Luminescence Processes in the KCl:Tl Phosphor in a Multidimensional Configuration Space

In order to explain the 3050Å and 4750Å emissions of KCl:Tl phosphor which are raised by excitation in any of the absorption bands, a multi-dimensional configuration coordinate model is introduced. This is achieved by taking into account the Jahn-Teller effect for the excited states of the thallium ion. The Jahn-Teller effect is treated by means of an intermediate coupling scheme within the 6s6p configuration, because the mixing of the 3 P and 1 P states by the spin-orbit interaction is essential to understand the optical properties. It is shown that introduction of a tetragonal distortion of the emission center is able to explain the luminescence processes of this phosphor both semi-quantitatively and qualitatively: Under the tetragonal and trigonal distortions, both the crossing of the pure spin singlet 1 P and triplet 3 P curves and the appearance of two minima in the 3 P 1 energy curves are expected. The former accounts for the non-radiative transition from the 1 P 1 state to the 3 P 1 state and the latter does for the two emission bands. The two types of distortions are equally able to give semi-quantitative agreement of the relative strengths of the absorption and emission bands with experiments, but only the tetragonal distortion explains the polarization of the 3050Å emission observed. Finally Johnson and Williams' model is criticized.