Showing papers on "Scattering published in 1993"

Recent advances in magnetic structure determination by neutron powder diffraction

Abstract: In spite of intrinsic limitations, neutron powder diffraction is, and will still be in the future, the primary and most straightforward technique for magnetic structure determination. In this paper some recent improvements in the analysis of magnetic neutron powder diffraction data are discussed. After an introduction to the subject, the main formulas governing the analysis of the Bragg magnetic scattering are summarized and shortly discussed. Next, we discuss the method of profile fitting without a structural model to get precise integrated intensities and refine the propagation vector(s) of the magnetic structure. The simulated annealing approach for magnetic structure determination is briefly discussed and, finally, some features of the program FullProf concerning the magnetic structure refinement are presented and discussed. The different themes are illustrated with simple examples.

Imaging standing waves in a two-dimensional electron gas

Abstract: ELECTRONS occupying surface states on the close-packed surfaces of noble metals form a two-dimensional nearly free electron gas1–3. These states can be probed using the scanning tunnelling microscope (STM), providing a unique opportunity to study the local properties of electrons in low-dimensional systems4. Here we report the direct observation of standing-wave patterns in the local density of states of the Cu(lll) surface using the STM at low temperature. These spatial oscillations are quantum-mechanical interference patterns caused by scattering of the two-dimensional electron gas off step edges and point defects. Analysis of the spatial oscillations gives an independent measure of the surface state dispersion, as well as insight into the interaction between surface-state electrons and scattering sites on the surface.

Raman study of CeO2: Second-order scattering, lattice dynamics, and particle-size effects.

Abstract: Polarized Raman-scattering spectra are obtained from oriented single crystals of ${\mathrm{CeO}}_{2}$. Second-order features are assigned to phonon overtones from the X and L points on the Brillouin-zone boundary on the basis of selection rules and by analogy to another fluorite structure compound, ${\mathrm{BaF}}_{2}$, in which the phonon dispersion curves are known. Complete selection rules for second-order scattering from these high-symmetry points are determined. A rigid-ion model is constructed for the phonon dispersion curves by fitting several of these features and the known zone-center optical modes. The ${\mathit{F}}_{2\mathit{g}}$ Raman-active mode at 465 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in ${\mathrm{CeO}}_{2}$ shifts and broadens with decreased particle size [G. W. Graham, W. H. Weber, C. R. Peters, and R. Usmen, J. Catalysis 130, 310 (1991)]. It is shown using the calculated dispersion curves that this particle-size effect cannot be explained with the spatial-correlation model.

Determining the optical properties of turbid media by using the adding–doubling method

Abstract: A method is described for finding the optical properties (scattering, absorption, and scattering anisotropy) of a slab of turbid material by using total reflection, unscattered transmission, and total transmission measurements. This method is applicable to homogeneous turbid slabs with any optical thickness, albedo, or phase function. The slab may have a different index of refraction from its surroundings and may or may not be bounded by glass. The optical properties are obtained by iterating an adding–doubling solution of the radiative transport equation until the calculated values of the reflection and transmission match the measured ones. Exhaustive numerical tests show that the intrinsic error in the method is <3% when four quadrature points are used.

Dynamic light scattering : the method and some applications

Abstract: List of contributors 1. Dynamic scattering from multicomponent polymer mixtures in solution and in bulk 2. Single photon correlation techniques 3. Noise on photon correlation functions and its effects on data reduction algorithms 4. Data analysis in dynamic light scattering 5. Dynamic light scattering and linear viscoelasticity of polymers in solution and in the bulk 6. Dynamic properties of polymer solutions 7. Application of dynamic light scattering to polyelectrolytes in solution 8. Simultaneous static and dynamic light scattering: application to polymer structure analysis 9. Dynamic light scattering from dense polymer systems 10. Dynamic light scattering from polymers in solution and in bulk 11. Dynamic light scattering from polymer gels 12. Dynamic light scattering from rigid and nearly rigid rods 13. Light scattering in micellar systems 14. Critical dynamics of binary liquid mixtures and simple fluids studied using dynamic light scattering 15. Application of dynamic light scattering to biological systems 16. Diffusing-wave spectroscopy Index

Partial-wave analysis of all nucleon-nucleon scattering data below 350 MeV.

