Optical characteristics of Tl+ centers in CsCaCl3, KCaCl3, and CsCl crystals

Abstract

We have investigated the absorption, luminescence, and excitation spectra of the Tl+ centers in CsCaCl3:Tl+, KCaCl3:Tl+, and CsCl:Tl+ crystals in the energy region up to the vacuum ultraviolet region. The A, B, and C absorption bands attributed to the intra-ionic transitions were observed in the transparent energy region below their respective fundamental absorption edge. The parameters characterizing the Tl+ centers were estimated by the energy positions of the A, B, and C absorption bands and the absorption intensity ratio of the C to A bands. These values are consistent with those of the Tl+ centers in other ionic chloride crystals. The luminescence bands attributed to the radiative transitions from several relaxed excited states were observed even under excitation of the A absorption band. Although the intensities of the luminescence bands change with an increase in temperature from low temperature, the total integrated luminescence intensity remained almost constant. The temperature dependence of the luminescence intensity can be explained by the thermally activated non-radiative transitions among the relaxed excited states and the radiative transitions from them. By including the Tl+ centers in alkali and ammonium chloride crystals, the features of the Tl+ centers in the resulting chloride compound crystals have been comprehensively discussed regardless of the different chloride-ionic configurations around the Tl+ ions.