituent particles, whilst crys­ tals, by contrast, exhibited perfectly ordered lattices. According to the present view, on the other hand, order and disorder often coexist inseparably from each other, though there are admittedly many cases in which "order" or "disorder" does describe quite accurately tbe actual state of affairs. Symptoms of disorder have recently been found in seemingly perfectly regular molecular structures, and symptoms of order in seemingly perfectly chaotic aggregations of particles. These dis­ coveries led to the formulation of new and important laws cor­ relating the structure of substances with their properties, and to tIlt' explanation of many phenomena in terms of changes in the degree of order.