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.

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.- 2.2 Wave Functions: Coordinate Representation.- 2.3 Wave Functions: Momentum Representation.- 2.4 Density Matrix and Distribution Function.- 3. Radiative Transitions and Form Factors.- 3.1 Probabilities of Radiative Transitions.- 3.2 Photoionization and Photorecombination.- 3.3 Transition Form Factors.- 4. Basic Approaches to Collisions Involving Highly Excited Atoms and Ions.- 4.1 Formulation of Problem.- 4.2 Born Approximation: Momentum Representation.- 4.3 Time-Dependent Approach: Impact-Parameter Representation.- 4.4 Semiclassical Approach in Action Variables.- 4.5 Impulse Approximation 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.- 5.3 Semiclassical Theory: Impact-Parameter Approach with Fermi Pseudopotential.- 5.4 Impulse Approximation for Rydberg Atom-Neutral Collisions.- 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.- 6.3 Orbital Angular Momentum and Energy Transfer: l-Mixing and n, l-Changing Processes.- 6.4 Ionization of Rydberg Atom by Atomic Projectile.- 6.5 Quenching of Rydberg States: Thermal Collisions with Atoms.- 6.6 Quenching and Ionization of Rydberg States: Thermal Collisions with Molecules.- 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.- 7.3 Quasi-molecular Approach: Basic Assumptions.- 7.4 Exchange of Rydberg Electron Energy with Translational Motion of Atoms.- 7.5 Resonant Excitation and Ionization.- 8. Inelastic Transitions Induced by Collisions of Rydberg Atom (Ion) with Charged Particles.- 8.1 Basic Problems.- 8.2 n-Changing Transitions.- 8.3 l-Changing Transitions.- 9. Spectral-Line Broadening and Shift.- 9.1 Classical and Quantum Treatments of Impact Broadening.- 9.2 Theory of Width and Shift of Rydberg Levels in Gas.- 9.3 Comparison of Theory with Experiment.- 9.4 Broadening of n - n? Lines in a Plasma.- List of Symbols.- References.