Thermally activated structural changes in KCl crystals containing color centers

Structural changes taking place in the temperature interval 300–750K in KCl crystals additively colored in Na vapor are studied by spectrophotometric and light scattering methods in conjunction with crystal lattice constant measurement.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 39–45, September, 1984.     

Study of radiation damage in quartz single crystals irradiated with protons

The effect of proton irradiation on quartz single crystals is studied. Positron diagnostics (the angular distribution of annihilation photons (ADAP)) and acoustical and spectrophotometric methods are used to study radiation-induced defects. It is shown that a narrow component with intensity f in the ADAP spectrum is caused by parapositronium and determines the high sensitivity of the method used in studying special features of the quartz crystal structure. In this case, any process leading to a decrease in the probability of positrinium (Ps) formation (the capture of positrons by charged defects and the interaction with impurity ions and lattice distortions) decreases the intensity of the narrow component. The concentration of radiation-induced defects is estimated and their kinetics of annealing up to 873 K is studied.     

Build-up of F and M color centers in KCl single crystals under combined electron and proton irradiation

The F and M color-center build-up kinetics in KCl crystals under combined irradiation with electrons of energy 15 and 100 keV and 100-keV protons have been studied in the flux range of 10¹³–10¹⁵ cm⁻² and at a flux density of 3×10¹¹ cm⁻² s⁻¹. It is shown that consecutive irradiation with electrons and protons produces results not obtainable under electron or proton irradiation alone. Fiz. Tverd. Tela (St. Petersburg) 40, 2015–2018 (November 1998)     

Rearrangement of defects in single crystals of cadmium sulfide irradiated with fast electrons

This study exaines CdS single crystals subjected to a special nonalloying treatment, alloyed with copper, and annealed in a cadmium atmosphere. Irradiation of the test specimens by electrons with E 1.2 MeV led to the formation of mobile defects and to decomposition of the original donor-acceptor associates. Over time, the primary radiation defects form secondary defects which act as fast recombination centers responsible for impurity photoconductivity. New donor-acceptor pairs are formed when the irradiated specimens are stored, these pairs being paramagnetic centers. Conclusions are made as to the nature of the secondary radiation defects, which are distributed both in the body of the specimens and near their surface.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 5–9, May, 1988.     

Oscillator strength calculations in color centers of diamond and the role of spin

A generalized Hubbard model based on a molecular approach is used to calculate many electron wavefunctions of diamond vacancies. We have calculated the oscillator strength of the dipole transition rates from the ground states of the neutral and negatively charged vacancies. The ratio of the oscillator strengths is in very good quantitative agreement with the reported optical spectroscopic data. Electronic configurations in the ground and dipole allowed excited states are presented. With the proposed picture, the much larger oscillator strength of the negatively charged vacancy with respect to other experimentally investigated color centers N-V, H3, N3 and H4 is explained.Received: 9 March 2004, Published online: 23 July 2004PACS: 61.72.Bb Theories and models of crystal defects - 61.72.Ji Point defects (vacancies, interstitials, color centers, etc.) and defect clusters - 71.55.-i Impurity and defect levels     

F-center accumulation in filar KCl crystals irradiated with X-rays

The article outlines research on the kinetics of F-center accumulation in filar KCl crystals of various diameters and various origins. The mechanism of dislocation-group formation and the crystal surface cannot account for the radiation-induced coloration of alkali-halide crystals at room temperature. Differences in the properties of filar crystals grown by various techniques were observed for the first time.     

Thermal expansion measurements for estimating vacancy concentration in X-ray irradiated KCl single crystals

A sensitive capacitance technique is used for measuring changes in length (Δl) of KCl single crystals with temperature in the region 30–300°C. These measurements have been taken on KCl in (i) as-cleaved (ii) X-ray irradiated (iii) quenched and X-ray irradiated conditions (X-ray irradiation was always done at room temperature (≈ 30°C). The linear coefficient of thermal expansion (α) of the as-cleaved sample is 40.8 × 10^-6°C^-1. Variation of Δl with temperature in X-ray irradiated crystal shows two regions: (a) 30–180°C where α is 48.1 × 10^-6°C^-1, (b) 180–300°C where α is 40.4 × 10^-6°C^-1. Similar behaviour is exhibited by quenched and later X-ray irradiated KCl the first region is up to 140°C, beyond which the second region takes over. From these data, concentration of vacancies in X-ray irradiated KCl at room temperature is calculated to be 3.4 × 10¹⁷ cm^-3 which is in fairly good agreement with the value obtained from F-band absorption measurements on the sample. An attempt has been made to understand these results.     

Short-lived color and luminescence centers in LiNbO3 crystals irradiated by pulsed electron beams

Data on spectra of short-lived optical absorption (SLOA) and luminescence induced in congruent crystals of lithium niobate by a pulsed electron beam (0.25 MeV, 20 ns, 15–160 mJ/cm²) in a temperature range of 80 to 350 K are presented. Anisotropic bands with maxima at 1.6 and 4.0 eV, originating from the capture of one and two conduction electrons by an (Nb_Nb-Nb_Li) complex, respectively, and weakly polarized bands at 2.5 and 3.3 eV due to holes located at Li and Nb vacancies are identified in the structure of SLOA spectra. Cathodoluminescence (CL) of lithium niobate crystals is characterized by fast (τ<4 ns) decay and a broad spectrum, which contains the same bands as the SLOA spectra. It is shown that the change in the initial amount of defects in reduced crystals at 830 K results in an identical change in both the CL and SLOA spectra. A model which treats the luminescence as the result of radiative nonphonon transitions that accompany the thermalization of charge carriers captured into the ground state of a polaron is discussed.     

Optical properties of silver colloidal centers in KCl crystals

The Ag colloidal centers in additively colored KCl: Ag crystals have been studied by optical and electron microscope methods. The transmission electron microscope studies reveal that the colloidal particles are spherical in shape and have a size distribution which can be described by the skewed zeroth order logarithmic distribution (ZOLD) function. The band shape of the experimental absorption spectra of colloidal particles agrees well with that calculated on the basis of Mie theory, if the optical constants are suitably modified and if the particle size distribution is taken into account. If the particle size is so small that the size distribution cannot be measured by electron microscope the optical absorption bands can be used to determine the size distribution parameters provided the distribution is well described by ZOLD, which in turn is used to calculate theoretical extinction bands.     

Transient color centers in GGG crystals

Electron pulse induced absorption and their decay kinetics have been investigated in samples of GGG crystals with different starting absorption spectra. It is shown that for all samples there appears a wide transient absorption (TA) band with two maxima in the region 14,000-17,000 v cm m 1 and 22,000-26,000 v cm m 1 . TA decay kinetics measurements in 14,000 v cm m 1 and 22,000 v cm m 1 are two-exponential (with half-time order several tens and several hundreds ns). Analyzing the obtained results, we can suppose that low and high energy TA bands are connected with the F + (or O m ) and F transient color centers (TCC) respectively.     

Laser stimulated luminescence in KCl and NaF containing color centers

A broad luminescence band was found in laser irradiated KCl and NaF crystal containing color centers. This emission extends on both sides of the laser line and is interpreted as “hot” fluorescence due to radiative retrapping of laser ionized color centers.     

Deformation Luminescence in gamma-irradiated KCl single crystals

Abstract

Deformation Luminescence(DL) is studied on KCl crystal colored by γ-irradiation. The spectra analysis of DL and thermoluminescence reveals that F-center plays a role as electron donor through the interaction with moving dislocation and V₂-center is a probable luminescence center. A theory is presented for understanding the deformation rate and temperature dependences of DL intensity.     

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.