H. KLEINPOPPEN AND J. F. WILLIAMS It has only very recently become possible to study angular correlations and coherence effects in different areas of atomic collision processes: These investigations have provided us with an analysis of experimental data in terms of scattering amplitudes and their phases, of target parameters such as orientation, alignment, and state multipoles, and also of coherence parameters (e. g. , the degree of coherence of excita- tion). In this way the analysis of electron-photon, ion-photon, atom-photon, or electron-ion coincidences from electron-atom, ion-atom, or atom-atom collisional excitation has led to a breakthrough such that the above quantities represent most crucial and sensitive tests for theories of atomic collision processes. Similarly, the powerful (e, 2e) experiments (electron-electron coincidences from impact ionization of atoms) have attracted much attention where improved experimental studies and detailed theoretical description provide a wealth of information on either the col- lisional ionization process or the atomic structure of the target atom. Interference effects, many-electron correlations, and energy and angular momen- tum exchange between electrons in a Coulomb field playa decisive role in the under- standing of postcollision interactions. New results on coherence effects and orienta- tion and alignment in collisional processes of ions with surfaces and crystal lattices show links to relevant interference phenomena in atomic collisions. In small-angle elastic electron-atom scattering the effect of angular coherence can be studied in a crossed beam experiment.

Table of Contents:
1. Theory of (e, 2e) Experiments.- 2. Impulsive (e, 2e) Experiments: A Tool to Test Different Ionization Theories and Electronic Structure of Atoms and Molecules.- 3. Low-Energy Electron Impact Ionization with Completely Determined Kinematics.- 4. Coincidence Investigations of Electron-Hydrogen Collisions.- 5. Correlation in Multiple Escape near Threshold.- 6. Interference Effects in the Multiphoton Ionization of Atoms.- 7. Correlation in Electron-Atom Excitation.- 8. Angular Correlation Parameters for Excitation of the 21P State of Helium and the 2p State of Atomic Hydrogen.- 9. New Aspects in Electron-Photon Coincidence Experiments.- 10. The Possibility of a Complete Determination of Scattering Amplitudes for S ? D Excitation by Electron-Photon Coincidence Measurements.- 11. Threshold and Pseudothreshold Excitation of Molecules by Electron Impact.- 12. Polarization Correlation Measurements in Ar and H2.- 13. Effect of Optical Potentials on Angular Correlation Parameters.- 14. The Calculation of Orientation and Alignment Parameters for the Electron Impact Excitation of the 31P State of He in the First-Order Many-Body Theory.- 15. Electron-Photon Correlations in Bremsstrahlung Processes.- 16. Angular Distribution and Polarization of x-Ray Radiation by Electron Impact on Free Atoms.- 17. Energy Dependence of the Alignment in Inner-Shell Ionization by Electron Impact.- 18. Identification of States Excited by Collision: Current State of Theory.- 19. Angular Correlation between Autoionization Electron and Scattered Ion in Slow He+-He Collision.- 20. Coherence Effects in Postcollision Interactions in Electron Impact Experiments.- 21. Collision-Stimulated Autoionization.- 22. Electronic Correlation in Electron-Atom Scattering.- 23. Observation of the Postcollision Interaction in the Scattered-Electron Spectra of Helium.- 24. Excitation of Autoionizing States in Noble Gases by Fast Electrons.- 25. The Postcollision Interaction as a Manifestation of Many-Electron Correlations.- 26. Coincidence Studies of Inner-Shell Excitation in Ion-Atom Collisions.- 27. Coherent Production of Positrons in Heavy-Ion Collisions.- 28. Alignment of Core-Excited Autoionizing Be+ Projectiles Excited in Single Gas Collisions.- 29. Interference Effects in K-Shell Vacancy Sharing.- 30. Coherent Excitation of Ionic States by Correlated Collisions in a Crystal Lattice.- 31. Alignment, Orientation, and the Beam-Foil Interaction.- 32. Polarized-Photon, Scattered-Ion Coincidence Study of Mg+ Collisions with He, Ne, and Ar.- 33. Coherence Effects in Heavy-Ion-Atom Collisions.- 34. Excitation of Cd(53P1) and Cd(51P1) in Na+-Cd Collisions: Optical Polarization and Population of Magnetic Sublevels.- 35. Spin Polarization in Ion-Atom Collisions Studied with Ion Storage Techniques.- 36. Scattering Experiments with Laser-Excited Atoms.- 37. Orbital Angular Momentum State Coherence in Charge Transfer to the n = 2 Hydrogen Levels by Fast Protons in Gases.- 38. Photon-Scattered Atom Coincidence Study with Vector Polarization Analysis for K-Noble-Gas Collisions.- 39. Alignment and Orientation Measurements in He+-He and He+-H2 Collisions.- 40. Simultaneous Electron-Photon Excitation of Atoms.- 41. Energy and Angular Momentum Exchanges between Two Electrons in a Coulomb Field.- 42. Oscillatory Structure of Excitation Functions in Atomic Collisions.- 43. Quantum Mechanical Phase Coherence and Polarization in Low-Energy Ion-Atom and Ion-Surface Collisions.- 44. Polarized Electron-Lithium Scattering.- 45. Polarization Effects Caused by Interference in Electron Scattering and Photoionization.- 46. Spin Polarization of Electrons by Resonance Scattering from Mercury.- 47. Spin-Dependent Inelastic Electron-Atom Scattering.- 48. Spin-Polarized Photoelectrons from Unpolarized Lead Atoms Exposed to Unpolarized Radiation.- 49. Total Cross Sections for the Scattering of Low-Energy Electrons by Excited Sodium Atoms.- 50. Orientation and Alignment in Scattering Processes from Laser-Excited Atoms.- 51. Photon Polarization Dependence of Superelastic Electron Scattering by Laser-Excited Barium.- 52. The Effect of the Angular Coherence of Each Beam, in a Crossed Beam Experiment, on Observations of the Small-Angle Elastic Scattering.- 53. Polarized-Beams Studies of Spin Exchange in Electron-Hydrogen Collisions.- 54. Ionization of Polarized Alkali Atoms by Polarized Electrons.- 55. Total Cross Section of Interference between Direct and Exchange Interaction in Polarized Electron Impact Excitation of Polarized Atoms.