Doped Alkali Halides Research Articles

Ion Beam Induced Luminescence of Alkali Halide Crystals

AbstractNew experimental results bearing on luminescence induced by ion beam on pure and doped alkali halide crystals are reviewed. In the optical spectra besides activator luminescence bands new intrinsic luminescence bands are observed. The mechanism of the recombination of electrons from the conduction band with V2 hole centre is discussed.

Afterglow and Thermoluminescence Response of Europium-Doped Alkali Halides Dosemeters

Abstract It is well known that Eu2+-doped alkali halides show a persistent luminescence, often referred to as afterglow, after being subjected to ionising radiation. Several mechanisms responsible for the observed afterglow and thermoluminescence have been proposed which are dependent on the type of alkali halide crystal, of radiation used and the aggregation - precipitation state of the impurity. We have carried out detailed afterglow and thermoluminescence measurements in europium-doped KC1, irradiated with X rays. Our conclusions support the occurrence of at least two main mechanisms originating in the existence of impurity vacancy dipoles and the recombination of F centres and other defects with interstitial halogen ions.

Afterglow and Thermoluminescence Response of Europium-Doped Alkali Halides Dosemeters

It is well known that Eu2+-doped alkali halides show a persistent luminescence, often referred to as afterglow, after being subjected to ionising radiation. Several mechanisms responsible for the observed afterglow and thermoluminescence have been proposed which are dependent on the type of alkali halide crystal, of radiation used and the aggregation - precipitation state of the impurity. We have carried out detailed afterglow and thermoluminescence measurements in europium-doped KC1, irradiated with X rays. Our conclusions support the occurrence of at least two main mechanisms originating in the existence of impurity vacancy dipoles and the recombination of F centres and other defects with interstitial halogen ions.

Optical Detection of magnetic resonance without microwaves via MCD of F-centers in doped alkali halides

Abstract By monitoring the magnetic circular dichroism (MCD) of F-centers in irradiated alkali halides EPR of impurity centers was detected without microwaves due to a cross-relaxation of the impurities with optically pumped F-centers. Cross-relaxation effects were also observed in magnetic resonance detected via MCD in absorption and the intensity of spin-dependent recombination emission in ionic crystals doped with transition and rare earth ions.

Peculiarities of the dislocation ensemble motion in doped alkali halide crystals

Abstract Correlation between the dislocation mobility in the stress field of a concentrated load and other plastic deformation parameters for doped NaCl crystals was studied. The good correlation between all parameters under investigation was found in the case of the impurity hardening and the disturbance of this correlation was established in the case of the impurity softening. The influence of impurity state on the low-temperature anomaly of dislocation mobility was revealed.

(F 2+) H-centers in doped alkali halides : Production and optical properties

A study of the F 2 + center production using different dopant anionic impurities (OH −, CO 3 2−, NO 3 − and SO 4 2−) has been performed in alkali halides. The chemical reactions during the additive coloration is discussed. The optical properties (absorption, luminescence, quantum efficiency and lifetime) of F 2 +-like centers have been investigated, and the photoconversion between different F 2 + centers has also been analyzed.

Electronic spectroscopy of KF:Cu +

A KF:Cu + crystal was grown and studied to develop a more comprehensive understanding of the spectral properties of the Cu + -doped alkali halides. Excitation, emission, vacuum ultraviolet absorption spectra, and emission lifetimes were obtained in order to examine the properties of the electronic transitions of the Cu + impurity. The ground state of Cu + was determined to be on-center in KF, although the 3E g excited state may be off-center. The spin-orbit split levels of the 3E g state were found to be separated by 46 cm -1, and the 3d→4s and 3d→4p transitions were observed at 38 000 cm -1 and 53 000 cm -1, respectively.

350 nm laser radiation effects on Eu 2+ in alkali halides

Europium doped alkali halides, when irradiated by a XeF excimer laser, exhibit changes in their absorption spectra similar to those induced by ionizing radiation. Initial growth rates of the laser-induced absorption show a 3-photon absorption mechanism which, in turn, is used to address the nature of excited state absorption of europium in alkali halide hosts.

Modelling of the Self-Trapped Excition π Luminescence in Alkali Halides

On the basis of recent works which show that the self-trapped exciton in alkali halides and other ionic halides are essentially a nearest neighbour pair of F – H centers, we show that the emission band peak energy and lifetime of the π band in pure and heavier halogen doped alkali halides are correlated to the off-center shift of V k -core. It is suggested that parameters of the π emission bands can be predicted on the basis of this correlation.

Thermally stimulated luminescence studies in plutonium- and americium-doped alkali halides

Abstract Thermally stimulated luminescence (TSL) studies were conducted on 239 Pu- and 241 Am-doped NaCl and KCl. Both self-irradiated (by α-particles) as well as externally γ-irradiated samples were investigated. In general, the TSL glow curves were found to be similar under these two irradiation conditions for both types of samples. In addition to the intrinsic emission of the host lattices, characteristic emissions of Pu 3+ /Am 3+ were also observed in the 239 Pu/ 241 Am doped alkali halides. The main glow peaks observed above room temperature are associated with the partial thermal destruction of radiation-induced F -centres and the total destruction of F - and M -centres. The trap parameters for the different glow peaks have been determined.

