Importance of Pseudo-Jahn-Teller Effect in the Excited State for (TL+)2-Type Centers in Alkali Halides

Abstract

The excited states for (Tl+)-type centers in alkali halides have been established to be characterized by the Jahn-Teller effect in both aspects of absorption and emission process. In the case of (Tl+)2-type centers, on the other hand, there is a general belief that the interaction between the paired ions lifts the orbital degeneracy of the active electrons and Jahn-Teller effect can not be expected any more. As a matter of fact, the absorption spectra for (Tl+)2-type centers have been successfully interpreted in terms of the interaction between the paired ions. The luminescence, however, is not so clear-cut to be understood with the same model. For the purpose of understanding the relaxed excited states (RES's) responsible for the luminescence in (Tl+)2-type centers, we have investigated their spectral and polarization characteristics and the temperature dependence of the radiative decay times in KI:(Ga+)2, KI:(In+)2, and KI:(Tl+)2, and concluded that some of the RES's are principally determined by the Jahn-Teller effect in (Tl+)2-type centers, just like in (Tl+)-type centers. In all the investigated emission bands, the degree of polarization measured as a function of azimuthal angle in the (001) plane is zero in the <110> direction and its absolute value is maximum in the <100> direction. Therefore, <100> is the direction connecting the paired ions. The emission band excitable by the A∑ and A∏ absorption bands in the paired-ion center is located close to the AT emission band in the corresponding single-ion center for KI:(Ga+)2 and KI:(In+)2, while that is close to the AX emission band for KI: (Tl+)2. Since the interaction between the paired ions is small in the triplet excited states, the interaction can be regarded as a perturbation to the RES's responsible for AT and AX emission bands due to the Jahn-Teller effect. The perturbation destroys the equivalence of the three tetragonal distortions for AT RES; one of them becoms slightly different from the other two. Thus the AT∑ and AT∏ emission bands are observed separately as in KI:(Ga+)2. Although these two bands could not be separated in KI: (In+)2, the polarization characteristics observed indicate that the 2.76 eV emission band consists of at least two component bands, AT∑ and AT∏.