Jahn-Teller Effect on the Structure of the Emission Produced by Excitation in theABand of KI: Tl-Type Phosphors. Two Kinds of Minima on theΓ4−(T1u3)Adiabatic Potential-Energy Surface

Emission spectra, excitation spectra, and decay times of ${\mathrm{Ga}}^{+}$, ${\mathrm{In}}^{+}$, and ${\mathrm{Ti}}^{+}$ in KI, KBr, KCl, and NaCl have been investigated at 1.8-300°K under excitation in the $A$-absorption band. There are three types of temperature variation of the emission spectra; these types are represented by KI:Tl, KBr:Tl, and KCl:Tl, respectively. In case of types (1) and (2), two emission bands, $A_T$ (high energy) and $A_X$ (low energy), are observed. The excitation spectrum for the $A_T$ emission is slightly different from that for the $A_X$ emission. The decay times of the $A_X$ emission are complex and temperature dependent, as exemplified in KI:Ga and KI:In. The $A_T$ emission is surely due to the inverse process of the $A$ absorption, ${{\Gamma }_4}^{-}\left({}_{}{}^3{}_{}{}^{}T_{1u}^{}{}_{}{}^{}\right)\to {{\Gamma }_1}^{+}\left({}_{}{}^1{}_{}{}^{}A_{1g}^{}{}_{}{}^{}\right)$. Four reasons are given why the $A_X$ emission is not due to the inverse process of the $D$ absorption as proposed by Illingworth and supported by Donahue and Teegarden. To explain the $A_X$ emission, the Jahn-Teller effect on the ${}_{}{}^3{}_{}{}^{}T_{1u}^{}{}_{}{}^{}$ state has been considered by assuming Russell-Saunders coupling. Since the correlation of polarization indicates the existence of tetragonal minima, adiabatic potential-energy surfaces (APES's) in the $E_g(Q_2, Q_3)$ configuration coordinate space have been obtained for the ${{\Gamma }_4}^{-}\left({}_{}{}^3{}_{}{}^{}T_{1u}^{}{}_{}{}^{}\right)$ and ${{\Gamma }_1}^{-}\left({}_{}{}^3{}_{}{}^{}T_{1u}^{}{}_{}{}^{}\right)$ states. When the spin-orbit interaction is of suitable value as compared with the Jahn-Teller (electron-lattice) interaction, the obtained APES's have two kinds of minima. Emission is considered to occur from these minima to the ground state, at least at low temperatures. The APES's can explain why there exist the three types of the emission spectra as well as the characteristic features of the emission spectra, excitation spectra, decay times, and polarization. The importance of these APES's for analyzing the stress, Stark, and Zeeman effects on the $A_T$ and $A_X$ emission is pointed out.