Keitel, Christoph H. Division Prof. Dr. Christoph H. Keitel, MPI for Nuclear Physics, Max Planck Society;
http://link.aps.org/doi/10.1103/PhysRevA.54.729 (Publisher version)
Watson, J. B., Keitel, C. H., Knight, P. L., & Burnett, K. (1996). Entropic measure of wave-packet spreading and ionization in laser-driven atoms. Physical Review. A, 54(1), 729-735. doi:10.1103/PhysRevA.54.729.