Electron-lattice interaction in the absorption spectra of thallium doped alkali halides

The electron-lattice interaction of NaCl:Tl⁺, KCl:Tl⁺, KBr:Tl⁺, and KI:Tl⁺ is discussed using the moments of the absorption bands. The discussion is based on a theory ofToyozawa andInoue andHonma. Consistency of the data is found for the absorption measurements. An analysis of the band shift under applied stress shows for theA-band in KCl and KBr and for theA- andB-band in KI that the electronlattice coupling constants derived from these data assuming next neighbour interaction differ considerably from those derived from the second moments of the bands.     

Photoelastic constants in silver and thallium halides

In the present communication the expressions for the photoelastic constants of the solids crystallizing in cesium chloride structure have been derived on the basis of lattice theory. These expressions have been utilized to predict the magnitudes of photoelastic constants in thallium halides. On the basis of these constants the validity of the Clausius-Mossotti model of electronic polarization in silver and thallium halides has been discussed.     

Absorption lineshape ofF A centres in alkali halides

Summary The lineshapes of the absorption bands ofF _A centres in alkali halides have been studied for the first time. The new method used for this investigation is based on the determination of the overlap between theF _A1 andF _A2 bands from luminescence measurements. The experimental results have been compared with calculated values deduced from the theoreticalF _A bands of different shapes. For bothF _A (I) centres in KCl: Na⁺ andF _A (II) centres in KCl: Li⁺ and RbCl: Li⁺ the absorption lineshape at low temperature is much closer to a sum of two Lorentzian curves than that of two Gaussian or Poissonian bands. This result shows an unexpected difference with theF centres, whose absorption lineshape is known to be Poissonian at the same temperatures.

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Riassunto Sono state studiate per la prima volta le forme di riga delle bande di assorbimento dei centriF _A in alogenuri alcalini. Il nuovo metodo usato per questa indagine è basato sulla determinazione attraverso misure di luminescenza della sovrapposizione fra le bandeF _A1 eF _A2 . I risultati sperimentali sono stati confrontati con i valori dedotti da bandeF _A teoriche di diverse forme. La forma di riga dell'assorbimento a bassa temperatura, sia per i centriF _A (I) in KCl: Na⁺ sia per i centriF _A (II) in KCl: Li⁺ e RbCl: Li⁺, è approssimata molto meglio da una somma di due curve Lorentziane che da quella di due bande Gaussiane o Poissoniane. Questo risultato rivela una differenza imprevista rispetto ai centriF, il cui assorbimento ha una forma di riga Poissoniana alle stesse temperature.

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Резюме Впервые исследуются формы линий зон поглощения дляF _A центров в щелочно-галоидных соединениях. Предлагается новый метод исследования. Этот метод основан на определении перекрытия междуF _A иF _A2 зонами из измерений люминесценции. Экспериментальные результаты сравниваются с вычисленными величинами, полученными из теоретическихF _A зон различной формы. ДляF _A (I) центров в KCl: Na⁺ иF _A (II) центров в KCl: Li⁺ и RbCl: Li⁺ форма линии поглощения при низких температурах оказывается ближе к сумме двух Лоренцевых кривых, а не двух Гауссовых или Пуассоновых зон. Этот результат обнаруживает неожиданное отличие отF центров, форма линии поглощения которых, как известно, представляет Пуассонову кривую при тех же температурах.

     

Colour centres in microcrystalline powders of alkali halides

Abstract

Powders of twelve alkali halides have been coloured in an electrodeless discharge. Due to the simultaneous production and bleaching of F centres, F aggregate centres could be produced in an adequate concentration. From their study it has been concluded that powders behave as deformed single crystals. It could also be shown from this study that the interaction between colour centres and dislocations is lattice dependent and varies regularly as cation changes from Li⁺ to Rb⁺, and anion changes from F ^?to I^?.     

New high-pressure polymorphs in sodium halides

Static compression experiments to 50 GPa, employing X-ray diffraction through a diamond cell, were made on NaF, NaBr and NaI. NaF was found to transform from its initial B1 (NaCl-type) to the B2 (CsCl-type) structure at 27 ± 1GPa on the ruby fluorescence scale with a volume change at the transition of ?8.9%. New high-pressure polymorphs showing birefringence under microscope were found both for NaBr and NaI at 29 ± 1 GPa and 26 ± 1 GPa, respectively. X-ray diffraction patterns of these high-pressure polymorphs could not be indexed as the B2 structure. The GeS-type structure (the distorted NaCl structure) was tentatively assigned to the high-pressure polymorph of NaI.     

