1. P. A. M. Dirac: The Principles of Quantum Mechanics, 1st edition (1930), 2nd edition (1958) (Clarendon Press, Oxford)
2. M. Goeppert-Mayer: Ann. Physik 9, 273 (1931)
3. W. Kaiser, C. G. B. Garrett: Phys. Rev. Letters 7, 229 (1961)
4. See, for example: Topics in Applied Physics, Vol. 1: Dye Lasers, ed. by F. P. Schäfer (Springer Berlin, Heidelberg; New York 1973)
5. L. S. Vasilenko, V. P. Chebotayev, A. V. Shishaev: JETP Letters 12, 113 (1970)