1. V. P. Klochkov and E. B. Verkhovskii, Opt. Spektrosk.85, 427 (1998) [Opt. Spectrosc. 85, 390 (1998)].
2. V. P. Klochkov and E. B. Verkhovskii, Opt. Spektrosk. 88, 778 (2000) [Opt. Spectrosc. 88, 692 (2000)].
3. V. P. Klochkov and E. B. Verkhovskii, Opt. Spektrosk.89, 114 (2000) [Opt. Spectrosc. 89, 103 (2000)].
4. A. V. Andreev, V. I. Emel’yanov, and Yu. A. Il’inskii, Cooperative Phenomena in Optics (Nauka, Moscow, 1988).
5. P. V. Zinov’ev, S. V. Lopina, Yu. V. Naboikin, and N. B. Silaeva, Fiz. Nizk. Temp. 10, 510 (1984) [Sov. J. Low Temp. Phys. 10, 266 (1984)].