Optical storage in doped microstructures of alkali halides

Peculiarities of colour centres production and their recombination in photostimulated processes in doped alkali halide microstructures were examined in connection with their practical use as active photostimulable media in miniaturised optoelectronic and photonic devices. The specific interaction of unrelaxed H-centres and electrons with the dopants in different valence and electronic states open a way for widening the scope of multifunctional (logical and mathematical) optical data processing and transfer.     

Direct determination of site symmetry forF A centres in alkali halides

Summary The evaluation of site symmetry distortion for an impurity ion in cubic crystals is a problem which obtained attention from a number of investigators during the last decade. So far, all approaches were based on multi-parameter fittings of the experimental results by some theoretical curves. A new technique, described in this paper, makes it possible to obtain the site symmetry distortion parameter in a straightforward fashion directly from the experimental data. The method is based on precise measurements of the light polarization azimuth for a beam transmitted by a crystal with reorienting centres. results are reported forF _A (Li) centres in KCl and RbCl.     

Off-axis configuration of FA(II) centres in alkali halides

Abstract

The experimental values of the off-axis angle of F_A(II) dipoles in KCl:Li⁺, RbCl:Li⁺ and KF:Li⁺ are in good agreement with those obtained from theoretical estimates of the off-centre displacements of the Li⁺ ion calculated with the shell model potentials. On this basis, one can quantitatively predict the off-axis behaviour of F_A(II) centres from the calculated off-centre positions of small impurity ions in alkali halides.     

Anomalous decay and breather formation in doped alkali halides

Anomalous decay of doped alkali halides has been ascribed to breather formation in the immediate neighborhood of the impurity. New results support this connection. We report experimental data for NaBr and RbBr crystals showing anomalies in their slow emission decay. These data complete the earlier picture, confirming that the decay anomaly becomes bigger as the host-lattice-anion/cation mass ratio increases. We show the correlation between the decay anomaly and the breather formation as a function of this ratio. By simulating finite-temperature effects (which do not just involve white noise) in the lattice dynamics, we demonstrate that with increasing temperature the breather weakens, as is experimentally observed for the decay anomaly.     

On the luminescence quenching of F centres in alkali halides

Non-radiative transitions from the excited state of F center to the ground state occur very efficiently around and above, over a rather large interval of energy, the crossing point of the two adiabatic potential energy curves. Taking this effect into account the criterion for luminescence quenching originally proposed by Dexter et al. is re-examined. The luminescence quantum efficiency is estimated for those alkali halides in which the luminescence is weak, and the results are in general accord with experiment. The border-line case of NaCl is also explained.     

Aggregation pathways and Suzuki phase formation in doped alkali halides

A model about Suzuki phase formation in Alkali Halides doped with divalent cationic impurities is presented. It considers rearrangement of small aggregates as dimers and trimers. Experimental data of Suzuki phase nucleation at room temperature in NaCl:Cd²⁺ corroborate this model.     

Theoretical investigations on structural, elastic and electronic properties of thallium halides

Theoretical investigations on structural, elastic and electronic properties, viz. ground state lattice parameter, elastic moduli and density of states, of thallium halides (viz. TlCl and TlBr) have been made using the full potential linearized augmented plane wave method within the generalized gradient approximation (GGA). The ground state lattice parameter and bulk modulus and its pressure derivative have been obtained using optimization method. Young's modulus, shear modulus, Poisson ratio, sound velocities for longitudinal and shear waves, Debye average velocity, Debye temperature and Grüneisen parameter have also been calculated for these compounds. Calculated structural, elastic and other parameters are in good agreement with the available data.     

Mollwo-Ivey relations for emission bands in copper doped alkali halides

The fluorescence spectra were measured of nine alkali halides with a low concentration of monovalent copper. Relations analogous to Mollwo-Ivey law for F-centers and for lead doped alkali halides were found to be satisfied by the measured wavelengths corresponding to the emission band maxima. The Stokes shifts, however, were here essentially larger as compared to those of lead doped alkali halides. Their order of magnitude is about the same as for F-centers.Dedicated to Prof. Dr. I. Tarján on occasion of his 60th birthday.Cukrovarnická 10, Praha 6, Czechoslovakia.The authors wish to express their appreciation to Dr. M. Lébl, CSc., and Ing. . Barta, CSc., for growing the crystals and to Mrs. I. Velická for the polarographic analysis.     

