Proton-induced F-centers in LiF and MgF2

The growth of F-centers in LiF irradiated at room temperature with 40- and 85-MeV protons and with ⁹⁰Sr electrons was found to be proportional to the square root of the absorbed energy over the range 0.5 to 2.3 Mrad which corresponds to an F-center density range of 1 × 10¹⁶ to 1.5 × 10¹⁷ per cm³. The production efficiency was 5 × 10³eV per F-center at an absorbed energy of 2.3 Mrad. The density of F-centers produced in MgF₂ by 40- and 85-MeV protons was measured over an absorbed energy range of 0.2 to 29 Mrad which corresponds to a maximum F-center density of 2 × 10¹⁶ per cm³. The production efficiency for MgF₂ was 4 × 10⁵eV per F-center at an absorbed energy of 16 Mrad.     

Polaron theory of the F-F′ conversion in alkali halides

Extending recent suggestions regarding the vibronic properties of F-centers in alkali halides, the following scenario is now proposed of the optical conversion of F-centers into F′-centers: An excited F-center electron, formed by photobleaching in the F-band, passes into a bound polaron state before passing on to an F-center to give rise to an F′-center. The detachment of the electron cloud from the excited F-center involves configurational tunneling in an exothermic process. However, both the tunneling barrier and the electron-transfer expectancy depend drastically on the host material leading to different activation energies and frequency factors of the conversion efficiency in different hosts. The Fcenter-polaron conversion is effected through the strong coupling to at least two modes, one being the LO lattice mode of the crystal. The F to F′ conversion efficiency for various hosts is calculated using a reaction-rate method and found to agree reasonably well with available experimental data.     

Experimental studies of electrical conduction mechanism of H2-reduced BaTiO3

ESR, resistivity and Seeback coefficient measurements have been performed on both ceramics and single crystals of reduced semiconducting BaTiO₃.From the results that the observed temperature dependence of the Seeback coefficient can be explained by the temperature dependence of carrier concentration estimated from the electric resistivity making use of the data of electron mobility, it is concluded that the electric conduction in reduced BaTiO₃ is due to the band conduction rather than to the hopping process. From the measurement of the temperature dependence of the ESR intensity of the F-center, the number of electrons trapped at the F-centers decreases exponentially with temperature, while the number of conduction electrons increases. This temperature dependence can not be simply explained as that of the unionized donors in semiconductor. Therefore, the ESR signal considered as that of the F-center may not be due to simple donors, even though some of the conduction electrons may be originated in them.     

Effect of Br− ions on the F-center formation in KCl crystals under u.v. light irradiation

The relative efficiency of the F-center formation, Y_F in KCl crystals has been studied as a function of photon energy of u.v. light between 5.7 and 10.0 eV. Y_F is maximum at the peak of the absorption band due to the localized excitons at Br^? ions, and increases with Br^? concentration. Results suggest an important role of the localized excitons in the F-center formation.     

Elektronenspinresonanz- und optische Untersuchungen zur FarbzentrenreaktionF⇄F′ in KCl-Kristallen

Simultaneous measurements of optical absorption and electron spin resonance of additively colored KCl crystals have been carried out during the optical bleaching ofF-centers at ?90°C. TheF-center resonance signal does not change in its significant properties e.g. line shape, line width,g-value, and saturation behavior, but it decreases proportional to the decrease of the opticalF-center absorption band. No ESR-signal due to theF′-center could be observed. The time dependence of theF?F′-center conversion has been investigated by ESR and optical measurements. It does not give any indication of a paramagnetism of theF′-center, too. The kinetics of the thermal decay of theF′-center at ?90°C have been studied. One can interpret the time dependence of this process, if one assumes that the mean lifetime of anF′-center depends on the distance of the nearest halogen ion vacancy. The expectation of life of theF′-centers therefore increases during the thermal decay, as the mean distance between remainingF′-centers and vacancies increases.     

Relativistic corrections and very small g-shifts in solids

The relativistic Breit-Margenau correction to the Zeeman-interaction has been calculated for a group of atoms in the periodic table with Herman Skillman wave functions and potentials. The results are applied to the S-state ions Mn²⁺ and Gd³⁺, to the F-center and V_k-center, and to the shallow donors in Si.     

