Elektronenspin-Resonanz und Photochemie des U2-Zentrums in Alkalihalogenid-Kristallen

The photochemical reactions involved in the production and in the bleaching of U₂ centers (interstitial hydrogen atoms) in KCl, KBr and NaCl crystals are investigated by measuring the optical absorption spectra of the crystals at low temperatures. The U₂ centers are prepared by UV irradiation of hydroxide- and hydride-doped crystals. The electron spin resonance of KCl, KBr and NaCl crystals with U₂ centers at low temperatures reveals a well-resolved hyperfine splitting due to interactions of the hydrogen atom with four neighbouring halogen ions. The U₂ center can be described as a tetrahedal [HX₄]^4?-molecule ion, in which one hole, bound mainly at the hydrogen atom and partially at the surrounding halogen ions, gives rise to the spin resonance.     

Photoluminescence and EPR studies on gamma irradiated Ce doped potassium halide single crystals

Electron Paramagnetic Resonance(EPR), Photoluminescence(PL), Thermoluminescence (TL) and other optical studies of γ-irradiated KBr, KCl:Ce³⁺ single crystals. Cerium when doped into the KBr, KCl is found to enter the host lattice in its trivalent state and act as electron trap during γ-irradiation, thereby partially converting itself to Ce²⁺. The Photoluminescence(PL) spectra of both KCl and KBr crystals doped with Ce exhibit the strong blue emissions of Ce corresponding to ⁵d(²D)→²F_5/2 and ⁵d(²D)→²F_7/2 transitions. The defect centers formed in the Ce³⁺ doped KBr and KCl. Crystals are studied using the technique of EPR. A dominant TL glow peak at 374, 422 K and KCl:Ce³⁺ at 466, 475 K is observed in the crystal. EPR studies indicate the presence at two centers at room temperature. Spectral distribution under the thermoluminescence emission(TLE) and optically stimulated emission(OSL) support the idea that defect annihilation process to be due to thermal release of F electron in KBr, KCl:Ce³⁺ crystals. Both Ce³⁺ and Ce²⁺ emissions were observed in the thermoluminescence emission of the crystals.     

Allgemein-relativistische Dynamik und Machsches Prinzip

The ESR of KCl and KBr single crystals doped with KSH or KSeH is investigated at low temperature. In these crystals interstitial hydrogen atoms are produced by irradiation with ultraviolet light at 20 °K. After annealing to 110 °K for some seconds and recooling to 20 °K the crystals show the resonance spectra of sulfur and selenium centers which are ascribed to S^?-, H₂S^?-, and Se^?-ions. For verifying the nature of the centers crystals with KSD as well as KSH, the sulfur being enriched by S³³ isotope, are studied. The components of theg- andA-tensors are determined. Two new types of sulfur centers are found in sulfide doped crystals after introduction of F-centers followed by irradiation with ultraviolet light at 370 °K. Furthermore all crystals being investigated reveal the ESR of HCN^?-ions.     

Photochemistry and reactions of OH− defects and F centers in Alkali Halides

Additively colored KCl:OH^? crystals show under combined UV and visible irradiation an irreversible destruction of F centers and visible absorption. This process, which is effective over the whole temperature range 4–300 K, was studied systematically in terms of the reaction efficiency and products using UV-visible and IR (local mode) spectroscopy. The UV photo-dissociation of OH^? defects and photo-ionization of F centers produce a combined net effect, in which at higher temperatures all F centers are converted into U centers, thus completely bleaching the colored crystal. By these photo-reactions, high contrast visible images can be produced which are stable under visible light at RT, and are thermally stable up to 650°C. Besides the optical information-storage aspect, these photo-reactions can be used for controlled production of U_A centers if the crystal contains alkali-ion impurities like Na⁺.     

Elektronenspin-Resonanz von Stickstoffzentren in Alkalihalogenid-Kristallen

Paramagnetic nitrogen centers are produced in nitrate doped single crystals of KCl, KBr, KJ, and NaCl by introduction of F-centers. The electron spin resonance is studied at low temperature. The hyperfine structure of the lines is caused by interaction with two N¹⁴ nuclei. The angle dependence of the resonance spectra is measured by rotating the crystals about several crystallographic axes. The results can be fitted to a spin-Hamiltonian with orthorhombic symmetry. The components of theg-tensor and theA-tensor are determined. The centers are believed to be N ₂ ^? -ions in negative ion vacancies.     

