This book presents an ac count of the NATO Advanced Study Institute on "Collective Excitations in Solids," held in Erice, Italy, from June 15 to June 29, 1981. This meeting was organized by the International School of Atomic and Molecular Spectroscopy of the "Ettore Majorana" Centre for Scientific Culture. The objective of the Institute was to formulate a unified and coherent treatment of various collective excitation processes by drawing on the current advances in various branches of the physics of the solid state. A total of 74 participants came from 54 laboratories and 20 nations (Australia, Belgium, Burma, Canada, China, France, F. R. Germany, Greece, Israel, Italy, Mexico, The Netherlands, Pakistan, Poland, Portugal, Romania, Switzerland, Turkey, The Uni ted Kingdom, and The United States). The secretaries of the course were: Joseph Danko for the scientific aspects and Nino La Francesca for the administrative aspects of the meeting. Fourty-four lectures divided in eleven series were given. Nine "long" seminars and eight "short" seminars were also presented. In addition, two round-table discussions were held.

Inhalt
Quantum Mechanical Description of Solids.- Abstract.- I. Introduction.- II. Adiabatic Approximation.- III. HartreeFock Approximation.- IV. Electronic Bands in Crystals.- V. Lattice Dynamics as Collective Excitations: Phonons.- VI. Collective Excitations of Electrons: Plasmons.- VII. Extrinsic States.- VIII. Some Effects of Applied Stress.- IX. Conclusions.- References.- to Collective Excitations in Solids.- Abstract.- I. Interactions in a TwoLevel System.- II. Collective Excitations.- III. Interaction of Radiation with Collective Excitations.- IV. Propagation of Radiation in a Dispersive Medium.- V. Examples of Collective Excitations.- References.- Quasi-Particles and Excitons: Models of Structure and Correlation.- Abstract.- I. Introduction.- II. Basic Concepts of Quantum Field Theory.- III. Models.- IV. Conclusions.- References.- Coherent Wavepackets of Phonons.- Abstract.- I. OneDimensional Elastic Line.- II. Beyond the Model of a Chain of Masses and Springs.- III. Motion of a Pulse in the Classical Limit.- IV. Motion of a Pulse in Quantum Mechanics.- V. Birth and Death of a Pulse.- References.- Appendix A: The Coherent States.- Appendix B: The Poisson Distribution.- References.- to Exciton Physics.- Abstract.- I. Introduction.- II. Electronic Structure.- III. Interactions with Phonons.- IV. Interactions with Photons.- V. Kinetics and Dynamics at Low and Intermediate Densities.- Acknowledgements.- References.- Excitons in Semiconductors.- Abstract.- I. Basic Properties of the Free Exciton.- II. Introduction to Bound Excitons.- III. Bound Exciton Satellite Structure.- IV. High Excitation Intensity Effects.- References.- Excitons in Insulators.- Abstract.- I. Introduction.- II. Theoretical and Experimental Background.- III. Excitons in Alkali Halides.- IV. Excitons inRare Gas Solids.- V. Concluding Remarks.- References.- Inelastic Scattering of Fast Particles by Plasmons.- Abstract.- I. Introduction.- II. Bulk and Surface Plasmon Hamiltonian.- III. Charge Particle Spectroscopies of Plasmons.- References.- From Magnons to Solitons.- Abstract.- I. Introduction.- II. Magnons.- III. MagnonMagnon Forces.- IV. TwoMagnon Bound States.- V. ManyBody Scattering Theory.- VI. XY Model: A Model Antiferromagnet.- VII. Heisenberg Antiferromagnet: Ground State.- VIII. Some Consequences.- IX. Effects of Surfaces on Magnons.- X. Magnons vs. Solitons.- XI. Soliton Solutions.- References.- Quasiparticles in Magnetic Metals.- Abstract.- I. Introduction.- II. Low Density Electron Gas.- III. Criterion for Magnetic Ground State.- IV. Quasiparticles.- V. Nearly HalfFilled Band: Nagaoka's Theory.- VI. Two or More Magnetic SubBands.- VII. Magnons in Metals.- VIII. Antiferromagnetism in Metals.- Polaritons.- Abstract.- I. Introduction.- II. Bulk Polariton Linear Response.- III. Experiments on Bulk Polaritons.- IV. Surface Polariton Linear Response.- V. Experiments on Surface Polaritons.- VI. Conclusion.- References 49.- Polarons.- Abstract.- I. Introduction.- II. The LandauPekar StrongCoupling Theory.- III. FieldTheory Formalism, Fröhlich's Hamiltonian and WeakCoupling Theory.- IV. Other Methods for Large (Fröhlich) Polarons.- V. Small Polarons and SelfTrapping.- References.- Long Seminars.- Surface Collective Excitations.- Collective Excitations in Concentrated Mn2+ Systems: Spectral Properties.- Optical Dynamics in Concentrated Mn2+ Systems.- Spectroscopy of Stoichiometric Laser Materials: Excitons or Incoherent Transfers?.- Exciton-Hole Droplets in Semiconductors.- Excitons and Plasmons: Collective Excitations in Semiconductors.- Picosecond ExcitonPhenomena in Chlorophyll Complexes (Abstract only).- Bose-Einstein Statics in Exciton Systems.- Small Polarons in Biological Systems (Abstract only).- Short Seminars (Abstracts).- Perspectives of Free Electron Lasers in Solids.- The Dispersion Curves of Excitonic Polaritons and Their Distortion with Increasing Density.- Motion of a Magnon Soliton About a Phonon Soliton in a Onedimensional Ferromagnet.- Interconfiguration Fluctuations.- Luminescence of Mn2+ IN RbMnxMg1xF3.- Effect of Hydrostatic Pressure on the Luminescence Spectra of the S2 Centre and the EPR of This S2 Centre for the Crystal Scapolite.- On the Calculation of Polaron Wavefunction in the Static Electronlattice Coupling.- Semiconductor Surface Inversion Layers and Their Collective Modes.- Concluding Article.- Present Trends in Collective Excitations in Solids.- Contributors.