H. J. BEYER AND H. KLEINPOPPEN We are pleased to present Part D of Progress in Atomic Spectroscopy to the scientific community active in this field of research. When we invited authors to contribute articles to Part C to be dedicated to Wilhelm Hanle, we received a sufficiently enthusiastic response that we could embark on two further volumes and thus approach the initial goal (set when Parts A and B were in the planning stage) of an almost comprehensive survey of the current state of atomic spectroscopy. As mentioned in the introduction to Parts A and B, new experimental methods have enriched and advanced the field of atomic spectroscopy to such a degree that it serves not only as a source of atomic structure data but also as a test ground for fundamental atomic theories based upon the framework of quantum mechanics and quantum electrodynamics. However, modern laser and photon correlation techniques have also been applied successfully to probe beyond the "traditional" quantum mechanical and quantum electrodynamical theories into nuclear structure theories, electro­ weak theories, and the growing field of local realistic theories versus quan­ tum theories. It is obvious from the contents of this volume and by no means surprising that applications of laser radiation again played a decisive role in the development of new and high-precision spectroscopic techniques.

of Part D.- 1 Laser-Microwave Spectroscopy.- 1. Introduction.- 2. Classification of Laser-Microwave Spectroscopy.- 3. Measurements Based on Optical Pumping.- 4. Measurements Based on Double Resonance.- 5. Measurements Based on Nonlinear Phenomena.- 6. Other Schemes.- 7. Laser-Microwave Heterodyne Techniques for Spectroscopic Purposes.- 8. Concluding Remarks.- References.- 2 Collinear Fast-Beam Laser Spectroscopy.- 1. Introduction.- 2. Basic Concept and Experimental Realization.- 3. Experiments Based on the Doppler Effect.- 4. Spectroscopic Studies.- 5. Spectroscopy on Unstable Isotopes.- 6. Conclusion.- References.- 3 Radiofrequency Spectroscopy of Rydberg Atoms.- 1. Introduction.- 2. Rydberg Atoms.- 3. Core Polarization and Penetration.- 4. Experimental Techniques.- 5. Overview of the Results Obtained.- References.- 4 Rydberg Series of Two-Electron Systems Studied by Hyperfine Interactions.- 1. Introduction.- 2. Experimental Techniques.- 3. Even-Parity Rydberg Series of Alkaline-Earth Elements.- 4. Odd-Parity Rydberg Series of Alkaline-Earth Elements.- 5. Hyperfine Structure and Isotope Shifts of Rydberg States of Other Two-Electron Systems.- 6. Conclusion.- References.- 5 Parity Nonconservation in Atoms.- 1. Introduction.- 2. Theory.- 3. Circular Dichroism and Optical Rotation-Rigorous Discussion.- 4. Optical Rotation Experiments.- 5. Stark-PNC Experiments; Cesium and Thallium.- 6. Discussion of Results; Conclusions.- References.- 6 Energy Structure of Highly Ionized Atoms.- 1. Introduction.- 2. General Energy Relations in Isoelectronic Ions.- 3. Survey of the Low Configurations in Isoelectronic Sequences.- 4. The n = 2 Configurations.- 5. The Neon Sequence (N = 10).- 6. Ions with Ground Configurations of 3s and 3p Electrons.- 7. The Configurations 3dk.- 8. TheCopper Sequence (N = 29).- 9. The Silver Sequence (N = 47).- References.- 7 Inner-Shell Spectroscopy with Hard Synchrotron Radiation.- 1. Introduction.- 2. Instrumental Details.- 3. X-Ray Absorption by Free Atoms.- 4. X-Ray Absorption by Bound Atoms.- 5. Induced X-Ray Fluorescence and Auger-Electron Emission.- 6. Scattering of X-Rays.- References.- 8 Analysis and Spectroscopy of Collisionally Induced Autoionization Processes.- 1. Introduction.- 2. Description of Autoionizing States.- 3. Experimental Methods.- 4. Line Shapes and Interference Effects in Autoionization Spectroscopy.- 5. Spectroscopic Data for Various Atoms.- 6. Correlated and Uncorrelated Angular Distributions of Autoionization Electrons.- 7. Electron Emission from Quasimolecules.- References.- 9 Near Resonant Vacancy Exchange between Inner Shells of Colliding Heavy Particles.- 1. Introduction.- 2. Theoretical Methods.- 3. Two-State Systems.- 4. Multistate Systems.- 5. Conclusions.- References.- 10 Polarization Correlation in the Two-Photon Decay of Atoms.- 1. Introduction.- 2. Theoretical Considerations.- 3. Experimental Work.- 4. Discussion.- References.