Magnetic circular dichroism in the luminescence ofF andF A centres in alkali halides

Summary The magnetic circular dichroism has been in the past an excellent tool to clarify the optical cycle of theF centres in alkali halides and the nature of the relaxed excited state. We have used such technique to study the weak doubleF emission found recently in NaI and the luminescence ofF andF _A centres (of both type I and II) in KCl. We have found that the long-wavelengthF emission in NaI and theF _A (I) emission in KCl∶Na⁺ behave like the well-known emission of theF centres, confirming the attribution of the above luminescence in NaI to the normal relaxed excited state. A completely different behaviour is displayed by the short-wavelengthF emission in NaI and by theF _A (II) emission in KCl∶Li⁺. The experimental data have been compared with the Ham and Grevsmuhl vibronic theory of the relaxed excited state.

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Riassunto Gli effetti di dicroismo magnetico circolare hanno costituito in passato un ottimo mezzo per chiarire il ciclo ottico dei centriF in alogenuri alcalini e la natura dello stato eccitato rilassato. Abbiamo usato tale tecnica per studiare la debole doppia emissioneF trovata recentemente in NaI e la luminescenza dei centriF edF _A (di tipo I e II) in KCl. Si è trovato che l'emissioneF a minore energia in NaI e l'emissioneF _A (I) in KCl∶Na⁺ si comportano come la ben nota emissione dei centriF, il che conferma l'attribuzione di tale luminescenza in NaI allo stato eccitato rilassato normale. L'emissioneF a energia maggiore in NaI e l'emissioneF _A (II) in KCl∶Li⁺ mostrano un comportamento completamente diverso. I dati sperimentali sono stati confrontati con la teoria vibronica di Ham e Grevsmuhl dello stato eccitato rilassato.

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Резюме Магнитный круговой дихроизм ранее применялся как инструмент для выяснения оптического циклаF центров в щелочных галогенидах и природы релаксирующего возбужденного состояния. В этой работе мы используем эту технику для исследования слабого двойного F излучения, обнаруженного недавно в NaI и при люминесценцииF иF _A центров (типа I и типа II) в KCl. Мы обнаружили, что длинноволновоеF излучения в NaI иF _A (I) излучение в KCl∶Na⁺ ведет себя как хорошо известное излучениеF центров, что подтверждает приписывание вышеуказанной люминесценции в NaI нормальному релаксирующему возбужденному состоянию. Полностью отличное поведение обнаруживает коротковолновоеF излучение в NaI иF _A (II) излучение в KCl∶Li⁺. Полученные экспериментальные данные сравниваются с вибрационной теорией Гама и Гревсмула для релаксирующего возбужденного состояния.

     

Optical and dosimetric properties of variously doped SrF2 crystals

Optical properties and irradiation effects of Nd³⁺-, Pr³⁺-, Tb³⁺- and Tm³⁺-doped SrF₂ crystals and their possible application to solid-state dosimetry were studied and compared to those induced in pure SrF₂ crystals. Optical absorption, thermoluminescence (TL), X- and light-induced luminescence (XL and PL) as well as optically stimulated luminescence and phototransferred TL (OSL and PTTL) were measured in the various samples. Special attention was given to effects of monochromatic vacuum ultraviolet (VUV) radiation. TL was excited in the pure and doped samples by X and β rays as well as by VUV radiation. TL peaks appeared after VUV irradiation at the same temperatures and with the same thermal activation energies as after X or β irradiation, indicating that they are due to the same processes. The VUV excitation spectra showed two maxima at about 145 and 130 nm. Comparison of the TL sensitivities of the various TL materials, showed that the sensitivity of SrF₂:Pr³⁺ was by more than an order of magnitude higher than that of the known LiF:Mg,Ti (TLD-100) phosphor. The sensitivity of pure and of the Nd, and Tb-doped SrF₂ were by a factor of 2–4 higher than that of the TLD-100 and that of SrF₂:Tm was slightly lower. The main emission bands of SrF₂:Pr³⁺ are located in a convenient spectral region between 460 and 640 nm, where most of the standard photomultipliers are sensitive. The dose dependence of the 460 K TL peak in SrF₂:Pr³⁺ is nearly linear in a wide range up to above 27 000 Gy.     

Absorption and luminescence of the bound exciton in thallium doped silicon

We report the photoluminescence and absorption spectra of the bound exciton in Si:Tl. Four lines are observed in both spectra in contrast to three lines observed in the Al, Ga, In doped Si. The total oscillator strength is approximately 4x10^-4 continuing a trend of increasing oscillator strength with binding energy.