Keitel, Christoph H. Division Prof. Dr. Christoph H. Keitel, MPI for Nuclear Physics, Max Planck Society;
http://iopscience.iop.org/0295-5075/24/7/006 (Publisher version)
Keitel, C. H., King, P. L., & Burnett, K. (1993). Relativistic High-Harmonic Generation. Europhysics Letters, 24(7), 539-544. doi:10.1209/0295-5075/24/7/006.