Energy Levels and Rate Processes in the Thallium Activated Potassium Chloride Phosphor

Abstract

The dependence of spectral distribution and intensity of emission of KCl: Tl on temperature and excitation wavelength are correlated with absorption data and theoretical calculations to provide a detailed picture of the energy relationships in this phosphor. The two principal excited states are the ¹P₁⁰ and ³P₁⁰ states of Tl⁺ and are found to be approximately in thermal equilibrium with each other. The equilibrium is disturbed by emission, and the 4750A emission is favored by 1960A excitation and the 3050A emission is favored by 2470A excitation. The former processes are attributed to ¹P₁⁰↔¹S₀ transitions; the latter, to ³P₁⁰↔¹S₀ transitions. The rate processes are formulated and the specific rate constants deduced. The presence of Tl⁺ or Cd⁺⁺ adjacent to the activator induces additional excitation and emission bands. The activation energy for radiationless de‐excitation has been computed theoretically and found to be in accord with experiment.