Neutron quasielastic scattering from dynamic disorder in PbF2 and BaF2

Coherent neutron quasielastic scattering has been observed from powder samples of PbF₂, and BaF₂, at temperatures above their transition to fast-ion conduction. The scattering is found to be similar to that previously observed in SrCl₂ and CaF₂, suggesting that it stems from a mechanism common to these fluorites in their fast-ion phase.     

基于delta-P1近似模型的空间分辨漫反射一阶散射参量灵敏度研究

以delta-P1近似光学模型为基础, 推导了双点源近似下空间分辨漫反射一阶散射参量μ_s'灵敏度的解析式, 并进行了数值分析和比较. 研究表明, 与混合漫射近似模型和漫射近似模型相比, delta-P1 近似模型能更好地描述强散射较强吸收情况下近光源区域生物组织漫反射光子的分布, 且在有效反照率a'>0.83时, 获得最佳优化距离ρ_opt, ρ_opt 随μ_s' 的增大而减小, 且在距光源约2.7—4个输运平均自由程处μ_s'的变化对测量吸收的影响最小. 这项研究对于优化传感器几何结构以及生物组织光学参量的测量具有重要意义.     

Multi-k multipole order analyzed by resonant X-ray scattering in the electric quadrupole transition

We introduce a convenient expression of the resonant X-ray scattering (RXS) amplitude, which is appropriate for the investigation of the multipole ordering phase in localized f electron system. The expression is applied to the calculation of the RXS spectra in the electric quadrupole transition around the Np M_2,3 edges from the longitudinal triple-k antiferrooctupolar ordering phase in NpO₂. The results demonstrate a possibility that analyzing the energy profile of the RXS spectrum is effective to identify the type of the order parameter.     

Correlated Huang scattering in hexagonal Jahn Teller compounds

This is a theoretical investigation on structural precursors in diffuse neutron scattering of disordered phases of cooperative Jahn-Teller compounds above their structural phase transitions. The theory is based on Huang scattering off elastic dipoles. Softening of shear modes in the smallq regime known from inelastic scattering predicts monoclinic and orthorhombic structures which are found in some of these compounds. For these smallq the Jahn-Teller effect looks dynamic and Huang scattering is not applicable. With larger wave vectors the dipoles look static and Huang theory works but one picks up neighbor correlations which have to be included. We believe to have identified the spiral correlation announcing the helical structure of CsCuCl₃ in its ordered phase. We also calculate diffuse scattering for RbCrCl₃ and the experimentally observed satellites announcing the cell doubling of the ordered phase.     

Experimental and theoretical study of charge transfer in hydrogen ion scattering from a graphite surface

Ion scattering spectrometry (ISS) with time of flight (TOF) analysis is employed to measure the ion fraction of positively charged hydrogen (H⁺) projectiles scattered from a well characterized highly oriented pyrolitc graphite (HOPG) surface at a 45° scattering angle, various ingoing/outgoing angles and two different incoming energies (4 and 5 keV). In the theoretical approach, the negative ionization probability is calculated by employing a Green's function formalism to solve the dynamic collisional process. Both theoretical and experimental results are analyzed and contrasted. The theoretical negative ion fraction evolution during the collisional process is described in detail.     

Diffuse scattering and lattice dynamics of superionic copper chalcogenides

The energy-resolved neutron diffraction and inelastic neutron scattering study of diffuse scattering in copper chalcogenides was performed in order to clarify the role of static disorder versus low-energy phonons. Neutron diffraction patterns taken from Cu_1.75Se, Cu_1.98Se and Ag_0.25Cu_1.75Se powders in superionic phase show a broad maximum related to diffuse scattering. This diffuse background is suppressed in the energy-resolved experiment which indicates a strong contribution from inelastic scattering coming from correlated thermal displacements of the ions in the superionic phase. Diffraction experiments on a single crystal of α-Cu_1.8Se have revealed an ordered structure with superstructure reflections at the G ± 1/2 <111> and G ± 1/3 <220> positions of reciprocal space at room temperature. In addition to superstructure spots, diffuse scattering was observed along the <111> direction which is considered as a possible diffusion path of mobile Cu ions. In inelastic neutron scattering measurements with this single crystal sample strong inelastic scattering was observed along <111>. This shows that the diffuse scattering found in conventional diffraction experiment is mainly inelastic and most probably comes from low-energy phonons.     

