Table of contents

Introduction 641 I. Atomic and Molecular Frequency and Time Standards 642 1. Molecular and Atomic Generators 642 2. Cesium Frequency Standards 647 3. Other Frequency Standards 649 4. Frequency Stabilization 651 II. Use of Highly Stable Frequency and Time Standards to Check General and Special Relativity 653 1. Possible Experiments to Check General Relativity 653 2. First-Order Experiments to Check Special Relativity 666 III. Use of Atomic and Molecular Frequency Standards for the Investigation of Cosmological Effects 669 Conclusion 670 Appendix. Derivation of Formula for Relativistic Red Shift of the Frequency of a Spectral Line 670 References Cited 671

Last March marked the seventieth birthday of the eminent physicist and leader of Soviet science, the great public figure, Academician Sergei Ivanovich Vavilov, who passed away ten years ago. The scientific community took note of this date by holding a series of scientific conferences. We print in this issue several papers read at the joint meeting of the Presidium of the U.S.S.R. Academy of Sciences and the scientific community on March 24. Some of the reports read at conferences held at the P. N. Lebedev Physics Institute and the S. I. Vavilov Optics Institute in connection with this occasion, are also here.

The interaction of electromagnetic radiation and charged particles with crystals may involve ionization, i.e., the production of excess current carriers. Experiments with single crystals of silicon have confirmed theoretical predictions on the effect of an externally applied electric field on the process of photoionization. A study of photoionization in the inner portions of the fundamental optical absorption bands of germanium and silicon has shown that at sufficiently high photon energies the quantum yield rises to values considerably greater than unity. For photons of energies many times greater than the width of the forbidden band, the quantum yield is proportional to the energy of the photon. In ionization due to fast charged particles, the energy lost per electron-hole pair produced is independent of the particle energy.

Introduction 781 III. Cerenkov Radiation in the Presence of Boundaries 782 1. Boundary Conditions 782 2. Radiation Produced by a Charge Moving Along the Axis of a Cylindrical Dielectric- Filled Channel 783 3. Motion of a Point Charge Parallel to the Axis of a Channel in a Dielectric 788 4. Radiation Produced by a Dipole Moving Along the Axis of a Cylindrical Channel 789 5. Cerenkov Radiation in Linear Periodic Structures 791 a) General Theory 791 b) Radiation of a Charge in an Iris-Loaded Waveguide 792 6. Cerenkov Radiation in Waveguides 793 a) Waveguide Filled with an Isotropic Dielectric 794 b) Waveguide Partially Filled with an Isotropic Dielectric 795 c) Waveguide Filled with an Anisotropic Dielectric 796 7. Field Produced by a Charged Particle Moving Parallel to the Boundary Between Two Media 799 Literature References 804