1. L. Ley, J. Azoulay andP. C. Kemeny: to be published.
2. See,e.g.,J. C. Phillips: inBonds and Bands in Semiconductors (New York, N. Y., 1973);D. Schiferl:Phys. Rev. B,10, 3316 (1974).
3. S. P. Kowalczyk, L. Ley, F. R. McFeely andD. A. Shirley:Journ. Chem. Phys.,61, 2850 (1974).
4. Lead salts: see ref. (3) ; SnTe:P. C. Kemeny andM. Cardona:J. Phys. C,9, 1301 (1976); GeTe: unpublished work of this laboratory.
5. C. J. M. Rooymans: inAdvances in High Pressure Research, edited byR. S. Bradley, Vol.2 (London, 1969), p. 1;D. Schiferl: private communication.