Thermal stability and spectroscopic properties of Er-doped lead fluorogermanate glasses

The thermal characterization and spectroscopic properties of Er³⁺-doped 0.6GeO₂-(0.4-x)PbO-xPbF₂ glasses were investigated experimentally. With the replacement of PbO by PbF₂ the thermal stability of glasses is improved and the infrared fluorescence intensity at 1530 nm is increased. The Judd-Ofelt intensity parameters, radiative transition rates, and fluorescence lifetimes of the excited ⁴I_13/2 level of Er³⁺ ions were calculated from Judd-Ofelt theory. The asymmetric ligand field around Er³⁺ ions resulted from the incorporation of PbF₂ into germanate glasses, broadens the infrared emission spectra at 1530 nm. Upconversion luminescence in the investigated glasses was observed at room temperature under the excitation of 976 nm laser diode. The glass 0.6GeO₂-0.3PbO-0.1PbF₂ exhibits the maximum upconversion emission intensity, while no frequency upconversion luminescence was observed in the 0.6GeO₂-0.4PbO glass. The quadratic dependence of the green and red emissions on excitation power indicates that two-photon absorption contributes to the visible emission under the 976-nm excitation.     

Chord length distributions and small-angle scattering

Chord length distributions describe size, shape and spatial arrangement of geometrical objects (particles). The chord length distribution is in principle proportional to the second derivative of the correlation function of small-angle scattering. It is calculable from a relative measurement of the scattering intensity I(h). In structure research, the characterization of numerous particle systems can be achieved by comparing experimental chord distributions with theoretical ones, provided the latter are available with sufficiently high precision for a lot of fundamental, universal shapes. Both sides of this concept are exemplified: – the step from a relative measurement of the scattering intensity of an isotropic two-phase sample to the chord length distribution (errors in and in , limited h-interval, corresponding to the region (1-2) nm < r in real space, must be observed); as well as the geometric matter of calculation of chord distributions as fingerprints for basic geometric figures, including the non-convex case. Received 15 March 1999 and Received in final form 26 April 2000     

Diffuse reflectance of TiO2 pigmented paints: Spectral dependence of the average pathlength parameter and the forward scattering ratio

Diffuse reflectance spectra of paint coatings with different pigment concentrations, normally illuminated with unpolarized radiation, have been measured. A four-flux radiative transfer approach is used to model the diffuse reflectance of TiO₂ (rutile) pigmented coatings through the solar spectral range. The spectral dependence of the average pathlength parameter and of the forward scattering ratio for diffuse radiation, are explicitly incorporated into this four-flux model from two novel approximations. The size distribution of the pigments has been taken into account to obtain the averages of the four-flux parameters: scattering and absorption cross sections, forward scattering ratios for collimated and isotropic diffuse radiation, and coefficients involved in the expansion of the single particle phase function in terms of Legendre polynomials.     

Nonlinear diffraction in spontaneous three-wave and coherent four-wave light scattering by polaritons

Spontaneous three-wave and coherent four-wave scattering by polaritons in crystalline media with periodically modulated nonlinear quadratic susceptibility have been studied both theoretically and experimentally. Phase matching conditions and an expression for the scattering spectral line shape with due account of cascade processes in nonlinear diffraction for coherent Stokes polariton light scattering have been derived. Measurements of the light intensity distribution due to the three-and four-wave scattering in a LiNbO₃:Nd:Mg crystal with a periodic domain structure are in good agreement with theoretical results. The prospects for using the effects of nonlinear diffraction in spectroscopic studies of media with periodic distributions of nonlinear optical parameters, specifically, in precision measurements of the IR refractive index dispersion and determination of the period and profile of the quadratic susceptibility distribution are discussed. Zh. éksp. Teor. Fiz. 112, 2001–2015 (December 1997)     

Spin relaxation rates of Kondo systems measured by diffuse neutron scattering

A method for the direct measurement of spin relaxation rates of dilute magnetic systems by diffuse scattering of unpolarized neutrons is discussed. The magnetic scattering from the single magnetic impurities results from a difference measurement of doped and undoped sample. To discriminate against the large background from nonmagnetic scattering the choice of special energy windows is necessary for the detection of the magnetic scattering.Neutron scattering experiments were performed on the Kondo system CuFe for concentrations between 600 and 4800 at. ppm Fe and in the temperature range 15 K to 300 K. The measured absolute values and the temperature dependence of the spin relaxation rates, which have been measured for the first time in this experiment, are found to agree well with theoretical calculations by Götze and Schlottmann. This supports the explanation of the Kondo effect as a dynamical phenomenon and is contradictory to the static picture of a quasiparticle as proposed by Heeger.     

Acoustic absorption in superionic PbF2 across transition temperature

In the present paper, we report a theoretical calculation of temperature variation of the ultrasonic absorption coefficient of PbF₂. The absorption can be explained by the TLS model, with parameters which simultaneously reproduce the specific heat, thermal expansion, bulk modulus and ionic conductivity. Our results agree well with recent experimental reports of ultrasonic attenuation in PbF₂.     

