1. Introduction.- 1.1 Physical Properties and Features of Rydberg Atoms and Ions.- 1.2 Scope of the Book.- 2. Classical and Quantum Description of Rydberg Atom.- 2.1 Classical Motion in a Coulomb Field.- 2.1.1 Orbital Electron Motion.- 2.1.2 Action Variables.- 2.2 Wave Functions: Coordinate Representation.- 2.2.1 Quantum Wave Function of Hydrogen-like States.- 2.2.2 JWKB Approximation.- 2.2.3 Semiclassical Approach in Action Variables.- 2.3 Wave Functions: Momentum Representation.- 2.3.1 Hydrogenlike Wave Functions.- 2.3.2 Momentum Wave Functions with Quantum Defect.- 2.4 Density Matrix and Distribution Function.- 2.4.1 Classical Distribution Functions.- 2.4.2 Coulomb Green's Function.- 2.4.3 Density Matrix.- 2.4.4 Wigner Function.- 3. Radiative Transitions and Form Factors.- 3.1 Probabilities of Radiative Transitions.- 3.1.1 General Formulas.- 3.1.2 Semiclassical and Asymptotic Approaches.- 3.1.3 Summed over Angular Quantum Numbers Line Strength. Kramers Approximation.- 3.2 Photoionization and Photorecombination.- 3.2.1 General Formulas.- 3.2.2 Asymptotic Approach.- 3.2.3 Kramers Formulas and Gaunt Factor.- 3.3 Transition Form Factors.- 3.3.1 General Formulas.- 3.3.2 n-n? Transitions: Quantum Expressions.- 3.3.3 n-n? Transitions: Asymptotic Expressions.- 3.3.4 nl - nl? Transitions: Semiclassical Expressions.- 3.3.5 Classical Approach.- 3.3.6 Angular Factors for Complex Atoms.- 4. Basic Approaches to Collisions Involving Highly Excited Atoms and Ions.- 4.1 Formulation of Problem.- 4.1.1 Features of Collisions with Neutral and Charged Particles.- 4.1.2 Stationary Problem of Scattering.- 4.2 Born Approximation: Momentum Representation.- 4.3 Time-Dependent Approach: Impact-Parameter Representation.- 4.3.1 Close Coupled Equations for Transition Amplitudes.- 4.3.2 Normalized Perturbation Theory.- 4.3.3 Connection with Momentum Transfer Representation.- 4.4 Semiclassical Approach in Action Variables.- 4.4.1 Classical Perturbation Theory.- 4.4.2 Relation between Classical and Quantum Values.- 4.4.3 Model of Equidistant Levels and Correspondence Principle for S-Matrix.- 4.5 Impulse Approximation Approach.- 4.5.1 Quantum Impulse Approximation.- 4.5.2 Binary Encounter Approach.- 5. Collisions of Rydberg Atom with Neutral Particles: Weak-Coupling Models.- 5.1 Quasi-free Electron Model.- 5.2 Scattering of Ultra-Slow Electrons by Atoms and Molecules.- 5.2.1 Electron-Atom Scattering.- 5.2.2 Electron-Molecule Scattering.- 5.3 Semiclassical Theory: Impact-Parameter Approach with Fermi Pseudopotential.- 5.3.1 Historical Sketch.- 5.3.2 Probabilities of the nl J ? n?l? J? and nl ? n?l? Transitions.- 5.3.3 Binary-Encounter Theory: nl ? n? and n ? n? Transitions.- 5.4 Impulse Approximation for Rydberg Atom-Neutral Collisions.- 5.4.1 Introductory Remarks.- 5.4.2 Fast and Slow Collisions.- 5.4.3 Cross Sections of Slow Collisions: General Expressions.- 5.4.4 Expressions Through the Form Factors and Scattering Length Approximation.- 5.4.5 Total Scattering Cross Section.- 5.4.6 Resonance on Quasi-discrete Level.- 5.4.7 Validity Criteria of Quasi-free Electron Model and Impulse Approximation.- 6. Elementary Processes Involving Rydberg Atoms and Neutral Particles: Effects of Electron-Projectile Interaction.- 6.1 Classification of Processes and Theoretical Treatments.- 6.2 Transitions between the Fine-Structure Components and Elastic Scattering.- 6.2.1 Weak Coupling Limit.- 6.2.2 Extension to Strong-Coupling Region.- 6.3 Orbital Angular Momentum and Energy Transfer: l-Mixing and n, l-Changing Processes.- 6.3.1 Semiclassical Unitarized Approach to Inelastic nl ? n? Transitions.- 6.3.2 Quasi-elastic Limit: l-Mixing Process.- 6.3.3 Effective Scattering Length.- 6.3.4 Scaling Laws.- 6.4 Ionization of Rydberg Atom by Atomic Projectile.- 6.5 Quenching of Rydberg States: Thermal Collisions with Atoms.