Keitel, Christoph H. Division Prof. Dr. Christoph H. Keitel, MPI for Nuclear Physics, Max Planck Society;
http://link.aps.org/doi/10.1103/PhysRevA.64.045402 (Publisher version)
Prager, J., Hu, S., & Keitel, C. H. (2001). High-order regime of harmonic generation with two active electrons. Physical Review. A, 64(4): 045402. doi:10.1103/PhysRevA.64.045402.