I am most pleased and, in a way, I feel honored to write the Foreword for the book The Hanle Effect and Level-Crossing Spectroscopy, which covers such a very wide range of applications not only in the initial areas of atomic and molecular physics, but also in solid state physics, solar physics, laser physics, and gravitational metrology. To link these fields together in a coherent way has been the merit of the editors of the book, who attracted most distinguished authors for writing the chapters. In retrospect to Hanle's discovery of quantum mechanical coherence between two quantum states about 65 years ago, this book demonstrates the enormous impact and central importance the effect has had, and most vividly still has, on modern physics. On the other hand, the concept of quantum mechanical coherence, which is an outgrowth of the linear super position principle of quantum states, has been evident through a consider able number of experimental methods beyond the original Hanle effect; some of these methods were only recently discovered or applied and they have indeed revolutionized research fields such as atomic collision physics.

Inhalt
1 The Hanle Effect and Level-Crossing SpectroscopyAn Introduction.- 1. Historical Survey.- 2. Classical Interpretation of the Hanle Effect.- 3. Quantum Mechanical Interpretation of the Hanle Effect.- 4. The Density Matrix Formalism for the Hanle Effect (Broad-Band Excitation).- 5. Laser Excitation and Pressure-Induced Coherences.- 6. Nonzero-Field Level Crossing.- 7. Conclusions.- References.- Appendix. Magnetic Effects on the Polarization of Resonance Fluorescence (original work by Wilhelm Hanle, translated by G. Moruzzi).- 2 The Hanle Effect and Atomic Physics.- 1. Introduction.- 2. Spectroscopic Applications.- 3. Collisions.- 4. Hanle Effect in Strong Laser Fields.- References.- 3 The Hanle Effect and Level-Crossing Spectroscopy on Molecules.- 1. Introduction.- 2. Molecular Level-Crossing Signal.- 3. Comparison with Quantum Beat Experiments.- 4. Excitation of Molecules.- 5. Lifetime Investigations.- 6. Landé g-Factors.- 7. Electric-Field Level Crossing.- 8. Stark-Zeeman Recrossing and High-Field Level Crossing.- 9. Hanle Effect on NO2.- 10. Conclusion.- References.- 4 The Nonlinear Hanle Effect and Its Applications to Laser Physics.- 1. The Nonlinear Hanle Effect and Its Experimental Observation.- 2. Saturation Intensity and Saturated Linewidth.- 3. The Three-Level Case: Homogeneously Broadened Lines.- 4. The Three-Level Case: Doppler-Broadened Lines and the Rate Equations.- 5. The General Case.- 6. The Rate-Equation Approach to the Nonlinear Hanle Effect in Inhomogeneously Broadened Transitions.- 7. The Nonlinear Hanle Effect with a Gaussian Laser Beam.- 8. The Nonlinear Hanle Effect in Absorption.- 9. The Nonlinear Hanle Effect in Laser-Active Media.- References.- 5 Applications of the Hanle Effect in Solar Physics.- 1. Introduction.- 2. Brief Review of theProperties of Solar Magnetic Fields.- 3. Overview of the Diagnostic Possibilities and Limitations of the Hanle Effect.- 4. Basic Theoretical Concepts for Applications in Astrophysics.- 5. Diagnostics of Magnetic Fields in Solar Prominences.- 6. Survey of Scattering Polarization on the Solar Disk.- 7. Diagnostics of Turbulent Magnetic Fields.- 8. Diagnostics of Magnetic Fields in the Chromosphere-Corona Transition Region and Above.- 9. Concluding Remarks.- References.- 6 Applications of the Hanle Effect in Solid State Physics.- 1. Introduction.- 2. The Hanle Effect on Free Electrons.- 3. The Hanle Effect on Excitons.- 4. The Hanle Effect on Impurity Centers.- 5. Summary.- References.- 7 Quantum Theory of the Hanle Laser and Its Use as a Metric Gravity Probe.- 1. Introduction.- 2. The Model.- 3. Linear Theory.- 4. Nonlinear Theory.- 5. Vanishing of Diffusion Constant for the Relative Phase.- 6. Applications as a Metric Gravity Probe.- 7. Discussion and Summary.- References.