1. H. Backe, L. Handschug, F. Hessberger, E. Kankeleit, L. Richter, F. Weik, R. Willwater, H. Bokemeyer, P. Vincent, Y. Nakayama, and J.S. Greenberg, Phys.Rev.Lett. 40 (1978) 1443
2. C. Kozhuharov, P. Kienle, E. Berdermann, H. Bokemeyer, J.S. Greenberg, P. Vincent, H. Backe, L. Handschug, and E. Kankeleit, Phys.Rev.Lett. 42(1979)376
3. J. Reinhardt, B. Müller, and W. Greiner, Phys.Rev. A24(1981)103
4. Quantum Electrodynamics of Strong Fields, edited by W. Greiner, Plenum, New York (1983)
5. T. Tomoda, and H.A. Weidenmüller, Phys.Rev. A26(1982)103