1. V. N. Fedoseev, V. I. Mishin, D. S. Vedeneev, and A. D. Zuzikov, J. Phys. B 24, 1575 (1991).
2. V. N. Gorshkov, V. A. Komarovskii, A. L. Osherovich, and N. P. Penkin, Opt. Spektrosk. 52, 935 (1982) [Opt. Spectrosc. 52, 562 (1982)].
3. Popular Library of Chemical Elements, Ed. by I. V. Petryanov-Sokolov (Nauka, Moscow, 1983), Vol. 2.
4. Laser Handbook, Ed. by A. M. Prokhorov (Sov. Radio, Moscow, 1978), Vol. 1.
5. V. A. Gerasimov, Opt. Spektrosk. 87, 156 (1999) [Opt. Spectrosc. 87, 144 (1999)].