Electron-beam irradiation effects on luminescence properties in subsurface regions of single-crystalline sapphires treated with and without hydrogen plasma exposures

Electron irradiation effects on various insulating sapphires treated with and without hydrogen plasma have been investigated mainly by means of cathodoluminescence (CL) measurements. The samples examined included Be-diffusion-treated natural sapphire (BNS) and two types of synthetic sapphires grown by Verneuil and Czochralski methods. For all the samples examined, on one hand, their CL intensities of the F⁺-center-related emission peaked at ≈3.8 eV rapidly increased with increasing the fluences of keV electrons, and were represented roughly by exponentially saturating curves. There occurred slight blue-shifts of the F⁺-center luminescence other than the intensity increases for some of the electron-irradiated specimens, suggesting possible presence of two components for the F⁺-center luminescence. On the other hand, a hydrogen plasma exposure to these sapphires resulted in sample-dependent changes in the optical property and in the beam-irradiation effect on the F⁺-center CL emission. Such variations were induced most strongly in the BNS sample, whose color changed from orange to pink due to substantial decreases in the absorbance after the hydrogen plasma treatment. Furthermore, the energy positions of both the Cr³⁺-center luminescence peaked at ≈1.8 eV and its satellite peaks were found to slightly shift for the untreated and H-plasma-treated BNS samples after the electron beam irradiations. Possible origins of these observations are discussed.     

Mechanism of <Emphasis Type="Italic">F</Emphasis>-center luminescence in anion-defective aluminum oxide single crystals

New experimental data illustrating the effect of deep traps on the luminescence properties of anion-defective α-Al₂O₃ single crystals are presented. It was established that deep traps have electronic nature and their filling occurs through photoionization of F centers and is accompanied by F → F⁺-center conversion. Model concepts were developed that describe the luminescence mechanism in anion-defective aluminum oxide single crystals with inclusion of thermal ionization of the excited F-center states. The validity of the model was supported by experimental data obtained in a study of thermoluminescence, thermally stimulated exoelectron emission, and thermally stimulated electrical conductivity.     

Vibronic problem in the higher excited states of F-center in alkali halides

Vibronic problem in the higher excited states of F-center in alkali halidesA new approach to the vibronic problems of bound polaron is proposed, taking advantage of the spherical symmetry of the system. By applying the obtained formula to the analysis of the transient absorption spectra due to the transition from the relaxed excited state to higher excited states of F-center in alkali halides, a good fit between the theory and the experiments is obtained and the final states of the transition are assigned to be 3p state and its phonon side bands perturbed through vibronic coupling.     

Effect of spin of F3-center on changes in its charge under the action of radiation

Methods of pulse spectroscopy with time resolution were used to investigate creation and annihilation processes of F₂ ⁺-, F₂-, F ₂ ^– -centers, which are noninertial with respect to the duration of an electron pulse of nanosecond duration acting on the crystal. It is concluded on the basis of these studies that the effect of radiation on an F₂-center is a change in its charge and spin states. It is established that the direction of change of an F₂ -center charged state is determined by the value of its spin: localization occurs of a band hole on an F₂-center in the singlet ground state, and of an electron in the on an F₂-center in the triplet state. An F₂-center triplet state is formed during the electron localization process on an F₂ ⁺-center or subsequent localization of band holes and an electron on an F₂-center as a result of a thermally activated intercombination transition from a higher state than the radiating singlet.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 109–120, October, 1991.In conclusion the author is grateful to V. A. Kravchenko for participation in the experiments, and to V. M. Lisitsyn and V. Yu. Yakovlev for interesting discussions.     

Development of a 2D dosimetry system based on the optically stimulated luminescence of Al2O3

A laser-scanning 2D dosimetry system based on the Optically Stimulated Luminescence (OSL) signal from Al₂O₃ films was built and demonstrated. The main challenge of using the OSL from Al₂O₃ for 2D dosimetry by laser scanning is the long lifetime (∼35 ms) of the main luminescence centers in this material (F-centers). In this work, we demonstrated the possibility of performing 2D dosimetry by laser scanning using a combination of the fast UV emission of F⁺-centers (lifetime <7 ns) and the slow F-center emission of Al₂O₃:C, and an algorithm to correct for the slow F-center luminescence lifetime. We also investigated the possibility of using Al₂O₃:C,Mg, to take advantage of its greater F⁺-center emission compared to Al₂O₃:C. Results from 6 MV photon beam irradiations from a clinical linear accelerator were compared to radiographic and radiochromic film profiles showing a good qualitative agreement.     

EPR of ultraviolet irradiated lithium hydride crystals

Results are presented of EPR measurements on lithium hydride single crystals at low temperatures. Loose aggregates of F-centers are formed during u.v. irradiation. The F-center production is interpreted as an excitonic process where the interstitials are stabilized as H₂-molecules.     

