1. For descriptions of the many-body theory of optical properties of solids see: G. D. Mahan, Many-Particle Physics, Plenum Press, New York, 1981; H. Stolz, Einführung in die Vielelektronentheorie der Kristalle, Akademie-Verlag, Berlin, 1974, and A. A. Abrikosov, L. P. Gorkov and I. Ye. Dzyaloshinskii, Quantum Field Theoretical Methods in Statistical Physics, Pergamon Press, Oxford, 1965.
2. For reviews see, e. g., the articles by R. J. Elliot and by H. Haken in: Excitons and Polarons, eds. C. G. Kuper and G. D. Whitfield, Oliver and Boyd, Edinburgh, 1963.
3. For reviews of the optical properties of highly excited semiconductors see E. Goebel and G. Mahler, Advances in Solid State Physics XIX, 105 (1979), and C. Klingshirn and H. Haug, Phys. Reports 70, 315 (1981).
4. For a review of the optical properties of excitonic molecules see, e. g., E. Hanamura and H. Haug, Phys. Reports 33C, 209 (1977).
5. Review articles on the properties of the e-h plasma in indirect-gap semiconductors are given by T. M. Rice, Sol. State Physics, Vol. 32 (Academic Press, 1977) p. 1; J. C. Hensel, T. G. Philips and G. A. Thomas, ibid p. 88.