LUMINESCENCE OF CSCLTL SINGLE-CRYSTALS

Spectral, polarization, and kinetic characteristics of CsCl:Tl luminescence are studied between 4.2 and 350 K. The A(X) band at 3.88 eV and the AT band at 3.65 eV are observed in the triplet emission spectrum of Tl+ centres. It is shown that in the triplet relaxed excited state (RES) the trigonal (X) minima are of higher energy than the tetragonal (T) ones. The symmetry of both the emitting and the metastable states in each (X or T) minimum is the same, and in many respects their structure is similar to that of other ns2 ions in alkali halides. However, some peculiarities are observed in the luminescence characteristics of CsCl:Tl crystals (e.g. very small quantum efficiency of triplet emission at excitation in high-energy absorption bands; population of the lower (T) minima mainly in the process of vibronic relaxation in the triplet excited state; too large polarization degree of the slow decay component of the A(X) emission, exceeding 1.5 times its theoretical limit; large difference in the spin-orbit splitting energies delta of the triplet RES in the X and the T minima as well as in radiative decay probabilities gamma1 of their metastable states; the absence of an off-centre displacement of the excited Tl+ ion from the crystal lattice site, etc.). Possible reasons for these features are discussed.