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.

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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.- I.A. QuantumMechanical Resonance.- I.B. Static Effects of Perturbation.- II. Collective Excitations.- II.A. Setting of the Problem.- II.B. Eigenfunctions.- II.C. Dispersion Relations.- II.D. Effective Mass.- II.E. Generalization to Three Dimension.- II.F. Periodic Boundary Conditions and Density of States.- III. Interaction of Radiation with Collective Excitations.- III.A. The Radiation Field.- 1. The Classical Radiation Field.- 2. The Quantum Radiation Field.- III.B. The Form of the Interaction.- III.C. Absorption and Emission Processes.- III.D. Interaction of Photons with Collective Excitations.- IV. Propagation of Radiation in a Dispersive Medium.- IV.A. Introduction.- IV.B. Dielectric Constant.- IV.C. Propagation of Electromagnetic Waves in a Dispersive Medium.- V. Examples of Collective Excitations.- V.A. Phonons.- 1. Summary of Properties.- 2. Infrared Absorption by Ionic Solids.- V.B. Excitons.- 1. General Theory.- 2. The Frenkel Exciton.- 3. The Wannier Exciton.- 4. The Intermediate Case.- 5. The PhotonExciton System.- 6. The PhotonExcitonPhonon System.- 7. Indirect Transitions.- V.C. Magnons.- 1. Setting of the Problem.- 2. Hamiltonian and Eigenstates.- 3. Semiclassical Treatment.- 4. Thermodynamics of Magnons.- V.C. Plasmons.- 1. Dielectric Response of an Ensemble of Oscillators.- 2. Dielectric Response of a Free Electron Gas.- 3. Transversal Optical Modes in a Plasma.- 4. Longitudinal Optical Modes in a Plasma.- References.- Quasi-Particles and Excitons: Models of Structure and Correlation.- Abstract.- I. Introduction.- II. Basic Concepts of Quantum Field Theory.- II.A. Quantization of Classical Fields.- II.B. Schrödinger Field.- II.C. Occupation Number Representation.- II.D. SelfInteraction.- II.E. Sources.- II.F. Effective Vacuum.- III. Models.- III.A. Empty Band Model.- III.B. Fermi Gas Model.- III.C. Tight Binding Model (TBM).- III.D. Models with Particle Interactions.- III.E. Random Cell Model (RCM).- III.F. Driven Systems.- IV. Conclusions.- References.- Coherent Wavepackets of Phonons.- Abstract.- I. OneDimensional Elastic Line.- I.A. Monatomic Array of Rigid Atoms.- I.B. Forces.- I.C. Energy 15.- 1. Potential Energy.- 2. Kinetic Energy.- 3. Total Energy.- I.D. Continuous Vibrating Line.- II. Beyond the Model of a Chain of Masses and Springs.- III. Motion of a Pulse in the Classical Limit.- III.A. General Features.- III.B. Pulse Shape and Dispersion.- IV. Motion of a Pulse in Quantum Mechanics.- IV.A. Variables.- IV.B. Phonons and Phonon Wavepackets.- IV.C. Propagation of a Pulse: Moment and Energy.- 1. Moment.- 2. Energy.- V. Birth and Death of a Pulse.- References.- Appendix A: The Coherent States.- 1. Definition of the Coherent State |?>.- 2. Some Properties.- 3. Significant Averages.- 4. Time Evolution.- 5. Coordinate Representation.- 6. Why Coherent.- 7. Why Minimum Uncertainty.- 8. Why Semiclassical.- 9. Why Displaced.- 10. Multimode Coherent State.- Appendix B: The Poisson Distribution.- 1. Poisson Distribution.- 2. Perfect Gas in a Container.- 3. Harmonic Oscillator in a Coherent State.- References.- to Exciton Physics.- Abstract.