Triplet relaxed excited state structure and luminescence of thallium‐doped alkali halides

For the first time a slow component is observed and studied in the decay kinetics of AT emission of TI+ centres in KBr, KI, RbCl, and RbI crystals. The existence is established of a singlet metastable level below the doublet emitting one in each Jahn‐Teller minimum of the triplet excited state of the TI+ centres. It is concluded that for all ns2 ions in alkali halides the structure of the relaxed excited state is qualitatively the same, and it may adequately be described in terms of a theory that considers the Jahn‐Teller effect as a main interaction in the relaxed excited states of the luminescence centres. The nature is discussed of the weak emission bands appearing on excitation in the A absorption band of TI+ centres besides the main AT and AX emission bands, earlier interpreted as hot luminescence of Tl+ centres.