Keitel, Christoph H. Division Prof. Dr. Christoph H. Keitel, MPI for Nuclear Physics, Max Planck Society;
http://www.maik.ru/full/lasphys/99/1/lasphys1_99p133full.pdf (Publisher version)
Szymanowski, C., Keitel, C. H., & Maquet, A. (1999). Influence of Zitterbewegung on Relativistic Harmonic Generation. Laser Physics, 9(1), 133-137.