Excitation of electroluminescence in alkali halide compounds

In ionic crystals the concentration of free carriers and the mean free path are usually small and the forbidden band is large. The field energy is in the main spent on heating the crystal. Therefore, it is difficult to accelerate an appreciable number of electrons to the energy necessary for the excitation of electroluminescence.One can overcome these difficulties by removing a thin layer of the luminophore. With the decrease in the layer thickness the development of the cascade breakdown becomes more difficult, and the thermal exchange with the electrodes becomes better. This makes it possible to increase the field up to the values necessary for tunnel ionization of the luminescence centres.We found electroluminescence of the sublimated layer of CsI-Tl.     

Brightness waves and brightness-current curves in alkali halide electroluminescence

A kinetic model has been formulated for electroluminescence (EL) in alkali halide crystals (AHC) on the basis of existing measurements and theoretical concepts on prebreakdown electroluminescence. An expression is derived for the intensity, which contains parameters S₁, S₂, and S₃, which are the probabilities of excitation, ionization, and stepwise ionization for the emission centers when an electron passes through section . If the current density is not more than 1 A/cm², the EL brightness waveform reproduces the exciting current pulse within the excited-state lifetime, with the amplitude directly proportional to the current density. As the current density increases, the wave becomes damped, because the ionized emission centers are excluded from the EL. The observed waves for KBr have been examined for current densities up to 1 A/cm² and agree with the theoretical ones. A method is proposed for determining S_x as a function of emission-center concentration from the initial parts of the brightness-current curves.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 82–86, April, 1988.     

Effective dielectric functions of alkali halide composites and their spectral representation

In the electrostatic approximation inhomogeneous samples with characteristic scale of inhomogeneities much smaller than the wavelength of light can be characterized by an effective dielectric function _eff. This paper analyzes in which cases simple mixing formulas can be used to calculate _eff from the dielectric functions of the consituents. These considerations are compared with reflectance measurements of alkali halide composites in the far infrared. In many cases it turns out that it is essential to employ the general ansatz of the Bergman spectral representation in order to describe all effects due to the geometrical arrangement of the phases in a proper manner. It is necessary to adjust Bergman's spectral density to measurements. This can be done by use of a Monte-Carlolike algorithm. Further on we try to answer the question how to extract the dielectric function of one of the constituents from measurements of the effective dielectric function.     

The influence of heating on the spectral characteristics of pure alkali halide NaCl crystals

The influence of high-temperature heating on the spectral characteristics of pure natural NaCl crystals is studied. It is shown that defects changing spectral characteristics of crystals are formed in them when heated. Temperature dependences of the luminescence intensity growth for these defects are described by the hyperbolic function. The action of X-rays on heated crystals results in the formation of new defects luminescent in regions of 435 and 570–585 nm. The efficiency of formation of these defects is obtained as a function of the temperature of heat pretreatment. Regularities in variations of the location and the intensity of luminescence of the defects obtained under the action of ultraviolet radiation and on long storage are determined.     

Dielectric properties of alkali halide and alkaline earth oxide crystals

The deviations of ionic charge and compressibility from their nominal values in alkali halides and alkaline earth oxide crystals have been discussed and explained in terms of a theory based on shell model and exchange charge interactions originally developed by Dick and Overhauser. The exchange charge polarization parameters have been evaluated in the present study using the appropriate values of overlap integrals. It has been concluded that the second neighbour short range forces have significnant magnitudes in alkaline earth oxides.     

Crystalline properties and colour centres in alkali halide thin films

Abstract

Thin films of KCl, KBr, RbCl have been obtained by thermal evaporation on amorphous substrates with different deposition parameters. The crystalline structure and orientation have been determined, and the films resulted to be policrystals with high uniformity of orientation. Production of colour centres, achieved by irradiation with low-energy electrons, leads to F center concentrations barely observed in large crystals. The colouration kinetics is similar to that in the bulk, and shows after a maximum an exponential decay at high doses because of centre aggregation coupled to thermal effects. The films exhibit a bleaching process of the colour centres at room temperature, whose kinetics depends on the irradiation damage.     

Bombardment of alkali halide crystals with medium-energy atoms

The sputtering characteristics of KCl and NaCl alkali halide single crystals by bombardment with argon and mercury atoms are investigated. The sputtering coefficients are measured in the energy range 0.5–3 keV. Anisotropy is found in sputtering at various incident angles of the beam. The yield of charged particles during bombardment of thin specimens (of the order of 8–20 m) of KCl single crystals is investigated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 55–59, July, 1984.     

On the structure of alkali halide solutions

Summary The structure factors of NaI solutions have been determined by employing both neutron and X-ray scattering. The results have been analysed in terms of the structure of hydration spheres of both Na⁺ and I⁻ ions. No concentration or temperature dependences of the size of the hydration shells have been evidentiated in the present experiment, while the structure of the free water appears to be affected, at least at small distances.

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Riassunto I fattori di struttura delle soluzioni acquose di NaI sono stati misurati impiegando la diffrazione di raggi X e neutroni. I risultati sono stati analizzati per caratterizzare le sfere di idratazione degli ioni Na⁺ ed I⁻. Nessuna dipendenza dalla temperatura o concentrazione è stata evidenziata per le sfere di idratazione, mentre alle piccole distanze si osservano effetti della concentrazione sulla sutruttura dell'acqua libera.

