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This book collects the lectures given at the NATO Advanced Study Institute on'Atoms in Strong Fields', which took place on the island of Kos, Greece, during the two weeks of October 9-21,1988. The designation'strong field'applies here to an external electromagnetic field that is sufficiently strong to cause highly nonlinear alterations in atomic or molecular struc­ ture and dynamics. The specific topics treated in this volume fall into two general cater­ gories, which are those for which strong field effects can be studied in detail in terrestrial laboratories: the dynamics of excited states in static or quasi-static electric and magnetic fields; and the interaction of atoms and molecules with intense laser radiation. In both areas there exist promising opportunities for research of a fundamental nature. An electric field of even a few volts per centimeter can be very strong on the atom­ ic scale, if it acts upon a weakly bound state. The study of Rydberg states with high reso­ lution laser spectroscopic techniques has made it possible to follow the transition from weak-field to strong-field behavior in remarkable detail, using static fields of modest lab­ oratory strength; in the course of this transition the atomic system evolves from one which can be thoroughly understood in terms of field-free quantum numbers, to one which cannot be meaningfully associated at all with the zero-field states of the atom.

This book is an outcome of the NATO institute on surface modification which was held in Trevi, 1981. Surface modification and alloying by ion, electron or laser beams is proving to be one of the most burgeoning areas of materials science. The field covers such diverse areas as integrated circuit processing to fabricating wear and corrosion resistant surfaces on mechanical components. The common scientific questions of interest are the microstructures by the different energy deposition techniques. and associated physical properties produced The chapters constitute a critical review of the various subjects covered at Trevi. Each chapter author took responsibility for the overall review and used contributions from the many papers presented at the meeting; each participant gave a presentation. The contributors are listed at the start of each chapter. We took this approach to get some order in a large and diverse field. We are indebted to all the contributors, in particular the chapter authors for working the many papers into coherent packages; to Jim Mayer for hosting a workshop of chapter authors at Cornell and to Ian Bubb who did a sterling job in working over some of the manuscripts. Our special thanks are due to the text processing center at Bell Labs who took on the task of assembling the book. In particular Karen Lieb and Beverly Heravi typed the whole manuscript and had the entire book phototypeset using the Bell Laboratories UNIX™ system.

Known and developed over the past twenty five years, lasers have been experimented in a variety of processes with an uneven success. Apart from fundamental physics experiments in which the various aspects of coherence are systematically exploited, applications in the field of Materials Science have been scattered recently over so many situations that it is apparently difficult today to conceive a comprehensive interpretation of all physical processes encountered. In some domains of research like photochemistry, development has been fast and rather self-supporting. In others, like solid-state processing, progress has been either very specific or deviated towards marginal applications, or else emerged as a joint-venture between physicists and chemists. This yielded a number of professional meetings, where day-to-day research activities are presented. In 1982, the Cargese ASI on'Cohesive properties of semiconductors under laser irradiation'was one of such meetings at which a prospective of the field was discussed at length in ebullient round-table sessions. Quoted from the proceedings,'the Institute helped to discern clearly the limits of existing theoretical approaches and the directions along which work is urgently needed within the next few years'. Four years have passed and the field has literally explo­ ded. It must be mentioned that some of the most striking developments over the past two years were accurately predicted at the Institute in Cargese.

It is a pleasure to write a few words as an introduction to the proceedings of the 1980 NATO ASI on'Physical Processes in Laser­ Naterial Interaction.'This ASI is the ninth course of a series devoted to lasers and their applications, held under the responsibility of the Quantum Electronics Division of the European Physical Society, and for this reason known as the'Europhysics School of Quantum Electronics.'Since 1971 the School has been operating with the joint direc­ tion of myself as representative of the academic research, and Dr. D. Roess (formerly with Siemens AEG, Munich, and now with Sick, Optik und Electronik, GmbH, Munich) for the industrial applications. Indeed the aim of the School is to alternate fundamental and applied frontier topics in the area of quantum electronics and modern optics, in order to introduce young research people from universities and industrial R&D laboratories to the new aspects of research opened by the laser.

The impact of Materials Science in our environment has probably never been as massive and decisive as it is today. In every aspect of our lives, progress has never been so dependent on the techniques involved in producing ever more sophisticated materials in ever larger quantities, nor so demanding for technologists to imagine novel processes and circumvent difficulties, or take up new challenges. Every technique is based on a physical process which is put into practice and optimized. The better we know that process, the better the optimization, and more powerful the technique. Laser processing of materials is inscribed in that context. As soon as powerful coherent light sources were made available, it was realized that such intense sources of energy could be used to'heat, melt and crystallize'materials, i.e., to promote phase transitions in atomic systems. As early as 1964, attempts in that direction were made but received very little (if any) attention. Reasons for this lack of interest were several. For one thing, laser technology was not fully developed, so that the process offered poor reliability and no versatility. Also, improving the existing techniques was believed to be sufficient to meet the needs of the time, and there was no real motivation to explore new ways. Finally, and more important, the fundamentals of the physics behind the scenes were, and continue to be, way out of the runni~g stream.

This volume contains eight invited papers and seventy-three contributed papers presented at the Fourth International Symposium on Gas Flow and Chemical Lasers. which was held in Stresa. Italy. from September 13 to 17. 1984. The purpose of the Symposium was to record and discuss current research developments and applications related to high power lasers. Papers were solicited from the entire spectrum of activities, includ­ ing basic physics, aerothermodynamics, new laser media, laser design, diagnostic techniques, laser propagation. interaction phenom­ ena, applications. The Symposium was an opportunity for scientists and engineers representing all these disciplines to come together to report their recent work, to exchange ideas and to provide an up-to-date account of international progress in these fields. The contributed papers were reviewed by the members of the Scientific Advisory CoEmittee. who also took responsibility for formulating the program of invited lectures. As editor I wish to express my appreciation and my gratitude to people and organizations that made this Symposium a success: the Members of the Scientific Advisory Committee, the Members of the Local Organizing Committee, the Sponsors and the Symposium Secretary. A grateful acknowledgement is expressed to Dr. M. Sandra Oggiano for assistance in the organization of the Symposium and publication of these proceedings.