Charge-transfer bands in crystals of alkaline earth fluorides with Eu<Superscript>3+</Superscript> and Yb<Superscript>3 + 1</Superscript>

The absorption, luminescence, and excitation spectra of CaF₂, SrF₂, and BaF₂ crystals with EuF₃ or YbF₃ impurity have been investigated in the range 1–12 eV. In all cases, strong wide absorption bands (denoted as CT₁) were observed at energies below the 4f ⁿ -4f ^{n ? 1}5d absorption threshold of impurity ions. Weaker absorption bands (denoted as CT₂) with energies 1.5–2 eV lower than those of the CT₁ bands have been found in the spectra of CaF₂ and SrF₂ crystals with EuF₃ or YbF₃ impurities. The fine structure of the luminescence spectra of CaF₂ crystals with EuF₃ impurities has been investigated under excitation in the CT bands. Under excitation in the CT₁ band, several Eu centers were observed in the following luminescence spectra: C _4v , O _h , and R aggregates. Excitation in the CT₂ bands revealed luminescence of only C _4v defects.     

Oscillator strengths and lifetimes of levels for ions of the rubidium isoelectronic sequence calculated by the Dirac-Fock method

The oscillator strengths f of the resonance transitions are calculated by the Dirac-Fock method for the first ten terms of the rubidium isoelectronic sequence. The variation of f values along the isoelectronic sequence is analyzed. The corresponding transition probabilities and lifetimes τ of the levels 5p ² P _1/2 and 5p ² P _3/2 are calculated. The dependence of τ on the ion charge is approximated by an analytical equation. The obtained values of f and τ are compared with experiment and other calculations.     

One-dimensional magnetophotonic crystals with magnetooptical double layers

One-dimensional magnetophotonic microcavity crystals with nongarnet dielectric mirrors are created and investigated. The defect layers in the magnetophotonic crystals are represented by two bismuth-substituted yttrium iron garnet Bi:YIG layers with various bismuth contents in order to achieve a high magnetooptical response of the crystals. The parameters of the magnetophotonic crystal layers are optimized by numerical solution of the Maxwell equations by the transfer matrix method to achieve high values of Faraday rotation angle Θ _F and magnetooptical Q factor. The calculated and experimental data agree well with each other. The maximum values of Θ _F =–20.6°, Q = 8.1° at a gain t = 16 are obtained for magnetophotonic crystals with m = 7 pairs of layers in Bragg mirrors, and the parameters obtained for crystals with m = 4 and t = 8.5 are Θ _F =–12.5° and Q = 14.3°. It is shown that, together with all-garnet and multimicrocavities magnetophotonic crystals, such structures have high magnetooptical characteristics.     

Schwingungszustände und optische Eigenschaften von Molekelkristallen

A discussion of the Born-Huang theory of dispersion in polar crystals with regard to the internal vibrations of molecular crystals (organic crystals, ionic crystals with complex ions) connects the imaginary part (?″) of the optical dielectric tensor (?)=(?′)?i(?″) with the eigenfrequencies of the vibrations active in absorption and with the direction and magnitude of the transition moments in crystals. The limits are shown, within which this theory gives a Lorentzian line shape, often found in experiment. The propagation of an electromagnetic wave in a strongly absorbing, anisotropic medium and its reflection at crystal boundaries are investigated with special emphasis on the experimental determination of (?). This treatment leads to expressions for (?), which are not limited to the case of weak absorption and which are valid for all crystal symmetries.     

