1. A preliminary version of the present work has been reported by F. De Martini, “An X-ray relativistic free-electron frequency unconverter,1 1 Proc. of the Workshop on Free-Electron Generators of Coherent Radiation, Telluride, Colorado, August 13–17, 1979, published in Physics of Quantum Electronics, Vol 7, Addison Wesley Pub. Co., Inc., Reading, Mass. 1979.
2. N. A. Vinokurov and A. N. Skrinsky, preprints INP 77.59 and 77.67 Novosibirsk 1977 and Proc. of the National Conf. on Charged Particle Accelerators, Dubna, 1978, Vol 2, p. 233.
3. R. Coisson, review paper on optical Klystron. To be published.
4. D. A. G. Deacon, R. L. Elias, J. M. J. Madey, J. J. Ramian, H. A. Schwettmann and T. I. Smith, Phys. Rev. Lett. 38, 892 (1972).
5. C. Shif and A. Yariv in Physics of Quantum Electronics, Vol. 7, (cfr. Ref. 1).