On the intrinsic nature of the blue luminescence in CaWO4

Luminescence and thermoluminescence properties of nominally pure LiF and CaWO₄ crystals are compared with those of the crystalline system LiF/CaWO₄. Evidence is found which clearly indicates that from the various emission bands of CaWO₄ crystals only the blue one is of intrinsic nature. Luminescence decay measurements are reported. Within the spectral range of the blue tungstate band in LiF/CaWO₄ four different decay times are obtained. This result is interpreted as arising from WO^2-₄ luminescence centres with different environments in the LiF host.     

Excitation and temperature dependence of the blue luminescence of ZnS crystals

The blue luminescence of pure and Al-, Au-, Ag-, Cu-, and Mn-doped ZnS crystals has been studied in the temperature range between LNT and RT and for different wavelengths of the exciting UV-light. All investigated crystals emit two blue bands. The energies of the maxima of these emission bands are independent of the temperature. The temperature dependence of the intensity ratio of the two blue luminescence bands can simulate a temperature shift of the blue emission. The experimental results support the assumption of an essentially uniform structure of the luminescence complex responsible for the visible luminescence of pure and doped ZnS phosphors.     

Influence of low and high temperature annealing on the blue and green emission bands of ZnS crystals

After annealing at 350 °C, in the luminescence spectra of pure and Cu-doped ZnS crystals a reversible blue-green conversion has been obtained. The blue-green conversion of luminescence is correlated with analogous reversible variations in the optical transmission and thermoluminescence properties. Besides these low temperature results, in the fluorescence spectra of undoped ZnS crystals, which are annealed at 1,000 °C in sealed evacuated quartz ampoules, a reversible blue-blue conversion has been observed. All these effects, induced by distinct heat treatments, seem to be mainly controlled by changes in the structural disorder or/and by the evaporation of lattice atoms from the surface of the crystals.Dedicated to Prof. Dr. Dr. Ing. E.h.W. Hanle on occasion of his 75^th birthdayPart of thesis, Gießen 1974 (D26)     

Leder,Pelze

Analytical and Bioanalytical Chemistry -     

Coexistence of Tellurium Cations and Anions in Phosphonium-Based Ionic Liquids

Elemental tellurium readily dissolves in ionic liquids (ILs) based on tetraalkylphosphonium cations even at temperatures below 100 °C. In the case of ILs with acetate, decanoate, or dicyanamide anions, dark red to purple colored solutions form. A study combining NMR, UV-Vis and Raman spectroscopy revealed the formation of tellurium anions (Te_n)²⁻ with chain lengths up to at least n=5, which are in dynamic equilibrium with each other. Since external influences could be excluded and no evidence of an ionic liquid reaction was found, disproportionation of the tellurium is the only possible dissolution mechanism. Although the spectroscopic detection of tellurium cations in these solutions is difficult, the coexistence of tellurium cations, such as (Te₄)²⁺ and (Te₆)⁴⁺, and tellurium anions could be proven by cyclic voltammetry and electrodeposition experiments. DFT calculations indicate that electrostatic interactions with the ions of the ILs are sufficient to stabilize both types of tellurium ions in solution.     

Zum Lumineszenz-Abklingverhalten von ZnS/Cu

The decay characteristics of ZnS/(10^?4) Cu have been investigated by ion pulse excitation. The excitation density could be varied by using ions of different masses. The decay is exponential. The decay times depend on the ion pulse duration and on the penetration depth of the used ions. The green emission shows two different decay times. The intensity of the slower component of the green emission could be altered by excitation with UV-light before decay measurement. In the case of the slower component of the green emission we suppose an interaction between the excited states of the luminescence centres and energetically deep electron traps.     

Reversible Blau-Grün-Umwandlung der Lumineszenz selbstaktivierter Zinksulfide

A blue-green-conversion of the luminescence of ZnS/Zn and ZnS/S which are annealed at 350 °C can be measured on phosphors with different thermal equilibria (room temperature or 350 °C). The blue-green-conversion of ZnS/S equals this of ZnS/Cu. The conversion depends on the amount of the short wavelength blue emission band in the luminescence of the phosphor. The high energy blue emission band and the blue-green-conversion of the luminescence seem to be coupled to disturbances of the sulphur sublattice.