Journal of Applied Spectroscopy, Vol. 69, No. 5, 2002
ABSORPTION SPECTRA AND THERMOSTIMULATED LUMINESCENCE OF IRRADIATED KBr CRYSTALS WITH A HIGH CONTENT OF AN Na IMPURITY A. T. Akylbekov, K. S. Baktybekov,* A. K. Dauletbekova, and K. O. Kaliakparov
UDC 548.4.03.535
The influence of high concentrations of the Na+ impurity on the photo- and thermostimulated luminescence in an x-irradiated KBr crystal has been studied. It is revealed that at an impurity concentration of 1 mol.% the absorption spectrum of the KBr:Na crystal possesses only the bands belonging to the (α, IA) and (F, HA) pairs of the centers and that the thermoluminescence spectrum consists of two peaks with a maximum at 140 and 165 K. Keywords: photo- and thermostimulated luminescence, crystal, impurity, absorption spectrum. Investigations were conducted using standard procedures such as absorption spectroscopy and the study of the thermostimulated luminescence spectra, and also the measurement of the recombination luminescence spectra. The absorption spectra were measured on a Specord UV VIS spectrophotometer in the region 1.5–6.2 eV. The crystals, previously irradiated with x-rays, were exposed to light from a DKsShRB 3000 xenon lamp through a ZMP-3 monochromator or an OI-24s incandescent lamp with a corresponding set of glass light filters. We grew KBr:Na single crystals by the Stockbarger method in ampoules made of synthetic quartz in a special furnace with automatic regulation of temperature. The crystals with a high concentration of Na (0.3–10.0 mol.% in a melt) were grown from the material freed only from OH groups without preliminary zone melting. The concentration of the Na impurity was determined by means of a semiquantitative spectrographic analysis. We found that if 0.3, 2.0, and 10.0 mol.% NaBr are added to a melt of KBr, then 0.005, 0.400, and 1.000 mol.% of the Na impurity is incorporated in the KBr crystal, respectively. It is evident from the absorption spectra (Fig. 1a) that, instead of the large set of various defects characteristic of the KBr crystal with a low impurity concentration (curve 2), in the absorption spectrum of KBr with a high Na concentration there are only bands belonging to the (α, IA) and (F, HA) pairs of centers (curve 1). To the simple absorption spectrum of KBr with high Na concentration there corresponds also the simple thermoluminescence spectrum (Fig. 1b, curve 1). It consists only of two peaks of the thermostimulated luminescence (TSL) with a maximum of about 140 and 165 K, whereas on the curve for the thermostimulated luminescence of KBr with low Na concentration (curve 2) there are many peaks corresponding to a large set of defects in this crystal. In the 120–180 K region, thermal delocalization of the H centers from impurity traps (thermal decay of the HA centers) occurs in the KBr:Na single crystal. Strictly in parallel with the HA band, the F band is also annealed. Therefore, following Tanimura, Okada, and Suita [1], we assign the thermostimulated luminescence peaks at 140 and 165 K to the recombination of the HA and F centers having the corresponding stages on the annealing curve. Since in this region of temperatures the annealing of the (α, IA) pairs is also observed, it is impossible to say in advance whether this thermoluminescence is the result of the direct recombination of the (F, HA) pairs or the result of a small chain of secondary processes occurring with participation of the (α, IA) pairs [2, 3]. *
To whom correspondence should be addressed.
E. A. Buketov Karaganda State University, 28 Universitetskaya Str., Karaganda, 470074, Kazakhstan; e-mail:
[email protected]. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 69, No. 5, pp. 643–645, September– October, 2002. Original article submitted December 12, 2001. 0021-9037/01/6905-0743$27.00 2002 Plenum Publishing Corporation
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Fig. 1. The optical density kd (a) and the thermostimulated luminescence curves of the KBr:Na crystals with an Na+ concentration of 1.000 (1) and 4.0 eV is absent. For the HA-illumination we failed to measure the entire emission band, because it is difficult to separate the luminescence light from the stimulating HA light. The recombination of defects during the F-illumination is accompanied by luminescence with a maximum at 2.75–2.80 eV and a halfwidth of about 0.8 eV. The maximum of the recombination luminescence accompanying the HA-illumination is located at hν ≥ 2.7 eV.