Abstract: We present a multienergy partial-wave analysis of all NN scattering data below ${\mathit{T}}_{\mathrm{lab}}$=350 MeV, published in a regular physics journal between 1955 and 1992. After careful examination, our final database consists of 1787 pp and 2514 np scattering data. Our fit to these data results in ${\mathrm{\ensuremath{\chi}}}^{2}$/${\mathit{N}}_{\mathrm{df}}$=1.08, with ${\mathit{N}}_{\mathrm{df}}$=3945 the total number of degrees of freedom. All phase shifts and mixing parameters can be determined accurately.

Reflection from layered surfaces due to subsurface scattering

Abstract: The reflection of light from most materials consists of two major terms: the specular and the diffuse. Specular reflection may be modeled from first principles by considering a rough surface consisting of perfect reflectors, or micro-facets. Diffuse reflection is generally considered to result from multiple scattering either from a rough surface or from within a layer near the surface. Accounting for diffuse reflection by Lambert’s Cosine Law, as is universally done in computer graphics, is not a physical theory based on first principles. This paper presents a model for subsurface scattering in layered surfaces in terms of one-dimensional linear transport theory. We derive explicit formulas for backscattering and transmission that can be directly incorporated in most rendering systems, and a general Monte Carlo method that is easily added to a ray tracer. This model is particularly appropriate for common layered materials appearing in nature, such as biological tissues (e.g. skin, leaves, etc.) or inorganic materials (e.g. snow, sand, paint, varnished or dusty surfaces). As an application of the model, we simulate the appearance of a face and a cluster of leaves from experimental data describing their layer properties. CR Categories and Subject Descriptors: I.3.7 [Computer Graphics]: Three-Dimensional Graphics and Realism.

Monte Carlo study of electron transport in silicon inversion layers.

Abstract: Electron transport in Si inversion layers at 300 K is studied using a self-consistent Monte Carlo solution of the Boltzmann transport equation coupled to the two-dimensional Poisson equation and the one-dimensional Schr\"odinger equation. Physical elements included in the model are (1) nonparabolicity effects to treat quantization in the inversion layer; (2) static screening of the Coulomb interactions accounting for the population of many subbands; (3) anisotropy of the deformation-potential interaction, shown to be quite important in the case of a two-dimensional electron gas (2DEG); (4) a careful analysis of the dynamic screening of the deformation-potential interaction, showing that the interaction between electrons and acoustic phonons can be approximated by the unscreened interaction in the nondegenerate limit of a 2DEG; and (5) the inclusion of interface ${\mathrm{SiO}}_{2}$ optical phonons. Up to ten subbands have been included to study the 2DEG together with a bulk-transport model employed to handle high-energy electrons. We have obtained mixed results: In the Ohmic regime, we have found a phonon-limited mobility that exhibits the correct dependence on carrier density, but which is about 20% larger than the experimental data. This still represents an improvement upon previous nonempirical theories, and even better quantitative agreement is obtained at the very low and very high carrier densities at which Coulomb scattering and scattering with surface roughness, respectively, control the mobility. At high longitudinal fields we find a bulklike saturated velocity, in agreement with some experimental results, but not with many others that we consider more reliable.

Double-integrating-sphere system for measuring the optical properties of tissue

Abstract: A system is described and evaluated for the simultaneous measurement of the intrinsic optical properties of tissue: the scattering coefficient, the absorption coefficient, and the anisotropy factor. This system synthesizes the theory of two integrating spheres and an intervening scattering sample with the inverse adding–doubling algorithm, which employs the adding–doubling solution of the radiative transfer equation to determine the optical properties from the measurement of the light flux within each sphere and of the unscattered transmission. The optical properties may be determined simultaneously, which allows for measurements to be made while the sample undergoes heating, chemical change, or some other external stimulus. An experimental validation of the system with tissue phantoms resulted in the determination of the optical properties with a <5% deviation when the optical density was between 1 and 10 and the albedo was between 0.4 and 0.95.

Direct observation of standing wave formation at surface steps using scanning tunneling spectroscopy.

