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.     

Alteration of the lattice constant and the composition of KCl and KBr crystals under macrogravity

In KCl and KBr crystals grown from supersaturated aqueous solutions under centrifuging (3g, 7g, and 12g) the lattice constant and the halogen content are altered significantly in comparison with original compounds, and the microhardness is increased by 1.5–2 fold. Fiz. Tverd. Tela (St. Petersburg) 39, 1792–1793 (October 1997)     

Correlation between deformation bleaching and mechanoluminescence in coloured alkali halide crystals

The present paper reports the correlation between deformation bleaching of coloration and mechanoluminescence (ML) in coloured alkali halide crystals. When the F-centre electrons captured by moving dislocations are picked up by holes, deep traps and other compatible traps, then deformation bleaching occurs. At the same time, radiative recombination of dislocation captured electrons with the holes gives rise to the mechanoluminescence. Expressions are derived for the strain dependence of the density of colour centres in deformed crystals and also for the number of colour centres bleached. So far as strain, temperature, density of colour centres, E _a and volume dependence are concerned, there exists a correlation between the deformation bleaching and ML in coloured alkali halide crystals. From the strain dependence of the density of colour centres in deformed crystals, the value of coefficient of deformation bleaching D is determined and it is found to be 1.93 and 2.00 for KCl and KBr crystals, respectively. The value of (D+χ) is determined from the strain dependence of the ML intensity and it is found to be 2.6 and 3.7 for KCl and KBr crystals, respectively. This gives the value of coefficient of deformation generated compatible traps χ to be 0.67 and 1.7 for KCl and KBr crystals, respectively.     

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.     

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.     

Off-center substitutional ions in alkali-halide crystals

The potential wells of alkali and halogen ions as impurities in alkali halides are investigated by means of a deformation-dipole model with single-ion parameters. The calculations indicate that Li⁺ is off center in seven and F^? in six of the sixteen rocksalt-type crystals considered. In about half of the off-center systems the configuration has D_4h symmetry if the impurity is kept at its lattice position. The minimum-energy configuration of the off-center system KCl:Li⁺ is calculated in detail, and the influence of hydrostatic pressure is investigated. Our results are in qualitative agreement with experiment.     

Optical absorption of the F center as function of the temperature

Abstract

The temperature shift of the F absorption band for KBr and KCl single crystals was experimentally and theoretically determined. The theoretical calculations are based on the pseudopotential method using different wave functions and assumed a linear thermal expansion coefficient for the crystalline lattice. The change of F-center absorption energy due to collisions with the thermally vibrating lattice is calculated. This contribution is small. The measurements agree fairly well with the results obtained using the present model.     

Paraelastische Defektelektronen-Zentren in Alkalihalogeniden

We have investigated the stress-induced alignment of Cl₂ ^? and Br₂ ^? molecular ions substituting for Cl^? and Br^? in KCl and KBr, respectively (so calledH-centers). By means of the dichroism induced in the optical absorption (H-band) by uniaxial stress we determined the static coupling to the lattice and the kinetics of the reorientation. Contrary to the O₂ ^?-center, which also substitutes for an anion, the reorientation-process of theH-center can be described by an Arrhenius-relation. TheH-centers freeze in between 10 and 13°K. In KBr a new center was found which is closely related to theH-center, but differs in the kinetics of the reorientation. It is presumably anH-center associated with another lattice defect. Further we have studied the paraelastic properties of the defects which in KCl and KCl:NaCl give rise to theV ₁-band. We found that depending upon the doping of the crystal different defects produce theV ₁-band.     

