LiF晶体F3+色心的实验研究

本文从实验上系统地研究了在不同条件下电子束辐照LiF晶体所形成的F₃⁺色心的光学特点。并且由荧光光谱分析了F₃⁺心和F₂心,的相对密度关系。实验表明,辐照温度对于色心的形成和密度的相对大小起着关键的作用。主要实验结果包括:1)在液氮温度下辐照,然后在暗处加热至室温可形成高密度的F₃⁺心,表现在发射光谱中F₃⁺心荧光占绝对优势。吸收光谱表明没有N心和R心。2)由动力学荧光谱可以看到低温辐照的样品在F₂⁺心衰变的同时,F₃⁺心密度迅速增加。而室温辐照的样品则是F₂心,与F₃⁺心的密度以近似相等的速率增加。3)详细观察了F₃⁺心530nm荧光激发带与F₂心670nm荧光激发带半宽度的变化和双峰结构。由此对M吸收区的发光特点作了解释。 关键词：     

Photocontrolled magnetic resonance plasticity of γ-irradiated KCl:Fe crystals

A photocontrolled resonance decrease in microhardness, which is due to the application of mutually perpendicular static and microwave fields, in γ-irradiated KCl:Fe crystals has been revealed. It has been found that the magnetic plasticity of unirradiated γ-KCl:Fe crystals is due to the resonance effect of magnetic fields on two types of impurity centers: first, centers containing Fe²⁺-ν _c ion-vacancy pairs and, second, centers containing Fe⁺ ions. The illumination of γ-KCl:Fe crystals with F-light (with a wavelength of λ = 500–600 nm) is accompanied by rearrangement of the spectrum of electron paramagnetic resonance detected by a change in microhardness. The effect of F-light on the spectrum of magnetic resonance plasticity is manifested as the suppression of the spectra of Fe²⁺-ν _c ions with effective g-factors of 7.0 and 3.5 due to their recombination with F-electrons and reconstruction to Fe⁺ centers with g-factors of 2.2 and 4.1.     

Spin lattice relaxation of F+ centers in neutron irradiated cao crystals

Abstract

We have measured the electron spin-lattice relaxation rates of F⁺ centers in CaO crystals at different F⁺ centers concentrations (from 5.8·10¹⁵ to 1.4·10¹⁷ cm^?3) in magnetic fields from 10^?3 to 4.8 T and temperatures from 0.4 to 4.2 K. At temperatures above 1 K the relaxation rate is nearly proportional to the F⁺ centers concentration. Spin diffusion to fast relaxing centers is essential in determining the F⁺ centers spin polarization decay.     

Ion bombardment-induced charge state effects CaO single crystals: F+ and F centers

The effects of heavy-and light-ion bombardment on defect formation in CaO have been investigated by UV-absorption spectroscopy and volume measurements. While 500 keV Ar or Ca implantation produces only F⁺ centers, 240 keVH produces both F⁺ and F centers at a F⁺ to F ratio of 5.6 to 1. On the other hand, when an argon implanted sample is subsequently bombarded with hydrogen, about 30% of the F⁺ centers anneal during 1 ×10¹⁴ H/cm²; at higher H fluences, new F⁺ and F centers are produced. An effect of energy partition between ionization and nuclear/atomic collision processes for the incident ions on the charge state of the resulting defect is thus clearly demonstrated.

The formation and annealing of these defects are accompanied by volume changes in the ion implanted surface layer which can be monitored in sltu with a cantilever beam technique. The measurements show volume expansion of the order of 1.5% following 10¹⁶ 500 keV Ar implantation; subsequent implantation of 10¹⁸ 240 keV H compacts the previously expanded material by 25 %. These results are in qualitative agreement with the optical data and seem to indicate that volume changes are associated with the formation and annealing of F⁺ centers.     