Abstract: Using scanning tunneling microscopy and spectroscopy, we have observed the formation of standing waves by the scattering of surface state electrons at steps and defects on the Au(111) surface. From the periods, peak positions, and intensities of the standing waves, the energy dispersion of the surface states, the scattering phase shifts as a function of energy and crystallographic direction, and coherence lengths could be determined.

Propagation of photon-density waves in strongly scattering media containing an absorbing semi-infinite plane bounded by a straight edge

Abstract: Light propagation in strongly scattering media can be described by the diffusion approximation to the Boltzmann transport equation. We have derived analytical expressions based on the diffusion approximation that describe the photon density in a uniform, infinite, strongly scattering medium that contains a sinusoidally intensity-modulated point source of light. These expressions predict that the photon density will propagate outward from the light source as a spherical wave of constant phase velocity with an amplitude that attenuates with distance r from the source as exp(-alpha r)/r. The properties of the photon-density wave are given in terms of the spectral properties of the scattering medium. We have used the Green's function obtained from the diffusion approximation to the Boltzmann transport equation with a sinusoidally modulated point source to derive analytic expressions describing the diffraction and the reflection of photon-density waves from an absorbing and/or reflecting semi-infinite plane bounded by a straight edge immersed in a strongly scattering medium. The analytic expressions given are in agreement with the results of frequency-domain experiments performed in skim-milk media and with Monte Carlo simulations. These studies provide a basis for the understanding of photon diffusion in strongly scattering media in the presence of absorbing and reflecting objects and allow for a determination of the conditions for obtaining maximum resolution and penetration for applications to optical tomography.

Atomic and Molecular Processes: an R-Matrix Approach

Abstract: Introduction. Summary of atomic and molecular R-matrix theory. Applications: Electron-atom scattering. Electron-ion scattering. Electron-molecule scattering H^T2, O^T2, HCl. Photoionization. Free-free transitions H-, C-. Computer program packages: Structure codes. Scattering codes. References not in the R-matrix bibliography. Key to the R-matrix bibliography. R-matrix bibliography. Reproduced papers.

Comparison of numerical models for computing underwater light fields

Abstract: Seven models for computing underwater radiances and irradiances by numerical solution of the radiative transfer equation are compared. The models are applied to the solution of several problems drawn from optical oceanography. The problems include highly absorbing and highly scattering waters, scattering by molecules and by particulates, stratified water, atmospheric effects, surface-wave effects, bottom effects, and Raman scattering. The models provide consistent output, with errors (resulting from Monte Carlo statistical fluctuations) in computed irradiances that are seldom larger, and are usually smaller, than the experimental errors made in measuring irradiances when using current oceanographic instrumentation. Computed radiances display somewhat larger errors.

The effects of photoionization on X-ray reflection spectra in active galactic nuclei

Abstract: Recent work on the X-ray spectra of Seyfert galaxies has shown the presence of a reflection component due to the hard X-ray continuum scattering from and causing fluorescence by cold matter. The exact location of the matter in relation to the hard X-ray source is unclear, but the strength of the reflection spectrum is usually consistent with the hard X-rays being emitted above a flat accretion disc. In this paper we calculate reflection spectra for cases where the hard X-rays photoionize the surface layers of the disc to the extent that the main X-ray absorbers are significantly ionized. The temperature, ionization state and spectrum are computed in a fully self-consistent manner for the inner, radiation pressure dominated part of an accretion disc

Rotator phases of the normal alkanes: An x‐ray scattering study

Abstract: We present results of a detailed x‐ray scattering study on the rotator phases of normal alkanes: CH3–(CH2)n−2–CH3 (20≤n≤33). We have characterized a new tilted rotator phase and determined the temperature and chain length dependence of the distortion, tilt, and azimuthal order parameters which characterize the time‐space averaged structures of the five rotator phases. We have shown that there is no strong even–odd chain length effect on the phase diagram within the rotator phases and have shown the continuity of that phase diagram in the 26‐27 carbon vicinity.

