Simplified Configuration Coordinate Model for KCl: Tl

The configuration coordinate model for KCl: Tl has been modified recently to include the effect of the crystalline field on spin-orbit interaction. We have now re-examined the spectral data on KCl: Tl and related phosphors and propose a simplified configuration coordinate model which also includes this effect. In the simplified model the two excited states are approximately parallel, the upper state being predominately singlet; the lower, triplet. In contrast to the earlier model, the pure ${}_{}{}^1{}_{}{}^{}P_{}^0{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}P_{}^0{}_{}{}^{}$ spin states do not cross, and thereby we avoid spin-dependent crystalline interactions which vary rapidly with configuration coordinate. The effects of crystalline interactions on the energy separation of the spin states and on the spin-orbit coupling constants are estimated theoretically. In the quantitative application of the one dimensional model to absorption and emission spectra neglecting configuration interaction with other crystal states, spin-orbit coupling constants which are smaller than the free ion values are used. The application of the model to other alkali halide phosphors and the origin of spectra not readily explained with the simplified model are discussed.