1. N. V. Karlov and A. M. Prokhorov, Usp. Fiz. Nauk,118, No. 4, 583?609 (1976).
2. V. S. Letokhov and S. B. Mur, Kvantovaya Elektron. (Moscow),3, 248?285 (1976).
3. R. V. Ambartsumyan, V. R. Kalinin, and V. S. Letokhov, Pis'ma Zh. Eksp. Teor. Fiz.,13, 305?307 (1971).
4. S. A. Tuccio, J. W. Dubrin, O. G. Peterson, and B. B. Swaleby, 8th Int. Quantum Elecctron. Conf. Q14, San Francisco, June 1974, p. 96.
5. U. Brinkman, W. Hartig, H. Telle, and H. Walther, Appl. Phys.,5, 109?110 (1974).