Quantifying absorption by aquatic particles: A multiple scattering correction for glass-fiber filters

Abstract: : Absorption spectra measured for aquatic particles concentrated onto glass-fiber filters require a correction for the increase in pathlength caused by multiple scattering in the glass-fiber filter. A multiple scattering correction was calculated from optical density spectral for 48 phytoplankton cultures of seven species representing a variety of cell sizes, pigment groups, and call-wall types. The relationship between optical density in suspensions and on filters was not wavelength-dependent. Differences between blank filters were always spectrally neutral. Small differences between relationships for single species were inconclusive. Given the absence of wavelength-dependent effects, we report a single general quadratic relationship, OD(susp)(lambda) = 0.378 OD(filt)(lambda) + 0.523 OD(filt)(lambda)2 (r2 = 0.988), for correcting glass- fiber filter spectra. For independent samples, the average error in predicting OD(susp)(lambda) with this algorithm at any wavelength was 2%. Greatest errors were in spectral regions of low absorption. Absorption spectra for particles concentrated onto glass-fiber filters can be quantitatively corrected for multiple scattering within this limit. Applicability of the algorithm to field samples of varied composition was enhanced by using a large number of spectra and a range of cell types in algorithm development. Optical oceanography, Physical oceanography, Absorption, Scattering.

Quantum Scattering Theory for Several Particle Systems

Abstract: Introduction 1 General Aspects of the Scattering Problem 2 Stationary Approach to Scattering Theory 3 The Method of Integral Equation 4 Configuration Space Neutral Particles 5 Charged Particles in Configuration Space 6 Mathematical Foundation of the Scattering Problem 7 Some Applications 8 Comments on Literature Bibliography Index

Nonlinear Thomson scattering of intense laser pulses from beams and plasmas

Abstract: A comprehensive theory is developed to describe the nonlinear Thomson scattering of intense laser fields from beams and plasmas. This theory is valid for linearly or circularly polarized incident laser fields of arbitrary intensities and for electrons of arbitrary energies. Explicit expressions for the intensity distributions of the scattered radiation are calculated and numerically evaluated. The space-charge electrostatic potential, which is important in high-density plasmas and prevents the axial drift of electrons, is included self-consistently. Various properties of the scattered radiation are examined, including the linewidth, angular distribution, and the behavior of the radiation spectra at ultrahigh intensities. Nonideal effects, such as electron-energy spread and beam emittance, are discussed. A laser synchrotron source (LSS), based on nonlinear Thomson scattering, may provide a practical method for generating tunable, near-monochromatic, well-collimated, short-pulse x rays in a compact, relatively inexpensive source. Two examples of possible LSS configurations are presented: an electron-beam LSS generating hard (30-keV, 0.4-\AA{}) x rays and a plasma LSS generating soft (0.3-keV, 40-\AA{}) x rays. These LSS configurations are capable of generating ultrashort (\ensuremath{\sim}1-ps) x-ray pulses with high peak flux (\ensuremath{\gtrsim}${10}^{21}$ photons/s) and brightness [\ensuremath{\gtrsim}${10}^{19}$ photons /(s ${\mathrm{mm}}^{2}$ ${\mathrm{mrad}}^{2}$), 0.1% bandwidth].

Dynamic conductance and the scattering matrix of small conductors.

Abstract: The current response to oscillating electric or magnetic fields acting on the carriers in the probes of a multichannel, mutlilead conductor is investigated. For a noninteracting system we find a frequency-dependent admittance matrix which is expressed in terms of scattering matrices. A self-consistent potential method is used to include Coulomb interactions. The low-frequency departure of the admittance away from the dc conductance is discussed in terms of phase-delay times and RC times.

Effect of strong scattering on the low-temperature penetration depth of a d-wave superconductor.

Abstract: For a pure superconductor in a d-wave-like state at temperatures T well below the critical temperature ${\mathit{T}}_{\mathit{c}}$, the deviation \ensuremath{\Delta}\ensuremath{\lambda} of the penetration depth from its zero-temperature value \ensuremath{\lambda}(0) is proportional to T. When the concentration ${\mathit{n}}_{\mathit{i}}$ of strongly scattering impurities is nonzero, \ensuremath{\Delta}\ensuremath{\lambda}\ensuremath{\propto}${\mathit{T}}^{\mathit{n}}$, where n=2 for T${\mathit{T}}^{\mathrm{*}}$\ensuremath{\ll}${\mathit{T}}_{\mathit{c}}$ and n=1 for ${\mathit{T}}^{\mathrm{*}}$T\ensuremath{\ll}${\mathit{T}}_{\mathit{c}}$. The crossover temperature ${\mathit{T}}^{\mathrm{*}}$ and the increase in \ensuremath{\lambda}(0) scale as \ensuremath{\surd}${\mathit{n}}_{\mathit{i}}$ up to logarithmic corrections when resonant scattering is dominant. We argue that this case is relevant to recent measurements on ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ and present specific results for a model pairing state with ${\mathit{d}}_{\mathit{x}}^{2}$-${\mathit{y}}^{2}$ symmetry.

