The Fourth USA-USSR Symposium. on The Physics of Optical Phenomena and Their Use as Probes of Matter, was held in Irvine, California, January 23-27, 1990. Participating in the Symposium were 22 scientists from the USSR and 29 from the USA. In addition, to provide an international dimension to this Symposium without, however, compromising significantly its essentially binational character, 7 non-US and non-USSR scientists were invited to take part in it. The present volume is the proceedings of that Symposium, and contains all manuscripts received prior to August 1, 1990, representing. scientific contributions presented. A few manuscripts were not received, but for completeness the corresponding abstract is printed.. Three previous USA/USSR Binational Symposia on related topics have been held, viz. "Theory of Light Scattering in Condensed Matter" (Moscow, 1975), "Light Scattering in Solids" (New York, 1979), and "Laser Optics of Condensed Matter" (Leningrad, 1987). These meetings were evaluated by the participants as highly successful and provided invaluable oppor tunities for researchers to exchange information and to initiate colla borative work which led to research visits by US physicist to Soviet laboratories, and vice versa, and which continue to the present day.

It is very rewarding for an author to know that his book is to be translated into another language and become available to a new circle of readers. The study of the optics and spectroscopy of activated crys stals has continued to grow. The development and first remarkable successes of light scattering by impurities in crystals have occurred in the comparatively short time since my original book was sent to press. After experimental observation of the sidebands (wings) in impurity infrared absorption spectra, interest in these spectra as a source of information on the vibrations of a crystal in the neigh borhood of an impurity has increased significantly. Therefore, in addition to making minor corrections, r have supplemented the section on the effect of anharmonicity (section25) and written two new sections and another Appendix on infrared ab sorption, scattering of light by an impurity center in a crystal, and the adiabatic approximation, respectively. The bibliography has received several dozen new entries, but it nevertheless does not pretend to be complete. r hope that the American edition is useful and in some de gree corresponds to the general deepening of our physical under standing of solids."

Almost fifteen years have now elapsed since the first observations of per sistent spectral hole-burning in inhomogeneously broadened absorption lines in solids. The fact that the spectral shape of an inhomogeneously broadened line can be locally modified for long periods of time has led to a large number of investigations of low-temperature photophysics and photochemistry that would not have been possible otherwise. Using hole burning, important information has been obtained about a variety of in teractions, including excited-state dephasing processes, host-guest dynam ics, proton tunnelling, low-frequency excitation in amorphous hosts, relaxation mechanisms for vibrational modes, photochemical mechanisms at liquid helium temperatures, and external field perturbations. At the same time, the possibility that persistent spectral holes might be used to store digital information has led to the study of materials and configura tions for frequency-domain optical storage and related possible applica tions. This is the first full-length book on persistent spectral hole-burning. The goal is to provide a broadly based survey of the scientific principles and applications of persistent spectral hole-burning. Since the topic is quite interdisciplinary, the book is intended for researchers, graduate stu dents, and advanced undergraduates in the fields of chemical physics, solid-state physics, laser spectroscopy, solid-state photochemistry, and high-performance optical storage and optical processing.