1. W.H. Fonger and C.W. Struck, J. Chem. Phys. 60, 1994 (1974).
2. F. Auzel in :Luminescence of Inorganic Solids (Ed. B. di Bartolo) Plenum Press, New York, 1978; B. Di Bartolo and R.C. Powell, Phonons and Resonances in Solids Wiley, New York, 1976.
3. T.H. Keil, Phys. Rev. A140 601 (1965).
4. C.W. Struck and W.H. Fonger, J. Chem. Phys. 60, 1988 (1974).
5. E. Gutsche, J. Lumin 24/25,689 (1981)