Analysis of temperature and concentration dependence of flow stress in KCl–KBr single crystals with special reference to solute distribution

Available data on the temperature and concentration dependence of critical resolved shear stress (CRSS) of KCl–KBr solid-solution crystals containing 9, 17, 27 and 45?mol% KBr in the temperature range 77–230?K have been analyzed within the framework of the kink-pair nucleation model of plastic flow in solid- solution crystals. It is found that CRSS τ decreases with increasing temperature T in accordance with the model relation lnτ?=?A???BT, where A and B are positive constants. The CRSS τ at a given temperature depends on solute concentration c as τ?∝?c^p , where exponent p has a value between 0.33 and 0.57 as temperature T rises from 0 to 230?K. The model parameter W _o, i.e. binding energy between the edge-dislocation segment L _o involved in the unit activation process and the solute atoms close to it (T?→?0?K), which is inversely proportional to B, increases with solute concentration c monotonically as W _o?∝?c ^0.33 up to a critical value c _m?=?35?mol% KBr, which is in reasonable agreement with the model prediction W _o?∝?c ^0.25. However, W _o decreases with an increase in c beyond c _m, which indicates somewhat ordered distribution of solute in the host lattice of concentrated KCl–KBr solid solutions with c?>?c _m.     

Influence of uniaxial compression at 80 K on the formation of radiation defects in KCl, KBr, and KI crystals

The influence of uniaxial compression at 80 K on the efficiency of formation of stable radiation defects in KCl, KBr and KI crystals is investigated by absorption spectroscopy. It is found that compression along the 〈100〉 directions does not alter the efficiency of radiation defect formation in KCl and KBr crystals, but in KI the efficiency drops by more than an order of magnitude. It is concluded from a semiquantitative analysis that the observed difference is attributable to the impossibility of H centers fitting into the compression-reduced interstitial voids in KI, whereas several multiples of ten-percent compression is required to produce the analogous effect in KCl and KBr. Fiz. Tverd. Tela (St. Petersburg) 40, 73–78 (January 1998)     

Non-steady-state tunnelling recombination in insulating solids,controlled by defect diffusion and rotation

Abstract

The experimental data for KCl—Ag, KBr, NaCl—Ag and KCl—SO₄ crystals as well as Na₂O.3SiO₂—Tb³⁺ glass are summarized which demonstrate the delayed increase (decrease) of the intensity of the thermally stimulated tunnelling recombination of radiation defects after step-wise defrosting (frosting) of their mobility. Both theory and experiment presented here allow to distinguish the cases when the recombination kinetics is controlled by the rotation of an anisotropic defects or their diffusion respectively. It can be achieved studying the non-steady-state stages of kinetics. The effect of relative spatial defect distribution upon the kinetics under study is considered; it is argued that the {F, V _k } recombination in KBr occurs within spatially correlated pairs.     

Surface vibrations of CO on W(100)

High resolution electron-energy-loss spectroscopy has been used to study the surface vibrations of CO on a W(100) surface at 300 K. For small exposures (β-CO) two losses at ～68 meV and ～78 meV are observed. This vibrational spectrum of β-CO is a clear indication of dissociative adsorption with the carbon and oxygen atoms in fourfold coordination sites each. With further exposure to CO two additional losses at 45 meV and 258 meV are observed, which represent the vibration of undissociated α-CO in upright position on top of a W atom. Furtheron results of coadsorption of H₂/CO and O₂/CO on W(100) are reported.     

Self-consistent analysis of absorption and emission data forF A centres in alkali halides

Summary A theory which explains self-consistently the emission and the polarization rotation of transmitted light has been developed forF _A centres in crystals with cubic symmetry. Azimuthal dependences ofF _A luminescence in alkali halides are very informative for determining the electron-phonon interaction as well as the parameters related to the structural and optical quality for coulour centre lasers crystals. The theoretical results are applied to the interpretation of the luminescence properties ofF _A (Li) centres in KCl, RbCl and KF.     

