Extent to which the Jahn-Teller model can account forT1+luminescence

Several mechanisms have been considered for obtaining coexistence of different kinds of Jahn-Teller minima on the excited-state potential surfaces of the impurity, but no one was able to give a satisfactory interpretation of ${\mathrm{T}1\mathrm{}}^{+}$ luminescence. These models are here critically reexamined, and an attempt is made for recognizing a scheme suitable to explain most of the experimental features.