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Резюме Используя дифракцию нейтронов и рентгеновских лучей, определяются структурные факторы растворов NaI. Полученные результаты анализируются в терминах структуры сферы гидратации ионов Na⁺ и I⁻. В проведенных экспериментах не обнаружено зависимостей от концентрации или темиературы для сферы гидратации, тогда как, по крайней мере, на малых расстояниях обнаружено влияние концентрации на структуру свободной воды.

     

On the structure of alkali halide solutions

Summary The adiabatic compressibility of NaCl, NaBr and NaI water solutions has been determined by measuring the sound velocity as a function of both concentration and temperature. A simple model has been employed to interpret the data in terms of size of hydration shells. Reasonably good agreement has been found with neutron and X-ray diffraction results.

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Riassunto La compressibilità adiabatica delle soluzioni acquose di NaCl, NaBr ed NaI è stata misurata determinando la velocità del suono in funzione di temperatura e concentrazione. Per interpretare i dati è stato impiegato un modello semplice che tiene conto delle sfere di idratazione degli ioni. Tale modello risulta in soddisfacente accordo con i dati ricavati da un esperimento di diffrazione di raggi X e neutroni.

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Резюме Определяется адиабатическая сжимаемость водньх растворов NaCl, NaBr и NaI, используя измерения зависимостей скорости звука от концентрации и температурь. Предлагается простая модель для интерпретации данньих в терминах размера сферьі гидратации. Получется хошее согласие с результатами дифракции нейтронов и рентгеновских лучей.

     

Photoelectric work function changes during heterogeneous adsorption of alkali halide molecules on alkali halide crystals

The changes of photoelectric work function during heterogeneous adsorption of alkali halide crystals had been observed. They were attributed to the non-zero effective dipole moment of the adsorbed layer, and were estimated using the Helmholtz formula. Measurements of this effect showed semi-quantitative agreement with the theoretical model. The divergences between experimental data and theoretical calculations were briefly discussed.     

Atomic structure of alkali halide surfaces

The atomic structure of surfaces of alkali halide crystals has been revealed by means of high-resolution dynamic force microscopy. True atomic resolution is demonstrated both on steps surrounding islands or pits, and on a chemically mixed crystal. We have directly observed the enhanced interaction at low-coordinated sites by force microscopy. The growth of NaCl films on metal surfaces and radiation damage in a KBr surface is discussed based on force microscopy results. The damping of the tip oscillation in dynamic force microscopy might provide insight into dissipation processes on the atomic scale. Finally, we present atomically resolved images of wear debris found after scratching a KBr surface. PACS 68.37.-d; 68.37.Ps; 75.55.Fv     

Lattice energy of alkali halide crystals

A new repulsive term in the ionic interaction potential ψ(r) = Ar^?ne^?r/gl, is suggested and the three unknown parameters A, λ and n are evaluated. Lattice energies of alkali halide crystals are calculated using this form. The results agree fairly well with the experimental values.     

Optical breakdown of alkali halide crystals

It is shown that the metallization of a dielectric in the region of its interaction with laser radiation is a substantiated mechanism of laser damage of wide-gap dielectrics. Calculations of the radiation pressure produced by high-power laser radiation and the pressure of external bulk compression at which the dielectric energy-gap width becomes zero and which is calculated on the basis of the self-consistent statistical electron theory of ionic crystals have shown that these pressures are of the same order of magnitude.     

Morphology of alkali halide thin films

Two alkali halide adsorbate-substrate systems were investigated by atomic force microscopy (AFM) working in contact mode. Adsorbate film orientation relative to the substrate was determined from the arrangement of the atomic steps of the substrate and the edges of the forming islands. In this work we present experimental results obtained for systems: NaCl/LiF(0 0 1) and LiF/NaCl(0 0 1), which exhibit a strong tendency of the self-assembly into regular structures.     

Debye temperature of alkali halide crystals

The Debye temperature _D is calculated from the elastic constants for 18 alkali halide crystals. Comparison of the results with calorimetric data _D reveals an agreement within the experimental error. The temperature dependence of _D is determined for NaCl, NaBr, KCl, and KBr crystals from the elastic constants, and this dependence is compared with the calorimetric data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 3, pp. 89–94, March, 1971.     

Electroluminescence of alkali halide crystals activated with europium ions

The prebreakdown electroluminescence of NaCl-Eu, KCl-Eu, and KBr-Eu crystals has been studied. The activator concentration in the crystals was on the order of 10¹⁸ ions per cubic centimeter. The emission spectra, the brightness wave during pulsed excitation, and the concentration dependence of the quantum yield were measured. The experimental results support the suggestion of an impact mechanism for the excitation of the electroluminescence of alkali halide crystals. Recombination and exciton mechanisms for energy migration play an insignificant role. There is no significant contribution from impurity-vacancy dipoles, which act as emission centers, in electric fields up to 10⁶ V/cm.Tomsk Academy of Control and Electronic Systems. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 105–109, February, 1994.