Keitel, Christoph H. Division Prof. Dr. Christoph H. Keitel, MPI for Nuclear Physics, Max Planck Society;
http://link.aps.org/doi/10.1103/PhysRevA.55.615 (Publisher version)
Gauthey, F. I., Keitel, C. H., Knight, P. L., & Maquet, A. (1997). Phase of harmonics from strongly driven two-level atoms. Physical Review. A, 55(1), 615-621. doi:10.1103/PhysRevA.55.615.