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Fig. 2. The absorption spectra (a) and the curves for thermal annealing of the IA band (5.4 eV) immediately after the irradiation (À) and after the total deexcitation of the F and HA centers (×) (b) in the KBr:Na crystal (1 mol.%) on x-irradiation at 80 K; the thermostimulated luminescence of KBr:Na (1 mol.%) after the irradiation (1) and after the total de-excitation of the F and HA centers (2) (c). Irradiation with F light at 80 K ionizes the F center with formation of the α center. The irradiation corre− sponding to the HA band leads to dissociation of the Br2 molecule of the HA center. One of the products of the photodissociation — an interstitial halogen atom mobile at 80 K — is bound to a new Br−2 molecule, i.e., the H center located at the crystallographic site adjacent to the initial position of the HA center. The migration of the H centers is terminated by the recombination of the H and F centers occurring in the ground states. It might be expected that as a result of the direct recombination of the F centers with the migrating H centers, an autolocalized exciton will be produced and π-radiation with a maximum at about 2.3 eV, characteristic for pure KBr, will arise. But in the spectrum of the luminescence accompanying the HA-illumination at 80 K the maximum at about 2.3 eV, typical of pure KBr, is not manifested. Thus, if the autolocalized exciton is produced at all in the recombination of the F and H centers, this process must occur slowly; otherwise π emission arises in the case of the HA-illumination. The thermostimulated recombination of the defects in KBr:Na (1 mol.%) is the more complex process, since the (α, IA) and (F, HA) pairs are annealed in the same region of temperatures (see Fig. 2). This coincidence of the temperatures was also noted in [1]. The thermoluminescence can arise as a result of both direct recombinations of the F and HA centers and secondary reactions between the (α, IA) and (F, HA) pairs. In the thermoluminescence spectrum the so-called impurity (Na) luminescence is observed. The presence of this luminescence can easily be explained in the scheme of secondary reactions. The I center migrates to the F center, with an electron releasing as a result of their recombination which then interacts with the HA center. The resulting (HA + e−)* center emits the luminescence observed. If the thermoluminescence is the result of direct recombinations of the F and H centers (the latter are released from the HA centers), then it is more difficult to explain the presence of the impurity luminescence, although there might be singularities in the crystal with such a high impurity concentration. The presence of two thermostimulated luminescence peaks can be caused by the recombinations of the weakly and strongly separated defects. This is the manifestation of the analogy between the recombination of the (F, HA) pairs
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produced at 80 K and the recombinations of the (F, H) pairs produced at 4.2 K for which the presence of different thermostimulated luminescence peaks for the separated pairs is also known [2, 3, 5]. At high impurity concentrations in an irradiated crystal new HAA and IAA centers can be produced along with the HA and IA centers. Then the presence of two thermostimulated luminescence peaks in the case of direct F and HA recombinations can be explained by the different height of the barrier for the H center near one Na+ ion and near two Na+ ions. The release of the I centers from various traps near one Na+ ion and near two Na+ ions can be considered as a scheme of secondary recombination. In this connection, using the MNDO quantum chemical method (a cluster consists of 217 ions, 107 of which are optimized using the geometrical parameters in energy minimization), we carried out simulation of delocalization of the H and I centers from the Na traps. If the H and I centers are localized near one Na+ ion, the height of the barrier of thermal delocalization is 0.26 and 0.30 eV, respectively. In the case of localization near two Na+ ions, the height of the energy barrier for the H and I centers is equal to 0.43 and 0.50 eV, respectively. It is evident that the heights of the barriers for the HA and IA centers, just as for the HAA and IAA ones, are nearly the same.
REFERENCES 1. 2. 3. 4. 5.
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K. Tanimura, T. Okada, and T. Suita, Solid State Commun., 14, No. 12, 107–111 (1974). D. E. Aboltin, V. J. Grabovskis, A. R. Kangro, Ch. B. Lushchik, A. A. O’Konnel-Bronin, I. K. Vitol, and V. K. Zirap, Phys. Status Solidi (a), 47, No. 2, 667–675 (1978). Ch. B. Lushchik, E. A. Vasil’chenko, and L. Ch. Lushchik, Vopr. Atomn. Nauki Tekhniki. Fiz. Radiats. Povrezhd. Radiats. Materialoved., No. 1 (15), 17–27 (1981). K. Tanimura and T. Okada, Phys. Rev. B, 21, No. 4, 1690–1697 (1980). N. Saidoh and N. Itoh, J. Phys. Chem. Sol., 34, No. 7, 1165–1171 (1973).
