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2. T. Nishino and Y. Hamakawa, in Optical Properties of Solids: New Developments, ed. by B. O. Seraphic (North-Holland, Amsterdam, 1976), p. 255.
3. C. Kunz, in ref. [2], p. 473; R. Haensel, Festkörperprobleme 15, 203 (1975).
4. F. C. Brown, Solid State Physics 29, 1 (1974).
5. Proceedings of the Summer School on Synchrotron Radiation Spectroscopy,, Alghero, Italy (1977, in press).