1. L. A. Lisitsyna, Izv. Vyssh. Uchebn. Zaved., Fiz., No. 10, 109?120 (1991).
2. L. A. Lisitsyna, V. A. Kravchenko, and V. M. Reiterov, Fiz. Tverd. Tela (St. Petersburg),33, 786?790 (1991).
3. L. A. Lisitsyna, V. A. Kravchenko, and V. M. Reiterov, ibid.,33, 2801?2805 (1991).
4. L. Bosi and M. Nimis, Phys. Status Solidi (b),152, 67?71 (1989).
5. V. A. Arkhangel'skaya, E. V. Guseva, G. M. Zinger, et al., Opt. Spektrosk.,61, No. 3, 542?549 (1986).