Keitel, Christoph H. Division Prof. Dr. Christoph H. Keitel, MPI for Nuclear Physics, Max Planck Society;
http://www.sciencedirect.com/science/article/pii/0030401891900134 (Publisher version)
Narducci, L. M., Scully, M. O., Keitel, C. H., Zhu, S. Y., & Doss, H. M. (1991). Physical origin of the gain in a four-level model of a Raman driven laser without inversion. Optics Communications, 86(3-4), 324-332. doi:10.1016/0030-4018(91)90013-4.