1. I. V. Ignat'ev and V. V. Ovsyankin, Spectroscopy of Crystals [in Russian], St. Petersburg (1983), p. 36.
2. J. P. Hurrel, Z. Kam, and E. Cohen, Phys. Rev. B.,6, No. 5, 1999?2013 (1972).
3. Yu. K. Voron'ko, A. L. Larionov, and B. Z. Malkin, Opt. Spektrosk.,40, No. 1, 86?92 (1976).
4. V. F. Zolin, V. I. Tsaryuk, V. M. Markushev, et al., Zh. Prikl. Spektrosk.,44, No. 4, 594?601 (1986).
5. V. I. Tsaryuk, V. F. Zolin, B. V. Lokshin, and Z. S. Klemenkova, Fiz. Tverd. Tela.,29, No. 4, 1157?1164 (1987).