1. See e.g. Levenson, M. D., and Song, J. J. in Coherent Nonlinear Optics, ed. Feld, M. S. and Letokhov, V. S. (Springer: Berlin) 1980.
2. See e.g. Demtröder, W. D. in Laser Spectroscopy, (Springer: Berlin) 1981.
3. Placzek, G., Handbuch der Radiologie, ed. Marx. E., (Akademische Verlagsgesellochaft: Leipsig) 1934.
4. Loudon, R., in The Quantum Theory of Light (Clarendon Press, Oxford) 1983.
5. Zinth, W., Lauberau, A. and Kaiser, W., Opt. Commun. 49, 935 (1978).