- I. Introduction.- II. Electronic Structure.- II.A. Excitons in a Static Crystal.- 1. Atomic and Molecular Approach.- 2. Effective Mass Approach.- 3. Unified Approach.- 4. Excitons in Disordered Materials.- 5. Effects of External Fields.- 6. Excitons as Quasiparticles.- 7. Manybody Considerations.- II.B. Excitons in Real Materials.- 1. Observation.- 2. Spectral Characteristics.- 3. Surface Excitons.- 4. Trapped and Localized Excitons.- III. Interactions with Phonons.- III.A. Formalism.- 1. Phonons.- 2. Linear ExcitonPhonon Coupling.- 3. Clothing of Excitons.- 4. Effects of HigherOrder Coupling.- III.B. Applications.- 1. Coupling Strengths.- 2. Scattering.- 3. SelfTrapping.- IV. Interactions with Photons.- IV.A. Semiclassical Radiation Theory.- 1. Transition Rates.- 2. Direct Transitions.- 3. Indirect Transitions.- 4. Giant TwoPhoton Effects.- IV.B. The Polariton.- 1. Background.- 2. Polariton Picture and Optical Properties.- 3. Direct Observation of Polariton Dispersion.- 4. Surface Polaritons.- IV.C. Special Topics.- 1. Davydov Splitting.- 2. Excitons in Mixed Crystals.- 3. Line Shapes and Widths.- 4. Urbach's Rule.- V. Kinetics and Dynamics at Low and Intermediate Densities.- V.A. Introduction.- V.B. Exciton Diffusion.- 1. Historical.- 2. Förster's Theories.- 3. TimeResolved Studies of Singlets Assumed to Diffuse.- 4. Triplet Exciton Diffusion.- V.C. Coherence.- 1. General Remarks.- 2. Coherence as Described by a Memory Function.- 3. Observation of Coherence.- V.D. Phenomena at Intermediate Densities.- 1. General Remarks.- 2. Annihilation.- 3. Biexciton Formation.- Acknowledgements.- References.- Excitons in Semiconductors.- Abstract.- I. Basic Properties of the Free Exciton.- I.A. Concept of the Free Exciton.- I.B. Energy States of the Free Exciton.- I.C. Frenkel and WannierMott Free Excitons.- I.D. Direct and Indirect Gap Semiconductors.- I.E. Phonon Assisted Free Exciton Absorption in Direct Transition.- I.F. Higher Energy Free Exciton States.- I.G. Zeeman Splittings and Diamagnetic Shifts.- II. Introduction to Bound Excitons.- II.A. History of Bound Excitons.- II.B. Typical Near Gap Bound Exciton Luminescence.- II.C. Trends of EBXHaynes' Rule.- II.D. Additional Aspects of jj Coupling and Other ZeroField Bound Exciton States.- II.E. MagnetoOptical Effects.- II.F. Auger Recombinations.- II.G. Giant Oscillator Strength.- III. Bound Exciton Satellite Structure.- III.A. Phonon Replicas.- III.B. Electronic Satellites.- III.C. The Use of Luminescence Excitation Spectra.- IV. High Excitation Intensity Effects.- IV.A. Stimulated Emission.- IV.B. Excitonic Molecules and BoseEinstein Condensation.- IV.C. Multiple Bound Excitons.- References.- Excitons in Insulators.- Abstract.- I. Introduction.- II. Theoretical and Experimental Background.- III. Excitons in Alkali Halides.- III.A. Absorption and Reflectivity Measurements.- III.B. Intrinsic Luminescence Measurements.- IV. Excitons in Rare Gas Solids.- V. Concluding Remarks.- References.- Inelastic Scattering of Fast Particles by Plasmons.- Abstract.- I. Introduction.- II. Bulk and Surface Plasmon Hamiltonian.- II.A. Classical Concepts.- 1. Bulk Modes.- 2. Planar Interface Modes.- 3. Thin Film Modes.- 4. Sphere and Void Modes.- II.B. Model Hamiltonians.- 1. Bulk Plasmon…