1. R. Graham andM. Höhnerbach:Z. Phys. B,57, 233 (1984);K. Zaheer andM. S. Zubairy:Phys. Rev. A,37, 1628 (1988).
2. J. Seke:Quantum Opt.,3, 127 (1991).
3. J. Seke:Nuovo Cimento D,13, 1429 (1991);J. Seke andO. Hittmair:Nuovo Cimento D,14, 101 (1992).
4. J. Seke andF. Rattay:Phys. Rev. A,39, 171 (1989).
5. S. Haroche andJ. M. Raimond: inAdvances in Atomic and Molecular Physics, edited byD. Bates andB. Bederson, Vol.20 (Academic Press, New York, N.Y., 1985), p. 347;D. Meschede, H. Walther andG. Müller:Phys. Rev. Lett.,54, 551 (1985);G. Rempe, H. Walther andN Klein:Phys. Rev. Lett.,58, 353 (1987).