1. See, for example, the recent review byY. Hamakawa andT. Nishino: inOptical Properties of Solids: New Developments, edited byB. O. Seraphin (Amsterdam, 1976), p. 255.
2. See, for example,F. C. Brown: inSolid State Physics, Vol.29, edited byH. Ehrenreich, F. Seitz andD. Turnbull (New York, N. Y., 1974), p. 1.
3. C. G. Olson, M. Piacentini andD. W. Lynch:Phys. Rev. Lett.,33, 644 (1974).
4. P. Thiry, R. Pincheaux, D. Dagneaux andY. Petroff: inProceedings of the Twelfth International Conference on the Physics of Semiconductors, edited byM. Pilkuhn (Stuttgart, 1974), p. 1324.
5. D. E. Aspnes andC. G. Olson:Phys. Rev. Lett.,33, 1605 (1974).