Peculiarities of the triplet relaxed excited-state structure and luminescence of a CsI:Tl crystal

Abstract

Four emission bands, all belonging to the main thallium centres, have been detected in CsI:Tl, and their spectral, polarization and kinetic characteristics have been studied at 0.35-320 K. A new model is proposed for the excited states responsible for the luminescence of thallium-doped caesium iodide. In this model, the main thallium centre is considered as a cluster consisting of a Tl⁺ ion and at least 12 I^- and 11 Cs⁺ ions. Two weak ultraviolet emission bands of CsI:Tl (3.31 and 3.09 eV) are ascribed to electronic transitions from trigonal and tetragonal Jahn-Teller minima of the triplet relaxed excited state, whose structure is similar to that of Tl⁺ centres in FCC alkali halides. Two intense visible bands (2.55 and 2.25 eV) are assumed to arise from two different ("weak" and "strong") off-centre configurations of a selftrapped exciton perturbed by the TI⁺ ion. The minima responsible for all the emission bands are located on the same adiabatic potential energy surface. The excitonic-like nature of visible emission could explain the high scintillation efficiency of CsI:Tl.