Origin of the absorption bands of cesium halides doped with Tl+ ions

We have systematically investigated the absorption spectra of CsX:TI+ (X = Cl, Br, I) and their mixed crystals with CsCl-type crystal structure. The lowest energy absorption bands, which are peaked at 5.18, 4.85, and 4.27 eV for CsCl:TI+ CsBr:TI+, and CsI:TI+, respectively, exhibit the similar temperature dependence as the A band of TI+ centres in NaCl-type alkali halides. In the higher energy region, several absorption bands are observed with no extremely different absorption intensity. The temperature evolution of the higher energy absorption bands is not characteristic of the TI+ centres in NaCl-type alkali halides. Absorption bands in CsBr1-xCsIx:TI+ mixed crystals can be classified into two groups: the rapidly disappearing and gradually fading absorption bands with increasing composition of the hetero-halogen ions. The origin of the higher absorption bands is discussed on the basis of the spectral changes in the CsBr1-xCsIx:TI+ (0.0 less than or equal to x less than or equal to 1.0) mixed crystals.