Surface-plasmon dispersion and size dependence of Mie resonance: Silver versus simple metals

Abstract: The recently observed blueshift of the surface plasmon of Ag with increasing parallel momentum and of the Mie resonance of small Ag particles with decreasing radius are discussed in terms of a model for the dynamical response of a two-component s-d electron system. In the case of flat Ag surfaces, the 5s conduction electrons are treated as a semi-infinite homogeneous electron gas while the influence of the fully occupied 4d bands is described via a polarizable medium which extends up to a certain distance from the surface. Using the time-dependent density-functional approach it is shown that the absence of the s-d screening interaction in the surface region leads to a positive dispersion of the surface plasmon in agreement with the data. A self-energy approach is introduced which allows us to establish a qualitative relation between the scattering processes at a flat metal surface and those at the surface of a spherical particle. Using this approach it is argued that the blueshift of the Mie resonance of Ag particles can also be understood in terms of a reduced s-d interaction in the region where the s electrons spill out into the vacuum. Finally, it is shown that the polarizability of simple metal particles exhibits above the Mie resonance a collective excitation which is the analogue of the dipolar surface plasmon observed on the flat surfaces of various simple metals. This feature seems to have been observed in recent absorption spectra on large K clusters.

Measurement of the optical properties of the skull in the wavelength range 650-950 nm.

Abstract: The optical properties of samples of bone from pig skull have been measured over the wavelength range 650-950 nm. The scattering phase function was measured on thin samples of the bone using a goniometer, and a value for the mean cosine g, of the scattering angle, was calculated. The scattering and absorption coefficients, mu s and mu a were then determined from measurements of diffuse reflectance and transmittance made with a pair of integrating spheres, by a step-wise search through a table of diffuse reflectance and transmittance versus mu a and mu s generated by a Monte Carlo model incorporating the measured scattering phase function. Values for g measured on six samples varied from 0.925 +/- 0.014 at 650 nm to 0.945 +/- 0.013 at 950 nm. Corresponding values for mu a and mu s measured on 18 samples were mu a = 0.04 +/- 0.002 mm-1, mu s = 35 +/- 0.7 mm-1 at 650 nm to mu a = 0.05 +/- 0.002 mm-1, mu s = 24 +/- 0.6 mm-1 at 950 nm.

Giant negative magnetoresistance in granular ferromagnetic systems (invited)

Abstract: Giant negative magnetoresistance (GMR) has been observed in a number of granular ferromagnetic systems [Co–Ag, Co–Cu, Fe–Cu, Fe–Ag, and (Fe–Ni)–Ag] with effect sizes as much as 85% at 5 K and 25% at 300 K. It is shown that the GMR is isotropic and is due to magnetic scattering of the conduction electrons by the nonaligned magnetic entities. The essential contribution to the resistivity is ρm[1−F(M/Ms)], where F(M/Ms) measures the spin disorder from ferromagnetic alignment and ρm is the magnetic resistivity that defines the size of the GMR. The magnitude of GMR is affected by the size and density of the magnetic entities which can be controlled by varying the composition and the process conditions. When the composition is varied, the maximum GMR is realized in systems with magnetic constituents of about 25%.

Conductivity and magnetoresistance in magnetic granular films (invited)

Abstract: The transport properties in magnetic granular films are modeled by considering the spin‐dependent impurity scattering within the granules and the interface roughness scattering at the boundaries of the granules The magnetoresistance for these films is derived by using the formalism developed for layered structures with currents perpendicular to the plane of the layers and which is applicable to random systems With this model, various features of the magnetoresistance observed in recent experiments can be explained and the optimal choice of parameters to maximize the magnetoresistance can be determined