Specific features of absorption and luminescence in CsBr: EuOBr crystals

The specific features of the absorption, photoluminescence, x-ray luminescence, thermally stimulated luminescence, and photostimulated luminescence spectra of CsBr: Eu²⁺ single crystals grown using the Bridgman method are investigated in the temperature range 80–500 K at the highest possible dopant content (0.1–0.4 mol % EuOBr in the batch) required for preparing perfect crystals. It is shown that an increase in the dopant content leads to a broadening of the absorption and photoluminescence excitation bands with maxima at wavelengths of 250 and 350 nm due to the interconfigurational transitions 4f⁷(⁸S_7/2) → 4f⁶5d(e _g , t_2g) in Eu²⁺ ions. The photoluminescence and photostimulated luminescence spectra of CsBr: EuOBr single crystals (0.1–0.4 mol % EuOBr) contain a band at a wavelength of λ_max=450 nm and bands at wavelengths of λ_max=508–523 and 436 nm. The last two bands are assigned to Eu²⁺-V_Cs isolated dipole centers and Eu²⁺-containing aggregate centers, respectively. It is revealed that the intensity of the luminescence associated with the aggregate centers (λ_max=508–523 nm) is maximum at an EuOBr content of less than or equal to 0.1 mol % and decreases with an increase in the dopant content. The possibility of forming CsEuBr₃-type nanocrystals that are responsible for the green luminescence observed at a wavelength λ_max=508–523 nm in CsBr: Eu crystals is discussed. The intensity of photostimulated luminescence in the CsBr: EuOBr crystals irradiated with x-ray photons is found to increase as the dopant content increases. It is demonstrated that CsBr: EuOBr crystals at a dopant content in the range 0.3–0.4 mol % can be used as x-ray storage phosphors for visualizing x-ray images with high spatial resolution.     

Determination of the scattering coefficient of biological tissue considering the wavelength and absorption dependence of the anisotropy factor

The anisotropy factor g, one of the optical properties of biological tissues, has a strong influence on the calculation of the scattering coefficient μ _s in inverse Monte Carlo (iMC) simulations. It has been reported that g has the wavelength and absorption dependence; however, few attempts have been made to calculate μ _s using g values by taking the wavelength and absorption dependence into account. In this study, the angular distributions of scattered light for biological tissue phantoms containing hemoglobin as a light absorber were measured by a goniometric optical setup at strongly (405 nm) and weakly (664 nm) absorbing wavelengths to obtain g. Subsequently, the optical properties were calculated with the measured values of g by integrating sphere measurements and an iMC simulation, and compared with the results obtained with a conventional g value of 0.9. The μ _s values with measured g were overestimated at the strongly absorbing wavelength, but underestimated at the weakly absorbing wavelength if 0.9 was used in the iMC simulation.     

Thermolumineszenz von KCl und NaCl nach Röntgen- und UV-Bestrahlung bei Heliumtemperatur

Glow curves of luminescence are recorded in the range from 10° K to 300° K. One gets characteristic changes by prior annealing the single crystals in O₂ or HCl. Also an increase of the lattice disorder causes new glow bands. In KCl a strong glow band always appears at 40° K after irradiating with X-rays or ultraviolet light in the range of the exciton bands. It is ascribed to trapped excitons, which become mobile at that temperature. For X-ray irradiated KCl the glow curve of luminescence is compared with the electrical glow bands and with the concentration change of known defects. The half widthΔT of all glow bands is found proportional to the temperature of the maximum:ΔT=(0.08±0.02)T _m .     

Crystallographic and dynamic characteristics of K<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Rb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiOPO<Subscript>4</Subscript> solid solutions in the low-temperature ranges

The unit cell parameters a and c of K_{1 ? x} Rb _x TiOPO₄ (x= 0, 0.3, and 0.5) crystals are investigated using x-ray diffraction in the temperature range 90–320 K. The thermal expansion coefficients along the principal crystallographic axes of the crystals are determined from the obtained temperature dependences of the unit cell parameters. It is found that the parameter a increases with increasing temperature, while the parameter c decreases. For the crystals under investigation, the elastic moduli c ₁₁ and c ₃₃ along the [100] and [001] directions are determined by the ultrasonic pulse-echo technique in the temperature range 100–350 K. It is shown that c ₃₃ > c ₁₁. The anomalies revealed in the temperature dependences of the crystallographic and dynamic characteristics of the samples at a temperature T ≈ 280 K indicate the occurrence of a phase transition. The temperature of the phase transition is found to increase with increasing rubidium content x.     

