1. U. Fano: Phys. Rev., 103, 1202 (1956).
2. S. I. Pekar: Sov. Phys. JETP, 6, 785 (1958); 11, 1286 (1960).
3. S. I. Pekar: Crystal Optics and Additional Light Waves (Benjamin/Cummings, Menlo Park, 1983).
4. J. L. Birman: in Excitons, edited by E. I. Rashba and M. D. Sturge (North-Holland, Amsterdam, 1982), p. 27.
5. J. J. Hopfield: Phys. Rev., 112, 1555 (1958); V. M. Agranovich: Sov. Phys. JETP, 10, 307 (1960).