- 6.5.1 Collisions with Rare Gas Atoms.- 6.5.2 Collisions with Alkali-Metal Atoms.- 6.6 Quenching and Ionization of Rydberg States: Thermal Collisions with Molecules.- 6.6.1 Quasi-resonant Energy Exchange of Rydberg Electron with Rotational Motion of Molecule.- 6.6.2 Ion-Pair Formation and Charge Transfer: Collision of Rydberg Atom with Electron-Attaching Molecule.- 7. Effects of Ion Core in Rydberg Atom-Neutral Collisions.- 7.1 Mechanisms of Perturber-Core Scattering.- 7.2 Separated-Atoms Approach: Shake-Up Model.- 7.2.1 General Treatment.- 7.2.2 Dipole Approximation.- 7.3 Quasi-molecular Approach: Basic Assumptions.- 7.4 Exchange of Rydberg Electron Energy with Translational Motion of Atoms.- 7.4.1 Matrix Elements of Transitions within One Electronic Term of Quasi-molecular Ion.- 7.4.2 Cross Sections of De-excitation and Ionization.- 7.4.3 Collisions of Highly Excited Hydrogen with Helium.- 7.5 Resonant Excitation and Ionization.- 7.5.1 Exchange of Electron Energy in Quasi-molecule.- 7.5.2 Dipole Transitions Between Symmetrical and Antisymmetrical Terms.- 7.5.3 Quadrupole Transitions in Nonsymmetrical Collisions of Rare Gas Atoms.- 8. Inelastic Transitions Induced by Collisions of Rydberg Atom (Ion) with Charged Particles.- 8.1 Basic Problems.- 8.2 n-Changing Transitions.- 8.2.1 Classical Approach.- 8.2.2 Born Approximation.- 8.2.3 Semiclassical Approach.- 8.2.4 Coulomb-Born Approximation.- 8.3 l-Changing Transitions.- 8.3.1 Born Approximation.- 8.3.2 Close Coupling Method.- 8.3.3 Method of Effective Magnetic Field.- 8.3.4 Fitting Formulas.- 8.3.5 Comparison with Experiment.- 9. Spectral-Line Broadening and Shift.- 9.1 Classical and Quantum Treatments of Impact Broadening.- 9.1.1 Impact-Parameter Method.- 9.1.2 Quantum Formulas.- 9.2 Theory of Width and Shift of Rydberg Levels in Gas.- 9.2.1 Mechanism of Core-Perturber Scattering.- 9.2.2 Mechanism of Electron-Perturber Scattering.- 9.3 Comparison of Theory with Experiment.- 9.3.1 Broadening and Shift in Rare Gases.- 9.3.2 Broadening and Shift in Alkali-Metal Vapors.- 9.4 Broadening of n - n? Lines in a Plasma.- List of Symbols.- References.

This monograph is devoted to the basic aspects of the physics of highly ex­ cited (Rydberg) states of atom's. After almost twenty years, this remains a hot topic of modern atomic physics. Such studies are important for many areas of physics and its applications including spectroscopy, astrophysics and radio astronomy, physics of electronic and atomic collisions, kinetics and di­ agnostics of gases, and low- and high-temperature plasmas. Physical phenom­ ena in radiative, collisional, and spectral-line broadening processes involving Rydberg atoms and ions are primarily determined by the peculiar properties and exotic features of highly excited states. The growth of interest and research activity in the physics of Rydberg the last two decades was stimulated by an extremely rapid de­ atoms over velopment of high-resolution laser spectroscopy, methods of selective excita­ tion and detection of highly excited states, atomic-beam techniques as well as radio astronomy. This has facilitated significant progress in the differ­ ent directions of the physics of highly excited atoms being of fundamental and practical importance. In particular, evident advances were achieved in studies of the structure and spectra of highly excited atoms, their behavior in static electric and magnetic fields, interactions with electromagnetic ra­ diation, spectral-line broadening and the shift of Rydberg series, collisions with electrons, ions, atoms, and molecules, etc. The principle objective of the present book is to reflect the most important physical approaches and efficient theoretical techniques in the modem physics of highly excited atoms and ions.