Temperaturabhängigkeit der Absorptionskoeffizienten für mittelschnelle Elektronen in polykristallinen Aluminium-, Silber- und Goldschichten

The absorption coefficientsμ _el andμ _unel for fast electrons (30–50 keV) due to elastic and inelastic scattering in polycrystalline foils of Al, Ag and Au have been measured at various temperatures between 160 and 600 °K. It is shown that,μ _unel decreases only slightly with temperature. On the other hand, the absorption coefficientμ _el, which is composed ofμ _R for Laue Bragg scattering andμ _TDS for thermal diffuse scattering increases with temperature. Asμ _R andμ _TDS depend on temperature in opposite sense, the resulting increase ofμ _el=μ _R+μ _TDS with temperature depends on the extent, to which the temperature dependence ofμ _R is reduced by dynamical extinction effects. For Al the measured temperature dependence ofμ _el is in good agreement with the theoretical calculation ofμ _TDS on the basis of the Einstein model and ofμ _R according to the two beam approximation of the dynamical theory. For Ag and Au the temperature dependence ofμ _el is much more pronounced than for Al. This is due to the facts that for heavy elements firstly the elastic scattering is stronger and secondly dynamical extinction effects are generally more pronounced. In order to study the influence of dynamical extinction, the crystal size of the foils was varied.     

Absorption of light by exchange-coupled ions in a 2D antiferromagnet

The optical absorption spectra of Rb₂Mn_xCd_1?xCl₄ crystals are experimentally studied in the vicinity of a magnon sideband of the exciton band at a manganese content x ranging from 1.0 to 0.4. Additional absorption bands are observed with an increase in the magnetic structural disorder upon replacement of manganese ions by cadmium ions. An analysis of the evolution of the additional absorption bands in a magnetic field during the spin-flop phase transition and the change in the intensity with variations in the manganese content x demonstrates that these bands are associated with the excitation of the exchange-coupled pairs of manganese ions located in different environments in a plane square lattice. The phase boundary between the antiferromagnetic and spin-flop phases is constructed using the results of optical measurements. The manganese content corresponding to the magnetic percolation point is evaluated.     

Refractometry of mechanically compressed (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>SO<Subscript>4</Subscript> crystals

The effect of uniaxial mechanical pressure σ _m ≤ 150 bar on the spectral (300–800 nm) dependence of the birefringerence Δn _i and refractive indices n _i of (NH₄)₂SO₄ crystals has been investigated. It is shown that the dispersion of n _i (λ) and Δn _i (λ) is normal and sharply increases with approach to the absorption edge. It is established that uniaxial pressure does not change the character of the dispersions dn _i /dλ and dΔn _i /dλ and only affects their magnitudes. It is shown that the increase in the refractive indices under uniaxial stress is mainly due to the increase in the refraction caused by the increase in the band gap and long-wavelength shift of the UV absorption band maximum.     

Magnon satellite bands in the optical spectrum of antiferromagnetic Rb<Subscript>2</Subscript>MnCl<Subscript>4</Subscript>

The evolution of optical absorption in a two-dimensional antiferromagnet is investigated in the range of the transition ⁶A_1g → ⁴A_1g, ⁴E _g (⁴G) observed in manganese ions in an external magnetic field inducing noncollinearity of the magnetic structure. It is revealed that hot and cold satellites of the exciton-magnon bands appear in the optical absorption spectrum and then increase in intensity. The shapes of the magnon satellite bands corresponding to a two-dimensional magnetic structure are calculated. It is demonstrated that magnons at the inner points of the Brillouin zone appreciably contribute to the absorption. The zero-point magnetic oscillations play a decisive role in the absorption associated with the magnon decay at low temperatures.     

Microwave spectroscopy of cold rubidium atoms

The effect of microwave radiation on the resonance fluorescence of a cloud of cold ⁸⁵Rb atoms in a magnetooptical trap is studied. The radiation frequency was tuned near the hyperfine splitting frequency of rubidium atoms in the 5S ground state. The microwave field induced magnetic dipole transitions between the magnetic sublevels of the 5S (F=2) and 5S (F=3) states, resulting in a change in the fluorescence signal. The resonance fluorescence spectra were recorded by tuning the microwave radiation frequency. The observed spectra were found to be substantially dependent on the transition under study and the frequency of a repump laser used in the cooling scheme.     

Method of determination of muon-catalyzed fusion parameters in H-T mixture

A method for measurement of the muon-catalyzed fusion (μCF) parameters in an H-T mixture is proposed. The kinetics of the μ-atomic and μ-molecular processes preceding the pt reaction in the ptμ molecule is described. Analytical expressions are obtained for the yields and time distributions of γ quanta and conversion muons formed in nuclear fusion reactions in ptμ molecules. It is shown that information on the desired parameters μCF can be found from the joint analysis of the time distributions of γ quanta and conversion muons to be obtained in experiments with the H-T mixture at three (or more) appreciably different atomic concentrations of tritium. The experiments with the H-T mixture at the meson facility PSI (Switzerland) were optimized to gain precise information about the desired μCF parameters.     