Aggregate color center formation processes in lithium fluoride crystals after irradiation

Lithium fluoride crystals were irradiated by different doses of gamma photons at a temperature of 77 K. We measured the aggregation kinetics for the color centers with different annealing temperatures above the temperature of anion vacancy mobility. We show that the lifetimes of the vacancies decrease while the lifetimes of the F₂⁺ F_2^{+} centers increase as the irradiation dose increases. We explain these types of dependences based on the aggregation processes for color centers in the post-radiation period. We determine the time constants and energies (analogous to activation energies in the Arrhenius equation) for the various processes involving rise and fall in the concentration of aggregate color centers. Based on the experimental data obtained, we have established the processes forming F ₂ and F₃⁺ F_3^{+} centers in the post-radiation period. The F ₂ centers are formed when vacancies ν_a add to F₁^- F_1^{-} centers. Vacancies arising during irradiation of the crystal participate in their creation in the first fast stage. In the long final stage, vacancies are used which appear in the post-radiation period on occurrence of the reaction F₂⁺ F_2^{+} + H → ν_a + fluoride ion at the lattice site, where H is an interstitial fluorine atom. The F₃⁺ F_3^{+} centers are formed both by merging F₂⁺ F_2^{+} and F ₁ centers and as a result of addition of vacancies to F ₂ centers. In this case, vacancies are used that are generated not only during irradiation of the crystal but also in the post-radiation period. The rise in the concentration of F₃⁺ F_3^{+} centers occurs faster than the rise in the concentration of F ₂ centers.     

Spectroscopic characteristics of anionic centers in α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> crystals bombarded by Cu<Superscript>+</Superscript> and Ti<Superscript>+</Superscript> ions

We have studied the effect of bombardment by Cu⁺ and Ti⁺ ions with energy 30 keV on the optical absorption and luminescence of F centers in oxygen-deficient aluminum oxide. We have shown that in the induced optical absorption spectra there are six components of gaussian shape, which can be assigned to absorption bands of F⁺, F², and F₂⁺ centers. We have established that bombardment of the samples by ion beams has a weak effect on the thermoluminescence parameters in the 3.0 eV and 2.4 eV bands, while in the 3.8 eV luminescence band for F⁺ centers, the thermoluminescent response increases considerably. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 3, pp. 422–424, May–June, 2008.     

Photoluminescence properties of anodic alumina membranes with ordered nanopore arrays

Photoluminescence (PL) of anodic alumina membranes (AAMs) with ordered nanopore arrays fabricated in oxalic acid has been investigated under different annealing temperatures. The PL intensity firstly increases, and at 500 °C reaches a maximum value, then decreases. The structural transition from amorphous to γ-Al₂O₃ in AAMs has been confirmed by X-ray diffraction. Thermogravimetric analysis results and electron paramagnetic resonance measurements revealed that the PL band of alumina membranes could be attributed to the oxygen-related defect centers (F⁺ centers) rather than the luminescent centers transformed from oxalic impurities.     

An Analysis of the Trigonal Center Structure of Yb3+ Ion in CsCaF3

The crystal field parameters determined from interpretation of optical spectra are used to analyze distortions of a crystal lattice in the vicinity of an impurity ion and vacancy at a Cs⁺ site compensating the excess positive charge in the trigonal centers of Yb³⁺ ions in CsCaF₃ crystal. Interactions of the impurity ion with the nearest neighbors (an octahedron of F^? ions) and the next nearest neighbors (a cube of Cs⁺ ions) are considered within the superposition model. It is established that, at formation of the trigonal center, three F^? ions of the nearest octahedron, placed symmetrically along the threefold axis on the side of the vacancy, move away from the impurity ion a little and significantly deviate from this axis. The second triangle of F^? ions, on the contrary, comes nearer to the impurity ion and nestles on the axis of the center a little. The three Cs⁺ ions, the second neighbors on the side of the vacancy, slightly come nearer to Yb³⁺ ion and considerably nestle on the center axis. The second triangle of Cs⁺ ions, from the opposite side of vacancy, also comes nearer to the paramagnetic ion and also nestles on the center axis a little. The Cs⁺ ion, lying on the center axis, comes considerably nearer to the impurity ion.     

FS and FS(OH) defect centers at the MgO(1 0 0) surface: cluster and periodic calculations

We present a model of a new paramagnetic defect center which results from the interaction of atomic hydrogen with the MgO(1 0 0) surface. DFT calculations have been performed using periodic supercells and embedded cluster models where long-range polarization effects are included explicitly. The H atom promotes the creation of an oxygen vacancy (F center) by formation of the F_S⁺(OH⁻) defect where an hydroxyl group is adsorbed near an electron trapped in an oxygen vacancy. This new center has some characteristics similar to those of the classical F_S⁺ centers but a smaller formation energy; furthermore, being globally neutral, it can be treated also with supercell methods.     