Total cross-sections for positron scattering by a series of molecules

The additivity rule is employed to obtain the total (elastic+inelastic) cross-sections for positron scattering from molecules including a number of diatomic, polyatomic molecules (H₂, N₂, HCl, CO₂, NH₃, SF₆, CH₄, C₂H₄ and C₃H₈) over an incident energy range of 10-1000 eV. The total cross-sections (TCS) of the constituent atoms of molecules are obtained by employing a complex optical model potential (composed of static, polarization and absorption potential). The present results are compared with experimental data and other theoretical calculations, good agreement is obtained in intermediate- and high-energy region. Received: 11 November 1997 / Revised: 23 March 1998 / Accepted: 16 June 1998     

Practical method for experimental effective atomic number in the coherent to Compton scattering ratio

A practical method of calculating Z_eff was developed for the coherent to Compton scattering ratio and the method is applicable for any material with known weight percentages of different elements in the compound. The coherent to Compton scattering ratio depends only on the mixture under study and provides a measurement of certain complicated functions of the atomic number Z and Z_eff. In order to measure the effective atomic number of Sb₂O₃, BaO, La₂O₃, CeO₂ and Nd₂O₃, the linear differential scattering coefficients of 59.5 keV γ-rays have been studied using a high-resolution Si(Li) semi-conductor detector. The coherent to Compton scattering ratio R is taken from the linear differential scattering coefficients μ(E, q). The results obtained have been compared with relativistic (RT) and non-relativistic (NRT) theoretical values.     

Resonant inelastic X-ray scattering and its magnetic circular dichroism

Magnetic circular dichroism in resonant inelastic X-ray scattering (MCD-RIXS) of ferromagnetic systems is discussed for the longitudinal geometry (LG), where the directions of incident X-ray and magnetization are parallel, and the transverse geometry (TG), where they are perpendicular. MCD-RIXS in LG is represented by two successive real processes, X-ray absorption and X-ray emission, while MCD-RIXS in TG is given by the interference process. We describe an application of MCD-RIXS in LG to the detection of Ce 4f² contribution in Ce, L₃ absorption edge of CeFe₂, and theoretical and experimental studies of MCD-RIXS in TG for Gd and Sm systems.     

Spectroscopic properties of Er/Yb codoped fluorophosphate glasses

Er³⁺/Yb³⁺ codoped fluorophosphate glasses were prepared and their thermal stabilities, Raman spectra, absorption spectra, and fluorescence spectra were measured. It is found that proper content of NaF or PbF₂ is helpful for the increase of stability against crystallization. The variation of Al(PO₃)₃ or NaF content in the composition affects not the maximum phonon energy but the phonon density. The introduction of PbF₂ decreases the phonon energy slightly. Intense green and red upconversion luminescence was observed for the fluorophosphate glass with low phosphate content. A glass matrix for upconversion luminescence requiring neither expensive raw material nor special atmospheric conditioned preparation is provided. Infrared luminescence around 1530 nm was researched. Fluorophosphate glasses with bandwidth properties and stimulated-emission cross sections better than tellurite, germanate and silicate glasses are obtained. Through the introduction of NaF, the bandwidth properties are decreased. Through the introduction of PbF₂ the gain properties are increased. On the whole, it is difficult to obtain a material with the best gain properties and bandwidth properties simultaneously. There should be a compromise between them according to the demand.     

The measurement of energy spectra of neutral particles in low energy ion scattering

An apparatus is described for low energy (0.1–10 keV) ion scattering (LEIS) experiments. A time of flight (TOF) spectrometer is incorporated in the system to be able to measure the energy of particles in the neutral state after scattering. The energy resolution ΔE/E of the TOF spectrometer is discussed and found to be 0.5% (FWHM). This is sufficient for our scattering experiments. An electrostatic analyzer (ESA) is used to measure the energy of scattered ions [ΔE/E=0.5% (FWHM)]. Experiments show that in general the ion dose needed to obtain a TOF spectrum (2×10¹⁰ ions/cm²) is much smaller than the dose needed for an ESA-spectrum (6×10¹³ ions/cm²). The ion spectra measured with the TOF spectrometer, by subtracting the neutral yield from the total yield, as well as with the ESA are found to agree quite well. This provides a way to calibrate the TOF spectrometer. The determination of the ion fraction of scattered particles is discussed [10 keV⁴⁰Ar⁺ on Cu(100), scattering angle 30°]. It is shown that the TOF spectrometer is able to measure light recoil particles (e.g. hydrogen) from a heavy substrate. In the analysing system is, in addition to the TOF spectrometer, also incorporated a stripping cell to measure the energy of neutral scattered particles. An energy spectrum of neutral scattered particles measured with both methods is shown.     

Inelastic neutron scattering study in cubic NaNbO3

Inelastic neutron scattering measurements performed on NaNbO₃ close to the cubic to tetragonal phase transition show that the 2D fluctuations earlier revealed by X-ray diffuse scattering correspond to an overdamped type dynamics. The sharp decrease in the spectral width observed at T-T_c =10° is interpreted in terms of the increase in correlation lengths and the cross-over between 2D and 3D behaviour of the fluctuations.