1. See, for instance, G. Harbake, in Optical Properties of Solids, edited by F. Abelès (North-Holland, Amsterdam, 1972), p. 21.
2. E. F. Gross and A. A. Kaplyanskii, Sov. Phys.-Solid State
2, 353 (1960).
3. F. Bassani and G. P. Parravicini, Electronic States and Optical Transitions in Solids (Pergamon, Oxford, 1975).
4. P. J. Dean and D. G. Thomas, Phys. Rev.
150, 690 (1966).
5. W. M. Yim, J. Appl. Phys.
42, 2854 (1971).