1. R. H. Dicke, Phys. Rev.,93, 99 (1954).
2. A. V. Andreev, V. I. Emel'yanov, and Yu. A. Il'inskii, Usp. Fiz. Nauk,131, 653 (1980).
3. Yu. V. Naboikin, V. V. Samartsev, P. V. Zinov'ev, and N. B. Silaeva, Coherent Spectroscopy of Molecular Crystals [in Russian], Naukova Dumka, Kiev (1986).
4. V. V. Samartsev, Izv. Akad. Nauk SSSR, Ser. Fiz.,37, 2155 (1973).
5. L. V. Keldysh, in: Problems of Theoretical Physics [in Russian], Nauka, Moscow (1972), p. 433.