Transient absorption induced by flash-light excitation in additively colored KCl: Na

Transient bands occuring between 600–1100 nm on flash-light illumination in the F-band near room temperature were investigated. Identified as the F′-andF′_A-bands, they varied substantially on repeated flashing: the F′_A-band intensified because of the F-F_A-conversion, while the F′-band attenuated as a result of the F-center bleaching.     

Anomalous F-center distribution in alkali halides irradiated with 1.5-MeV protons to high fluences

Crystals of KCl and KI irradiated to fluences of the order of 10¹⁶ protons/cm² exhibit an anomalous dip in F-center profile near the end of the proton penetration depth. This effect is shown to arise from the destruction of F centers by hydrogen capture, forming U centers in a distribution whose peak coincides with the proton range.     

Ion bombardment of alkali halides,II.: Effects observed at high fluence

Color-center effects unique to crystals irradiated to high fluence : 10¹⁶ particles/cm²) are reported for KCl and other alkali halides. These include (1) a minimum in F-center concentration at the end of the proton range and (2) an n-center distribution profile which changes with time in the dark at room temperature after irradiation. The first effect is due to annihilation of F centers by hydrogen atom capture (U-center formation) and the second is due to F-center diffusion and aggregation in a heavily damaged region of the crystal.     

M-Zentrenbildung in röntgenbestrahltem KCl

The equilibrium betweenF- andM-center concentration in KCl after X-irradiation at temperatures between 77 °K and 298 °K has been studied. At low temperatures (<200 °K) onlyF-centers which are formed statistically as nearest neighbours give aM-center. At higher temperaturesF-centers within a certain sphere of interaction with a radiusr formM-centers. The temperature dependance of this radiusr has been evaluated. The equilibrium constantK depends on the temperature in the form K=K₀exp(-Q/kT). The activation energy was determined to be Q=(0.5±0.1) eV.     

Electronic structure of F,-center in MgO

The embedded-cluster numerical variational method has been developed to calculate the electronic structure of perfect MgO, F and F⁺-centers in MgO. The energy band, bulk density of states has been calculated by cluster Mg₁₄O₁₃, Mg₁₄O₁₂F⁺ and Mg₁₄O₁₂F. The calculated absorption energy for F⁺ and F centers is in good agreement with experimental data. In our calculated defect energy levels, that the first excited state of F⁺-center is at CB-3.46 eV, indicates the necessity of a large photoelectron yielding energy. We also calculate the radius of color center electron, and plot the map of charge-density distribution of valence electrons in which the structure of the color center is shown directly. Received 22 May 1998     

Experimental study of Raman scattering by NaI crystals containing F centers

Experimental Raman spectra of Na I containing F-centers are described and compared to the calculated spectra, in the off-resonance approximation. The position of the strong resonant mode, almost of A_1g symmetry, is interpreted using a decrease of 67% in the longitudinal force constant between the F-center and its nearest neighbours and of 20% between the nearest and the fourth neighbours.     

Formation and stabilization of F centers following direct generation of self-trapped excitons in KCl crystals

The spectrum of luminescent F centers generated in high-purity KCl crystals by 7–10.2-eV photons has been measured at 230 K. The pulsed annealing of these centers (250–550 K), as well as the dependence of the efficiency of stable F-center generation on irradiation temperature (80–500 K) has been studied. The efficiencies of F ⁻ and Cl ₃ ⁻ -center generation are maximum under direct optical creation of self-trapped excitons in the region of the Urbach intrinsicabsorption tail. Besides the exciton decay with formation of F centers and mobile H centers, a high-temperature exciton decay channel which involves creation of cation defects stabilizing the H centers has been revealed. Fiz. Tverd. Tela (St. Petersburg) 41, 433–441 (March 1999)     

The photodamping of dislocations in KCl single crystals

The effect of F-light (λ = 550 nm) on the dislocation amplitude-dependent internal friction (dislocation photodamping) and dislocation charge in KCl crystals of different purity containing F-centers has been investigated. The photodamping process in all the crystals under study has been found to be due to the optical generation of new pinning points. The conditions of photopinner formation are found to depend on the magnitude of dislocation charge, the manner of the F-center introduction (γ-irradiation, additive coloration), and crystal purity. A model of photopinner formation has been proposed. The results obtained are used to analyse the binary systems in which the effect of dislocation photodamping is likely to occur.