1. R. Kleim: Thesis, Strasbourg (1968).
2. T. Skettrup andL. P. Lidholt:Sol. State Comm.,6, 589 (1968).
3. C. J. Wang:Journ. Appl. Phys.,42, 4408 (1971);Phys. Rev. B,8, 646 (1973).
4. U. Heim andP. Wiesner:Phys. Rev. Lett.,30, 1205 (1973);P. Wiesner andU. Heim:Phys. Rev. B,11, 3071 (1975).
5. B. Meyer, C. Schwab, A. Goltzené andS. Nikitine:International Conference on Color Centers in Ionic Crystals (Sendai, 1974);B. Meyer, A. Goltzené, C. Schwab andS. Nikitine:Proceedings of the XII European Congress on Molecular Spectroscopy, Strasbourg, July 1–4, 1975 (Amsterdam, 1976), p. 77.