Luminescence and radiation-induced color centers in anion-defective alumina crystals after high-dose irradiation

A method of UV spectroscopy was used to measure photoluminescence (PL) spectra and photoluminescence excitation (PLE) spectra in anion-defective alumina crystals exposed to high doses of gamma-radiation. An additional emission band in the range of 1.6–2.75 eV appears in the exposed crystals. Aggregate F₂-type centers in different charge states are responsible for this band. It was found that growing intensity of PL aggregate centers occurs at doses corresponding to saturation of dose response and is accompanied by a sharp drop in the intensity of F⁺-band in the PL spectrum resulting from combination of F⁺-centers into aggregates. Uncharged F₂-centers are formed when electrons are trapped by F₂⁺ and F₂²⁺-centers. The main role of F⁺-centers in radiation-induced transformations of color centers under high-dose irradiation of anion-defective alumina crystals was indicated.     

FA(Ga+,In+,Tl+) tunable laser activity and interaction of halogen atoms (F,Cl,Br,I,At) at the (001) surface of KCl crystal: ab initio calculations

An attempt has been made to examine F_A(Ga⁺,In⁺,Tl⁺) tunable laser activity and adsorptivity of halogen atoms (F,Cl,Br,I,At) at the (0 0 1) surface of KCl crystal using an embedded cluster model, CIS and density functional theory calculations with effective core potentials. The ion clusters were embedded in a simulated Coulomb field that closely approximates the Madelung field at the host surface. The nearest neighbor ions to the defect site were then allowed to relax to equilibrium. Based on the calculated strength of electron–phonon coupling and Stokes-shifted optical transition bands, The F_A(Tl⁺) center was found to be the most laser active in agreement with the experimental observation that the optical emissions of F_A(In⁺) and F_A(Ga⁺) centers were strongly quenched. The disappearance of the anisotropy and np splitting observed in the absorption of F_A(Ga⁺,In⁺,Tl⁺) centers were monotonically increasing functions of the size of the impurity cation. The F_A(Ga⁺,In⁺,Tl⁺) defect formation energies followed the order F_A(Ga⁺)>F_A(In⁺)>F_A(Tl⁺). The Glasner–Tompkins empirical relationship between the principal optical absorption of F centers in solids and the fundamental absorption of the host crystal was generalized to include the positive ion species. As far as the adsorptivity of the halogen atoms is concerned, the F and F_A(In⁺,Tl⁺) centers were found to change the nature of adsorption from physical adsorption to chemical adsorption. The adsorption energies were monotonically increasing functions of the electronegativity of the halogen and the amount of charge transferred from the defect-free surface. The calculated adsorption energies were explainable in terms of the electron affinity, the effective nuclear charge and the electrostatic potentials at the surface. The spin pairing mechanism played the dominant role in the course of adsorbate–substrate interactions and the KCl defect-free surface can be made semiconducting by F or F_A(In⁺,Tl⁺) surface imperfections.     

The optical and paramagnetic absorption spectra of MgO:Mn single crystals irradiated in a reactor

The optical and paramagnetic absorption spectra of MgO:Mn crystals of different origin, irradiated in a reactor, have been investigated. It is found that, in spite of only a small amount of Mn²⁺ impurity ions in the investigated crystals, their influence on the efficiency of formation and accumulation of F⁺-centers is dominant. The dependence of the number of F⁺-centers both on irradiation dose and on manganese concentration is studied.     

低温下电子束辐照产生F2+心及其荧光发射

用自制的低温装置保持LiF晶体的温度为77K,同时用电子束辐照,产生了浓度高达10¹⁷cm^-3的F₂⁺心。并对F₂⁺心荧光发射特性进行了实验观测和理论分析。 关键词：     

Spatial structures based on color centers created by electrons in lithium fluoride crystals: Absorption and luminescence characteristics

We have shown that in spatial structures based on color centers created by electrons in a lithium fluoride crystal, the distances between centers reach 1.6 nm and 3.6 nm for F₁ and F₂ centers respectively. This suggests considerable potential opportunities for using electron technology to form structures in the crystals with spatial resolution of such an order of magnitude. We measured the decrease in fluorine content on the irradiated surface of the crystal. We found the concentrations of F₁, F₂, F₃⁺, F₃(R₂), and F₄(N₁) centers. We established that the specific characteristics of color center formation by electrons leads to an increase in the efficiency of creation of F₃ and F₄ centers. We determined the decrease in the average luminescence lifetimes of F₂ and F₃⁺ centers as a result of concentration quenching. We observed distortion of the luminescence contour for F₂ centers as a result of absorption of its short-wavelength portion by other centers and emission of radiation by the latter in its long-wavelength portion. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 102–110, January–February, 2008.     

