Masakazu Ishiguro

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.

The Luminescence in CsI : Tl

The luminescence in pure CsI and CsI : Tl crystals containing various concentrations of Tl have been measured upon excitation by beta rays over the temperature range from -180°C to +20°C. The emission spectra in pure CsI at low temperatures have two peaks whose temperature dependence are different each other. In the case of CsI : Tl, the temperature dependence of the luminescence near 400 mµ is quite different from that of longer wavelength luminescence. In the longer wavelength region around 500 mµ, several glow peaks are observed from the samples of higher concentration of Tl. The process of luminescence in CsI : Tl are roughly explained by using the self trapped hole model.

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.

Hypersonic Attenuation in Sb Doped Germanium

Hypersonic attenuation in Sb doped Ge is measured. In the attenuation vs. temperature curve, a large peak is found at 1.8°K. A hypersonic sournd used is longitudinal waves of 1 GHz propagating along the axis of Ge crystal which contained about 1×1014, 1×1015 and 1×1016/cm3 donors. The above attenuation peak seems to be due to the electronic relaxation between the singlet and the triplet states of the donors in Ge, as discussed by M. Pomerantz in the case of As- or P-doped Ge.

Pressure effects on emmission in alkali halide-thallium phosphors with CsCl-type structure

Abstract Hydrostatic pressure effects on the A emission bands of CsBr:Tl and CsI:Tl have been studied at 1.5 K in comparison with those of CsCl phase of KI:Tl and RbI:Tl. It seems that in the CsCl phase there exist two similar neighboring excited states with the A-like adiabatic potential energy surface (APES).

Pressure Effects of Absorption Bands in NaBr: Cu+

Absorption spectra of NaBr: Cu + have been investigated over the pressure range from 1 bar to about 6 kbar at a low temperature. There occurs a critical pressure-induced local phase transition (off-center to on-center transition), accompanying appearance of new absorption. bands on the lower energy side of the known absorption bands. The above pressure-induced absorption bands are assigned to the A band (spin-forbidden transition) and the B band in order of increasing photon energy, refering to the absorption spectrum of an on-center system, NaCl: Cu + . The origin of the off-center is discussed in connection with the pressure effects.

Dichroism of Z4 and F Bands

To investigate the dichroic property of the Z 4 band, the experiment on the anisotropic bleaching was made. It was found that the symmetry axis of Z 4 center is parallel to direction and the Cole and Friaufs A model is in conformity with this symmetry. It was also found that there exists the same correlation in dichroism between F and Z 4 band as that between F and M band.

Dichroism of Z Bands

The dichroic properties of Z bands and their correlation to that of the F band have been studied on NaCi:Sr and KCl:Sr single crystals. From the experiment on the anisotropic bleaching, the Z 1 center was found to be a center of high symmetry in agreement with the result of paramagnetic resonance experiment. The Z 2 band showed dichroism when irradiated with [010] polarized light in the Z 2 or F band at room temperature, accompanying the dichroism of opposite sign in the F band. This is another example of the correlation in dichroism between two absorption bands as shown in the F - M system which has been found by Van Doorn and Haven. It is, however, questionable to simply decide the symmetry of the Z 2 center from the dichroism mentioned above, because in the case of KCl:Sr its peak position is shifted to shorter wavelength than the normal Z 2 band.

Experimental Studies of Uniaxial Stress-Induced Polarization of F Center Luminescence

The degree of polarization of F center luminescence, P S , induced by a uniaxial stress has been investigated in KF, KCl, KBr, KI, RbF, RbCl, RbBr, and CsBr crystals. Polarizations associated with \(\varGamma_{\alpha}\) mode coupling, \(P_{S}(\varGamma_{\alpha})\), were observed under the relevant experimental geometries. The observation of \(P_{S}(\varGamma_{5})\) and its temperature dependence in the range from 2 K to 77 K were first presented. This temperature dependence was found to be contrasted to that of \(P_{S}(\varGamma_{3})\) which was also precisely observed in this work. Three sorts of linear stress coefficients, that was \(P_{S}(\varGamma_{\alpha})\) per unit stress applied, were tabulated for α=3, 5, and a combination of 3 and 5. A vibronic parameter of \(\varGamma_{5}^{+}\)-mode phonons was estimated to be small compared with that of \(\varGamma_{3}^{+}\)-mode.

Magnetic Field Effect on the Hypersonic Attenuation by Shallow Donors in Ge

The magnetic field effect on the hypersonic attenuation in Sb-doped Ge has been measured over the range from 0 to 200 kG at liquid He temperatures. The attenuation was observed to decrease as the magnetic field was increased at high field. The magnetic field dependence of the attenuation in the case of 1×10 15 /cm 3 doped specimen differs from that of 1×10 16 /cm 3 doped specimen. The experimental results are not quantitatively explained by the theoretical treatment which takes into consideration only the Zeeman effect on the single and the triplet states. If it is assumed that the effective Bohr radius of the donor electron is larger than the known values by other experimental methods, the semiquantitative agreement is obtained by calculating the shrinkage effect of the donor wave function.