Gitterfehlordnung in KCl mit Farbzentren

Color centers were created in KCl single crystals by additive coloration and X-irradiation at room temperature. Change of lattice parameter was determined by an X-ray method and change of mass density by a hydrostatic flotation method. Combination of these results, together with optical absorption measurements, led to the following results: F centers in additively colored KCl are Schottky defects. They cause a lattice expansionΔ v/v=0.58 perF center.F centers in X-irradiated KCl are parts of Frenkel defects. During irradiation atoms are displaced to interstitial lattice sites. The volume expansion isΔ v/v=1,15 perF center.     

Anionische Silberzentren („B-Zentren“) in Alkalihalogeniden

It is proposed that negatively charged silver ions on anion sites are responsible for theB bands found in silver doped alkalihalides. Experimental investigations confirming this model of theB center are presented. Optical absorption and emission ofB centers were measured in seven alkalihalides in the temperature range from 450 to 4 °K. Configuration coordinate diagrams were obtained for KCl, KBr, and KI. A chemical method was used to determine the charge of theB center. Thermal or optical excitation causes theB centers to dissociate into neutral silver atoms andF centers. The Ag⁰ centers are bound to interstitial positions at low temperature. At high temperature they collect together forming colloidal centers. TetragonalB _A centers were formed during optical bleaching of theB band in mixed crystals of the type KCl+ΔNaCl. Their absorption and emission spectra, optical orientation, and polarized emission were investigated. These properties are shown to be similar to those ofF _A centers.B _A fluorescence polarized almost completely in the [100] direction was observed. A Jahn-Teller splitting of theB absorption band was resolved in RbCl at low temperature. The splitting confirms the configuration5s² for the Ag^? ion. The results are compared with those for the isoelectronic centers In⁺ and Sn⁺⁺. Thin films of alkalihalides containing small amounts of silver, copper, or thallium were condensed simultaneously with alkali vapor. New bands in the UV region were found, possibly due to Cu^? and Tl^? centers.     

Dependence of superconducting properties of underdoped Bi-2201 single crystals on the charge composition and growth conditions in gas-filled cavities

High-quality Bi_{2 + x} Sr_{2 ? y} CuO_{6 + δ} (Bi-2201) single crystals with the ratio Bi/Sr = 1.4–2.0 were grown by the free growth method in gas-filled cavities in KCl solution-melt in a range of doping levels, which provides variation in superconducting properties from insulators to optimally doped crystals. The charge composition Bi : Sr : Cu = 1.7 : 2.3 : 2.5 with excess Sr and Cu and synthesis conditions provided growth cavity formation in the KCl solution at the crystal growth stage. Lamellar single crystals and whiskers were grown under quasi-equilibrium conditions of lowered growth temperatures and partial oxygen pressure, achieved in closed gas-filled cavities.     

Oscillator strength of electron-type color centers in KCl single crystals irradiated with electrons and protons

The absorption spectra of KCl single crystals irradiated with electrons and protons at energies of 15 and 100 keV and a particle flux ranging from 5×10¹² to 10¹⁵ cm^?2 are investigated. The absorption bands attributed to simple (F, F _a, K) and complex (M, R ₂, R ₄, N) color centers are identified in the spectra. The correlation dependences of the absorption coefficients for M, R ₂, and R ₄ centers on the absorption coefficient of F centers and the correlation dependences of the absorption coefficients for R ₂ and R ₄ centers on the absorption coefficient of M centers are established. The oscillator strengths are calculated for M, R ₂, and R ₄ color centers.     

Decay modes of the 3000 Å emission band in KCl: Tl+

The time behavior of the 3000 Å luminescence signals of KCl: Tl crystals is complex. Three different components can be resolved in the temperature range investigated (80–340K). These results that are typical for the A_x emission of phosphors like Kl:Tl⁺ and KCl:Pb²⁺, cannot be explained with the available theoretical models for KCl:Tl system.     