Random Interaction among Dipolar Impurities in Alkali Halide: KI1-x(NO2)x

The effect of random interaction among dipolar impurities on the dielectric properties of doped alkali halide KI 1- x (NO 2 ) x was investigated through the dielectric measurement. The dielectric constant shows a maximum at a temperature T F , which depends on both the measuring frequency and the NO 2 concentration x . The dielectric dispersion data represented in the conventional. Cole-Cole plot show characteristic asymmetry, and the data having the same value of T / x fall on a universal curve. These features of the obtained Cole-Cole plot agree qualitatively with theoretical predictions based on a Glauber model with random dipole-dipole interaction.

X‐Ray Induced Relaxation Band Due to Higher Aggregates in KCl:Pb

AbstractAlthough aggregation of impurity centres in doped alkali halides upon X‐irradiation is proposed earlier, direct evidence of such centres having dipole moment is reported here for the first time by the method of ionic thermocurrents (ITC). The norma Idipolar relaxation band in KCl: Pb is located around 224 K, while sometimes a small band at about 160 K is found in the ITC spectrum of heavily doped systems. Exposure to X‐rays increases this band around 160 K at the cost of the 224 K dipolar band. Optical absorption in the A‐band of KCl:Pb, along with ITC measurements indicates that higher aggregates of I–V dipoles are easily formed in heavily doped systems. Formation and growth of the 160 K ITC band upon X‐ray irradiation are explained from the fact that the effect of X‐irradiation is to increase the rate of aggregation of I–V dipoles, through temporary electron trapping by lead ions.

Identification of a Tl dimer centre in alkali halides by ODMR

In Tl+-doped alkali halides laser-active centres can be produced by ionising irradiation. This treatment inevitably produces other Tl-associated centres, which might degrade the laser performance. One of these centres that absorbs in the wavelength region where the laser centre is pumped was identified in the hosts KCl, RbCl and KBr by measuring its magnetic circular dichroism (MCD) tagged by electron spin resonance (ESR). The positions of the lowest-energy absorption and emission bands as well as sign and magnitude of the MCD resemble closely those of the laser centre. From the analysis of the optically detected ESR the structure of this new centre was determined. It consists of two neighbouring Tl+ cations adjacent to an anion vacancy, which share an electron. The ESR spectrum is explained by a fast hopping of the unpaired electron between the two Tl+ positions.

A theoretical study of the formation and aggregation of vacancy-impurity dipoles in divalently doped alkali halide crystals

Abstract Computer simulation techniques based on generalized Mott-Littleton methods have been used to investigate defect structures in systems resulting from the incorporation of Mn2 +, Cd2+ and Pb2+ ions into NaCl, KC1 and KBr crystals. Two-body interionic potentials based on the simple shell model were used in the calculations. The (110) nearest-neighbour (n.n.) and (200) next-nearest neighbour (n.n.n.) simple impurity-vacancy dipoles were found to be of comparable stability, although definite trends with ion size are evident, whereas the (211) complex invariably is found to be less stable. The additional binding energies which arise when the n.n. and n.n.n. dipoles aggregate into a variety of dimers, trimers and tetramers have been calculated, as has the stability of the Suzuki phases. In the present work the results are compared with available experimental data and possible pathways for the formation of the Suzuki phase are discussed.

Thermoluminescence in alkali halides

Thermoluminescence studies in alkali halides (NaCl type) gained momentum in the last two decades with several groups working all over the world. Vigorous efforts are made to understand the TL mechanisms and the exact processes that precede the TL emission. This is a brief review of recent work on alkali halides with stress on explaining the complete mechanism of thermoluminescence in some of the pure and impurity doped alkali halide systems studied in the author's and other laboratories.

Luminescence in pure and Tl + doped alkali halides, excited by synchrotron radiation

Vacuum ultraviolet excitation of intrinsic luminescence and of the Tl + emission in alkali halides yields information on energy transfer processes. 7

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:Cd 2+ corroborate this model.

Light scattering by decorated dislocations in alkali halide crystals

The light scattering width of a dislocation decorated with lattice vacancies and impurities in doped alkali halide crystals is calculated. The contributions of the strain fields of dislocation and point defects to the change in dielectric constant are also considered. The calculations show that the decoration does not significantly change the light scattering from dislocations and it is the strain field of the dislocation that contributes most to the light scattering. Recent light scattering data and their interpretations are discussed in the light of the results of the present calculation.

ODMR study of Tl0centres in KC1

Abstract Recently Lüty and coworkers1 achieved efficient laser action from a colour centre in Tl doped alkali halides, which they described as F A (T1) centres. Also recently Goovaerts et al. 2 identified by ESR a T10 atom next to an anion vacancy, which was called Tl0(l) centre. We proved with ODMR experiments unambiguously the identity of both centres. An “optical excitation” spectrum of the ODMR lines reveals several absorption bands of this centre peaking at 1040, 725, 550, 405, 335, 290 and 254 nm. The magnetic circular dichroism of the two low energy bands can be quantitatively explained with the wave functions of a Tl0 atom in the axial field produced by the anion vacancy.

Optical gain measurements in doped alkali-halides

In order to verify the possibility of obtaining laser action in doped alkali-halides crystals, optical amplification measurements have been performed by the amplified spontaneous emission (ASE) technique. Results obtained in CsI : In(Tl) crystal are reported and discussed.