1. For reviews on excitons in semiconductors see R.S. Knox, Theory of Excitons in Solid State Physics, F. Seitz and D. Turnbull eds. (Academic Press, New York, 1963) Suppl. 5
2. J. O. Dimmock in Semiconductors and Semimetals, R.K. Willardson and A.C. Beer, eds. (Academic Press, New York, 1967) Vol.3, p. 259.
3. For reviews on excitons in external magnetic fields see: H. Hasegawa in Physics of Solids in Intense Magnetic Fields, E.D. Haidemenakis, ed. (Plenum, New York, 1969), p. 246
4. F. Bassani and A. Baldereschi Surface Sience 37 304 (1973)
5. H. J. Zeiger and G.W. Pratt, Magnetic Interactions in Solids (Clarendon Press, Oxford, 1973) ch. 7