EPR study of charge compensation of chromium centers in the strontium titanate crystal

Chromium centers and their charge compensation in two single strontium titanate crystals, i.e., SrTiO₃: Cr (0.05 at %) and Sr_0.9995TiO₃: Cr_0.0005 grown with strontium deficiency, have been studied by the electron paramagnetic resonance method. The crystals have been investigated both immediately after growth and after oxidation and reduction procedures. Oxidation and reduction are performed by crystal annealing in a corresponding gas atmosphere at high temperature. Chromium centers associated with oxygen vacancy (Cr³⁺-V _O) are detected in the reduced crystals. It is shown that strontium vacancies are formed in the crystal grown with strontium deficiency, which leads to a lowering of the tetragonal symmetry of (Cr³⁺-V _O) and Cr⁵⁺ centers to the orthorhombic symmetry. Possible compensation mechanisms for charges of various chromium centers are considered.     

Hydrogen maser atomic beam resonances

Au^? ions at anionic places are formed in gold doped crystals by a reducing treatment withF centers. The ultraviolet absorption consists of 4 bands, which are namedA, B, C, andD in analogy to the isoelectronic centers of the s² type, like Tl⁺. TheB band oszillator strength strongly increases with temperature in accordance with a phonon allowed transition. The ratio of the dipole strength of theC band to that of theA band as a function of the relative position of theB band is compared with Suganos prediction. Zero phonon lines are found at helium temperatures for theA band in NaCl (2,985 Å), KCl (3,068 Å), and KBr (3,145 Å) and for theC band in KCl (2,329 Å). In KCl the Huang-Rhys factor isg=3.4 for theA band. The vibronic structure comes from the relatively large radius 6s ² state of the negative ion. Uniaxial stress splits the zero phonon line. The results definitely agree with the stress splitting behaviour of a degenerateΓ ₁→Γ ₄ transition. Inversion symmetry of the center is confirmed by the absence of a linear Stark effect.     

Untersuchung von Neutronenreaktionen mit Emission von α-Teilchen an Na23 im Energiebereich von 12,6 bis 18,7 MeV

TheA band of Tl⁺ centers in mixed crystals of KCl—KBr, KBr—KI, and KCl—KI is split into several subbands at low temperatures. The degree of the splitting increases with the difference of the anion radii. The relative size of the subbands strongly depends on the mixture ratio and on the thermal history. Each of the subbands gives its own fluorescence spectrum. As in pure crystals it consists of two subbands. The shorter wavelength band dominates at low temperatures. We interpret the results by considering complexes of the general form TlX_nY_6?n (n=1, ..., 5). Their symmetry is lower than in a uniform surrounding (n=0; 6). As a consequence the degeneracies of the excited electronic states are lifted. In KCl with a sufficient low content of KI we get only TlICl₅ in addition to TlCl₆. From the equilibrium at different temperatures we find 0.07 eV for the energy of association of I^? to Tl⁺. TlICl₅ centers absorb in theA ₀ band, which is equivalent to theA band of the pure TlCl₆, and in anA ₁ band at longer wavelength. TheA _n1 fluorescence can be linearly polarized up to 60% in [100] direction. By applying uniaxial stress to the crystal in [100] direction at higher temperatures the TlICl₅ complexes can be partially oriented parallel to [100]. This gives a slight dichroism in theA ₁ band.     

Luminescence studies on gamma irradiated KCl: Ce crystals

This paper reports thermoluminescence(TL), optical absorption and TL emission studies that are made on Ce³⁺ doped KCl single crystals irradiated at room temperature. The glow curve and optical absorption studies indicate the participation of Ce³⁺ ions in the TL process. The TL study suggests the presence of low concentration of Ce³⁺ ions which reduces the TL efficiency with respect to pure KCl samples. On F bleaching γ irradiated crystals Z₁ centers are observed. A broader and strongly intense violet blue emission at 290, 370, 423 and 488 nm has been observed with 240 nm excitation. This emission has been attributed due to the transition from 5d(2D) excited energy level to the 4f¹ ground stable energy level (2F_5/2 and 3F_7/2) of Ce³⁺ doped KCl crystals.     

Farbzentrenassoziate mit (100)-Symmetrie (A und B-Zentren)

By light absorption in additively colored KCl A-centers can be developped under certain conditions. Irradiation with (100) polarized F-light produces a reorientation of these centers, which is observed by a characteristic dichroic absorption. From these measurements the symmetry and the two optical transitions A₁ und A₂of the A-center are derived. Experiments with Na-doped crystals suggest as a model for the A-centeran F-center assoziated to a single Na ⁺ ion in the KCl lattice. From A-centers a new center can optically be developped, which has again (100) symmetry and two optical transitions. This ?B-center“ is interpreted as the assoziation oftwo F-Centers to a Na ⁺ion in KCl.     