Optical properties and stable,broadly tunable cw laser operation of new FA-type centers in Tl+-doped alkali halides

A new group of complex color centers with F_A-type properties, involving simple center production and high thermal and optical stabilities, has been found in six Tl⁺-doped alkali halides: NaCl, KCl, RbCl, KBr, RbBr and RbI. In its first tested examples, KCl and KBr, broadly tunable cw laser operation over the 1.4 to 1.7 μm range has been obtained, with output powers in the 100 mW range. In contrast to already existing F⁺₂ and F⁺₂-like centers, operating in the same wavelength range, the new F_A(Tl⁺) lasers are optically stable and do not show any bleaching effects under laser operation.     

Dependence of blue-emitting property on nanopore geometrical structure in Al-based porous anodic alumina membranes

Porous anodic alumina (PAA) membranes were fabricated using anodization of Al foils. Based on morphology observations and photoluminescence (PL) measurements, it is revealed that the peak position of the observed blue PL band has an evident dependence on nanopore geometrical structure. The blue PL band is asymmetrical and can be Gaussian divided into two subbands, which originate in two kinds of oxygen-deficient centers, F⁺ and F centers. On raising the anodic voltage, the two subbands red shift and the intensity of the subband from the F⁺ centers relative to that from the F centers increases. This suggests that the increases of nanopore size and porosity make partial F⁺ centers convert into F centers. The mechanism of this conversion is further discussed on the basis of the PL behavior of the PAA membrane under ultraviolet irradiation. This work will be beneficial to the better understanding of the PL property of the PAA membrane. PACS 78.55.-m; 82.45.Cc; 76.30.Mi     

(F2+)A centers in lithium-doped KCl

Optical properties of the lithium (F₂⁺)_A center are reported and compared to those of the sodium (F₂⁺)_A and the intrinsic F₂⁺ centers.     

Deconvolution of Powder ESR Spectra: Ge Centers in Natural Quartz

A procedure for the deconvolution of complicated electron spin resonance (ESR) powder spectra into a set of spectral contours of separate paramagnetic centers is applied for the analysis of a polycomponent system of paramagnetic Ge centers in natural silica. The changes in the Ge centers system occurring during the thermal transformation of natural quartz into cristobalite are studied. The annealing at about 1300 K transforms the [GeO₄/Li⁺]⁰ centers partly into the [GeO₄/Na⁺]⁰ form in α-quartz and the further temperature increase leads to the capture of both defects by the cristobalite structure. A lag effect of the inheritance of the isomorphic impurities present in the α-quartz by cristobalite is established. The natural metamorphic transformations of vein quartz bodies also stimulate the repartitions of alkaline ion compositions in quartz structure. This can be detected by ESR as corresponding modifications in the compositions of Ge paramagnetic centers. ESR investigations of compositions of paramagnetic Ge centers in quartz from altered bodies reveal the enrichment of the quartz lattice of strained and granulated parts of veins by interstitial sodium. This process is caused by dissolution of sodium-containing fluid inclusions in the quartz lattice. Author's address: Vladimir P. Lutoev, Institute of Geology, Komi Scientific Center, Russian Academy of Sciences, Pervomaiskaya ulitsa 54, Syktyvkar 167982, Russian Federation     

Trimers of bivalent copper impurity ions in CaF<Subscript>2</Subscript> crystals: The structure and mechanism of formation

Crystals of CaF₂: Cu (with a copper impurity content higher than 0.1 at. %) grown by the Czochralski method from a melt in a mixed helium-fluorine atmosphere are investigated using electron paramagnetic resonance (EPR) spectroscopy. It is found that the crystals contain paramagnetic centers whose magnetic properties at low temperatures are identical to those of [CuF₄F₄]^6? (S=1/2) single centers. The magnetic properties of the centers exhibit a qualitative change in the temperature range 77–300 K. These changes are described within a model according to which the center is treated as a cluster composed of three [CuF₄F₄]^6? impurity complexes involved in exchange interactions and interactions occurring in the field of Jahn-Teller lattice distortions.