Untersuchung über die gasionisierende Strahlung einer Entladung II

This paper gives a report on measurements of the coefficient of absorption and intensities of a gas-ionizing radiation, which is emitted by a gas discharge. Earlier measurements in oxygen with a discharge in a cylindrical electrical field had yielded components of radiation with values ofμ≈550,μ≈250, andμ= 38 cm^?1 according to 760 mm Hg. By new measurements using a spark discharge an additional component with a value ofμ=2,5 has been found, which already has been measured by other authors. Measurements with mixtures of oxygen and nitrogen are compared with results of measurements in air. From this it may be deduced that the gas-ionizing radiation effective in air is essentially emitted by nitrogen and ionizes oxygen, the coefficient of absorption being aboutμ≈5 cm^?1. The number of ionizing quantums emitted per ionizing collision of electrons in the discharge tube is about 10^?3 for oxygen, nitrogen, and air. In oxygen this figure proves to be essentially independent ofE/p. In air, however, this figure is reduced with increasing values ofE/p. The absorption of the ionizing radiation in air is increased by the addition of methane (μ=960 cm^?1 for 760 mm Hg of methane). In carbon dioxyde several components of ionizing radiation are found, with coefficients of absorption in the range 200<μ<800.     

Overtone transitions of diatomic dimers XH<Superscript>−</Superscript>-XH<Superscript>−</Superscript> in ionic crystals

The resonance interaction of molecular defects XH^– in ionic crystals is considered. In the dipole approximation, the frequencies and intensities of vibrational transitions of diatomic dimers ^mXH^?-ⁿXH^? are found for the exact (m=n) and quasi-exact (m≠n) resonances. The interaction-atrix elements are obtained with regard to the mechanical anharmonicity in the second order of the theory of perturbation for the case of a linear dependence of the dipole moment of a diatomic molecule on the vibrational coordinate. The expressions obtained are applied to the calculation of overtones in the absorption spectrum of dimers ^mSH^?-ⁿSH^? in a KCl crystal.     

Magnetic Resonance Properties of a Nanogranular Ni<Subscript><Emphasis Type="Italic">m</Emphasis></Subscript>C<Subscript>100 − <Emphasis Type="Italic">m</Emphasis></Subscript> Film Structure

Magnetic resonance properties characteristic of nanogranular film structure Ni_mC_{100 ? m} with different concentrations m of its magnetic phase are studied. Samples are subjected to annealing at temperatures of 200 and 300°C. With tangential magnetization in the planes of films with m ranging from 70 to 89.8 at %, anisotropy is observed for the resonance field and the width of the absorption line. This anisotropy is explained by the uneven shapes of magnetic granules along different directions in the films.     

Spin dynamics and phase transitions in quasi-2D antiferromagnets <Emphasis Type="Italic">R</Emphasis><Subscript>2</Subscript>CuO<Subscript>4</Subscript> (<Emphasis Type="Italic">R</Emphasis>=Pr,Nd, Sm,Eu, Gd)

The spin and lattice dynamics of the R₂CuO₄ quasi-2D antiferromagnetic crystals (R=Pr, Nd, Sm, Eu, Gd) were studied in the millimeter-range electromagnetic wave band. Strong variations of the absorption coefficient were observed to occur at temperatures T≈T₀. Absorption lines of electrical nature due to lattice dynamics were also revealed near the T₀ temperatures. The observed anomalies are assumed to originate from phase transitions at T≈T₀, which entail changes in the structural and magnetic properties.     

Shallow thermal donors in nitrogen-doped silicon single crystals

Czochralski-grown nitrogen-doped silicon crystals contain shallow thermal donors (STD) which are not present in reference crystals. In the course of annealing at 600 or 650°C, the STD concentration reaches saturation and this concentration scales with nitrogen content N as N^1/2. This implies that an STD includes only one nitrogen atom and that the most likely model of the STD defect is the NO_m complex of an interstitial nitrogen atom with m oxygen atoms. The number m is estimated as, on the average, m=3 from data on the temperature dependence of the equilibrium constant for the complex formation reaction