Photoinduced dichroism in additively coloured KCl: Na at room temperature

Spectral and kinetic investigations of the reversible photoinduced dichroism in KCl : Na containing F_A and F_A + F_B centers are done. It is suggested that in presence of F_B centers the observed 〈100〉 and 〈110〉 dichroism is connected with the F_B(II) centers.     

Effect ofV K centers on positron annihilation in KCl and Ag-doped KCl

Positron lifetime spectra have been measured at 77 K for KCl and Ag-doped KCl before and after x-irradiation at 77 K and after annealing at room temperature. Radiation at 77 K reduces the intensity of the intermediate lifetime (τ₂) component. Radiation-induced defects were monitored optically and by ESR. The experiment shows that the changes observed in the positron decay are associated with the presence ofV _K centers.     

Specific features of absorption and luminescence in CsBr: EuOBr crystals

The specific features of the absorption, photoluminescence, x-ray luminescence, thermally stimulated luminescence, and photostimulated luminescence spectra of CsBr: Eu²⁺ single crystals grown using the Bridgman method are investigated in the temperature range 80–500 K at the highest possible dopant content (0.1–0.4 mol % EuOBr in the batch) required for preparing perfect crystals. It is shown that an increase in the dopant content leads to a broadening of the absorption and photoluminescence excitation bands with maxima at wavelengths of 250 and 350 nm due to the interconfigurational transitions 4f⁷(⁸S_7/2) → 4f⁶5d(e _g , t_2g) in Eu²⁺ ions. The photoluminescence and photostimulated luminescence spectra of CsBr: EuOBr single crystals (0.1–0.4 mol % EuOBr) contain a band at a wavelength of λ_max=450 nm and bands at wavelengths of λ_max=508–523 and 436 nm. The last two bands are assigned to Eu²⁺-V_Cs isolated dipole centers and Eu²⁺-containing aggregate centers, respectively. It is revealed that the intensity of the luminescence associated with the aggregate centers (λ_max=508–523 nm) is maximum at an EuOBr content of less than or equal to 0.1 mol % and decreases with an increase in the dopant content. The possibility of forming CsEuBr₃-type nanocrystals that are responsible for the green luminescence observed at a wavelength λ_max=508–523 nm in CsBr: Eu crystals is discussed. The intensity of photostimulated luminescence in the CsBr: EuOBr crystals irradiated with x-ray photons is found to increase as the dopant content increases. It is demonstrated that CsBr: EuOBr crystals at a dopant content in the range 0.3–0.4 mol % can be used as x-ray storage phosphors for visualizing x-ray images with high spatial resolution.     

Generation of color centers when annealing LiF crystals irradiated by Kr ions with an energy of 150 MeV

The effect thermal annealing has on F and F _n centers in LiF crystals irradiated by Kr ions with an energy of 150 MeV is studied with allowance for fluence and ionic currents. It is found that annealing at a temperature of 400 K using crystals irradiated at a fluence of ≥10¹³ iom/cm² reduces the concentration of F centers (due to annihilation with H centers) and raises the concentration of complex F _n centers.     

O−−-Lücken-Dipole in Alkalihalogenidkristallen

Dielectric loss measurements are reported for KBr, KCl, and NaCl crystals containing O^?? ions. The frequency and temperature dependences can be well represented by a Debye curve. The dipole moment found immediately after quenching is interpreted as being due to O^?? ions associated with anion vacancies. Activation energies for reorientation of the ion are E₀=0.59 eV for KCl, 0.54 eV for KBr and 0.61 eV for NaCl. Fluorescence bands were found in freshly quenched KCl crystals at 470 nm and 1.05 μm and are attributed to isolated dipoles. Dielectric losses decreased and the optical absorption changed during annealing due to the collection of centers into aggregates. A detailed analysis of the kinetics for KCl indicates the formation of double dipoles as the first step. The energy of association is E_A=0.85 eV. Earlier measurements of absorption, fluorescence and photochemical coloring are reinterpreted on the basis of the proposed model in which an O^?? center is thought of as a O^?? ion associated with an anion vacancy.