Optical properties of atomic thallium centres in alkali halides

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Abstract
The three-lowest-energy absorption bands of the Tl0(1) centres of KF, NaCl, KCl, RbCl, RbCl, and KI were identified by the technique of measuring their magnetic circular dichroism tagged by ESR and/or by measuring the excitation spectrum of the near-infrared emission band. The transition energies of the two lowest-energy absorption bands and the emission band as function of the lattice parameter can be explained by the atomic model with the change of the crystal-field splitting. The temperature dependence of the absorption and emission bands could be measured in all of the compounds except for NaCl and KF. Its analysis reveals two different effective phonon frequencies in each electronic state. The low frequency is due predominantly to an atomic Tl motion along the centre axis, while the higher frequency is associated with modes in which mainly the host-lattice ions are moving. A comparison of the frequencies in the ground and excited states suggests that the Tl atom is more relaxed towards the vacancy in the ground state than in the excited state.