Shoji Masunaga

Optical Properties of CsI : Tl and CsBr : Tl

Absorption, excitation and emission spectra and luminescent decay of CsI:Tl have been investigated over the temperature range from 2°K to 300°K. Six absorption bands are observed in contrast with four absorption bands A, B, C, and D in alkali halide-thallium phosphors having the NaCl structure. There exists no other absorption band having a character of the forbidden line as the B band. At 10°K, four emission bands are observed. The luminescent decay is composed of the fast and slow components and is somewhat complicated. For the purpose of comparison, the experiments on CsBr:Tl are also described.

Hydrostatic Pressure Effects on the Triplet Relaxed Excited States of KI: Tl + -Type Phosphors through the Quadratic Jahn-Teller Interaction

Pressure effects on the emission spectra excited by the A absorption have been observed at various temperatures between 168 and 293 K in KI, KBr, and KCl doped with Tl + , In + , or Ga + . When both of the A T and A X bands appear, the A T band grows at the expense of the A X band as the pressure increases. Particularly, in KBr: Tl + and KBr: In + , the A X band disappears almost completely under 7.5 k bar. When only the A T band appears as in KCl: Tl + and KCl: In + , no drastic change has been observed up to 7.5 k bar. The pressure effects have been discussed on the basis of the quadratic Jahn-Teller effect by correlating the relaxed excited states for the A T and A X bands to the tetragonal and trigonal minima on the \(\varGamma_{4}^{-}\) (A) and \(\varGamma_{1}^{-}\) adiabatic potential energy surfaces respectively.

Relaxed excited states determined by Jahn-Teller effect in Ti+-type centers in alkali halides

Abstract Effects of magnetic field and hydrostatic pressure on the A T and A X emission bands have been investigated. Results show the importance of the quadratic Jahn-Teller effect, indicating that the A T RES is related to the tetragonal minima and the A X RES to the trigonal minima on the Г - 4 (A) and Г - 1 adiabatic potential energy surfaces (APES). Cross sections of the APESs along tetragonal and trigonal distortions have been calculated to explain the luminescent properties systematically.

Absorption Spectra of Rubidium Halide-Thallium Phosphors with the CsCl-Type Structure

Absorption spectra of RbI:Tl, RbBr:Tl, RbI:Pb, and RbBr:Pb having the CsCl-type lattice structure under high pressures have been observed. At the phase transition from the NaCl-type to the CsCl-type structure, in RbI:Tl, the A band shows a discontinuous red shift and a new absorption band appears on the short wavelength side of the A band. This new absorption band does not show a character of the forbidden line such as the B band. In RbBr:Tl where the electron affinity of halogen ions is large than that in RbI:Tl, however, the A band shows a discontinuous blue shift and there does not appear any new absorption band. In the case of Pb ++ activator ions, at the phase transition, the A band shows a discontinuous red shift and a new absorption band appears in RbI and besides, in RbBr. These results are discussed on the basis of the possibility of the configuration interaction between the excited states of the activator ions and electron transfer states.

Pressure Effect of Emission Spectra in Alkali Halide Crystals Containing Tl+–Ions

Emission and excitation spectra of KCl:Tl, KBr:Tl, KI:Tl, NaCl:Tl and NaBr:Tl have been measured up to 36 kbar at room temperature. In the case of potassium halide, when the host crystal is transformed from the NaCl-type to the CsCl-type structure by applied pressure, the emission spectra show a discontinuous blue shift and grow significantly on their high energy sides, resulting in the overlap of the excitation and emission spectra. In the case of NaCl:Tl, the excitation and emission spectra also partially overlap each other with increasing pressure. It is discussed that the parameter S of Haung and Rhys in the Tl + -center decrease with increasing pressure.

Structures of Absorption Bands in Alkali Halide Crystals Containing Tl + -Like Ions under High Pressure

Absorption spectra of NaCl, KCl, KBr and KI crystals doped with In + or Sn ++ have been studied under high pressure. In the case of In + , the splitting energy of the A band and that of the C band become larger as pressure increases. The separation between the center of the A band and that of the C band remains almost constant within the applied pressure in contrast to the increase of the separation between the centers of the A band and the B band. In the case of Sn ++ , the splitting energy of the A band and that of the C band remain constant. The B and C bands hardly show any shift. The pressure dependence of the splitting energies of these bands is discussed with the Toyozawa-Inoues theory based on the dynamical Jahn-Teller effect.

Temperature Dependence of the A Band of Tl^+-Type Centers in Alkali Halide Crystals under High Pressure

Absorption spectra of alkali halide crystals doped with In + or Sn ++ have been investigated over the temperature range from 170 to 300 K under high pressure. In the case of In + , when the host crystal lattice of the potassium halides is converted from the NaCl-type to CsCl-type structure by the application of pressure, the separation between the A 1 and A 2 bands decreases by about 20–30%. The temperature dependence of the A band shows the characteristics of the dynamical Jahn-Teller effect in both the NaCl-type and CsCl-type structures. In the case of Sn ++ the separation between the A 1 and A 2 bands decreases gradually with pressure. The temperature dependence of the A band under high pressure also shows the characteristics of the dynamical Jahn-Teller effect.

Pressure Dependence of Direct Band Gap at Γ Point in Solids

The absorption spectra of the alkali and the cuprous halides under hydrostatic pressure were measured at room temperature. The first exciton peak shows a linear blue shift with increasing pressure. Ionic index I for ionic crystals and cubic semiconductors is defined and estimated. It is found that the ionic index I is almost 1 for the alkali halide. As the ratio of the covalent bond to the ionic bond increases, the ionic index I becomes smaller than 1.

Pressure Dependence of Γ Excitons in Alkali Bromide Thin Films

Pressure dependence of optical absorption spectra of Γ excitons in alkali bromide thin films, NaBr, KBr and RbBr, was measured at room temperature. The exciton energies showed linear blue shifts while the absorption intensities decreased with increasing pressure. In order to further understand the experimental results, the pressure dependence of the absorption intensities is discussed in terms of the pressure dependence of Bohr radius by Elliotts theory and exchange interaction between electron and hole by Onodera and Toyozawas theory. We speculate that the Bohr radii increase and the exchange energies decrease as the pressure increases for the Γ3/2 and the Γ1/2 excitons in the alkali bromides.

Quadratic Jahn-Teller Effect in the Line Shapes of the A-Emission and A-Absorption Bands in KBr:Tl^+-Type Phosphors

The adiabatic potential energy surfaces (APESs) of Tl + -type impurities in alkali halides, \(^{3}T_{1u}(\varGamma_{4}^{-})\) and \(^{3}A_{1u}(\varGamma_{1}^{-})\) have been calculated taking account of the spin-orbit interaction and both the linear and quadratic Jahn-Teller effect with the α 1 g and e g vibrational modes. It has been found that the 3 T 1 u APESs have one kind of minima or the two kinds of minima, depending on whether we assume the linear or both the linear and quadratic Jahn-Teller interactions. A trap level 3 A 1 u lies just below both the minima. The line shapes of the A -emission and A -absorption bands in KBr: Tl + are calculated by using the Franck-Condon and Condon approximations. It has been found that the theoretical line shape of the A -emission band has the asymmetric doublet structure and reproduces the experimental emission bands A T and A X . This shows that the large energy differnce between the A X and A T bands is caused by the quadratic Jahn-Teller terms involving Q 1 ...