Sum frequency generation and density functional studies of CO-H interaction and hydrogen bulk dissolution on Pd(1 1 1)

CO-H interaction and H bulk dissolution on Pd(1 1 1) were studied by sum frequency generation (SFG) vibrational spectroscopy and density functional theory (DFT). The theoretical findings are particularly important to rationalize the experimentally observed mutual site blocking of CO and H and the effect of H dissolution on coadsorbate structures. Dissociative hydrogen adsorption on CO-precovered Pd(1 1 1) is impeded due to an activation barrier of ∼2.5 eV for a CO coverage of 0.75 ML, an effect which is maintained down to 0.33 ML CO. Preadsorbed hydrogen prevented CO adsorption at 100 K, while hydrogen was replaced from the surface by CO above 125 K. The temperature-dependent site blocking of hydrogen originates from the onset of hydrogen diffusion into the Pd bulk around 125 K, as shown by SFG and theoretical calculations using various approaches. When Pd(1 1 1) was exposed to 1:1 CO/H₂ mixtures at 100 K, on-top CO was absent in the SFG spectra although hydrogen occupies only threefold hollow sites on Pd(1 1 1). DFT attributes the absence of on-top CO to H atoms diffusing between hollow sites via bridge sites, thereby destabilizing neighboring on-top CO molecules. According to the calculations, the stretching frequency of bridge-bonded CO with a neighboring bridge-bonded hydrogen atom is redshifted by 16 cm⁻¹ when compared to bridging CO on the clean surface. Implications of the observed effects on hydrogenation reactions are discussed and compared to the C₂H₄-H coadsorption system.     

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.     

Conductivity of ionic crystals when they are irradiated by picosecond electron beams

This paper discusses the pulsed electron conductivity σ of KCl, KBr, and NaCl crystals when they are excited by an electron beam (0.2 MeV, 50 ps) with current densities in the interval j=(30–10⁴) A/cm². It is shown that the lifetime of the electrons in the conduction band is τ≪100 ps. To explain the experimental σ(j) dependences, a model is proposed that includes electron capture by structural defects and stable radiation defects at low excitation densities and electron capture predominantly by unstable radiation defects generated by the excitation pulse at high excitation densities. Fiz. Tverd. Tela (St. Petersburg) 41, 1200–1203 (July 1999)     

Effect of absorbing microinhomogeneities on optical damage to Alkali-Halide crystals

Micropores up to 30?C100 ??m in size (bulk density ??10⁵ cm^?3) are obtained in NaCl, KCl, KBr, and RbI via the effect of a pulse of a CO₂ laser with power density 10⁶?C10⁷ W cm^?2 up to 5 ??s long. When a pore appears, plasma formation is initiated. The average temperature is ??5500 K and the pressure is ??10⁴ kg cm^?2. The dependence of the average weight of the material removed from the pore on the energy of the crystal lattice is found. Pore formation occurs mainly via the effect of the radiation pulse, due to evaporation at the absorption wave front (velocity, 5 m s^?1) and plastic deformation. The annealing kinetics of micropores and the effect of the ionizing radiation on the bulk pore formation in NaCl crystals are investigated.     

Temperature dependence of anomalous Rutherford scattering of fast electrons in crystals

In contrast to the channeling of positively charged particles, the effect of the crystal temperature on orientational scattering of fast electrons has not been investigated up to now. Only in [1] did they observe a decrease in the efficiency of the inverse scattering of ^–-particles upon the heating of <100> KCl and KBr crystals, which they explained by a decrease in the efficiency of focusing of the electrons on chains of atoms. In this article the effect of thermal vibrations of atoms on the interaction of electrons with single crystals is discussed within the framework of the model of bound states of fast electrons with atomic arrays and crystal planes, which consistently explains both the high penetration of particles into the lattice and the anomalous Rutherford scattering under channeling conditions [2, 3]. Detailed discussion is carried out for <111> and (100) Si crystals and electron energy E=(1.0–1.5) MeV.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 29–35, July, 1978.     

The applicability of the Debye model to thermal conductivity

The thermal conductivity of KBr crystals containing known concentrations of KNO₂ is studied at low temperatures. The conductivity of the doped crystals is greatly reduced and can be described with a simple relaxation time using the model of the thermal conductivity proposed byDebye. Crystals of KBr containing small concentrations of KI show a decrease in thermal conductivity very similar to that observed in mixed crystals with KCl as host lattice. This observation supports the model proposed byWagner that quasi localized modes act as phonon scatterers.