Showing papers on "Scattering published in 1995"

Electron Energy-Loss Spectroscopy in the Electron Microscope

Abstract: 1. An Introduction to Electron Energy-Loss Spectroscopy.- 1.1 Interaction of Fast Electrons with a Solid.- 1.2. The Electron Energy-Loss Spectrum.- 1.3. The Development of Experimental Techniques.- 1.4. Comparison of Analytical Methods.- 1.4.1. Ion-Beam Methods.- 1.4.2. Incident Photons.- 1.4.3. Electron-Beam Techniques.- 1.5. Further Reading.- 2. Instrumentation for Energy-Loss Spectroscopy.- 2.1. Energy-Analyzing and Energy-Selecting Systems.- 2.1.1. The Magnetic-Prism Spectrometer.- 2.1.2. Energy-Selecting Magnetic-Prism Devices.- 2.1.3. The Wien Filter.- 2.1.4. Cylindrical-Lens Analyzers.- 2.1.5. Retarding-Field Analyzers.- 2.1.6. Electron Monochromators.- 2.2. The Magnetic-Prism Spectrometer.- 2.2.1. First-Order Properties.- 2.2.2. Higher-Order Focusing.- 2.2.3. Design of an Aberration-Corrected Spectrometer.- 2.2.4. Practical Considerations.- 2.2.5. Alignment and Adjustment of the Spectrometer.- 2.3. The Use of Prespectrometer Lenses.- 2.3.1. Basic Principles.- 2.3.2. CTEM with Projector Lens On.- 2.3.3. CTEM with Projector Lens Off.- 2.3.4. Spectrometer-Specimen Coupling in a High-Resolution STEM.- 2.4. Recording the Energy-Loss Spectrum.- 2.4.1. Serial Acquisition.- 2.4.2. Electron Detectors for Serial Recording.- 2.4.3. Scanning the Energy-Loss Spectrum.- 2.4.4. Signal Processing and Storage.- 2.4.5. Noise Performance of a Serial Detector.- 2.4.6. Parallel-Recording Detectors.- 2.4.7. Direct Exposure of a Diode-Array Detector.- 2.4.8. Indirect Exposure of a Diode Array.- 2.4.9. Removal of Diode-Array Artifacts.- 2.5. Energy-Filtered Imaging.- 2.5.1. Elemental Mapping.- 2.5.2. Z-Contrast Imaging.- 3. Electron Scattering Theory.- 3.1. Elastic Scattering.- 3.1.1. General Formulas.- 3.1.2. Atomic Models.- 3.1.3. Diffraction Effects.- 3.1.4. Electron Channeling.- 3.1.5. Phonon Scattering.- 3.2. Inelastic Scattering.- 3.2.1. Atomic Models.- 3.2.2. Bethe Theory.- 3.2.3. Dielectric Formulation.- 3.2.4. Solid-State Effects.- 3.3. Excitation of Outer-Shell Electrons.- 3.3.1. Volume Plasmons.- 3.3.2. Single-Electron Excitation.- 3.3.3. Excitons.- 3.3.4. Radiation Losses.- 3.3.5. Surface Plasmons.- 3.3.6. Single, Plural, and Multiple Scattering.- 3.4. Inner-Shell Excitation.- 3.4.1. Generalized Oscillator Strength.- 3.4.2. Kinematics of Scattering.- 3.4.3. Ionization Cross Sections.- 3.5. The Spectral Background to Inner-Shell Edges.- 3.6. The Structure of Inner-Shell Edges.- 3.6.1. Basic Edge Shapes.- 3.6.2. Chemical Shifts in Threshold Energy.- 3.6.3. Near-Edge Fine Structure (ELNES).- 3.6.4. Extended Energy-Loss Fine Structure (EXELFS).- 4. Quantitative Analysis of the Energy-Loss Spectrum.- 4.1. Removal of Plural Scattering from the Low-Loss Region.- 4.1.1. Fourier-Log Deconvolution.- 4.1.2. Approximate Methods.- 4.1.3. Angular-Dependent Deconvolution.- 4.2. Kramers-Kronig Analysis.- 4.3. Removal of Plural Scattering from Inner-Shell Edges.- 4.3.1. Fourier-Log Deconvolution.- 4.3.2. Fourier-Ratio Method.- 4.3.3. Van Cittert'1. An Introduction to Electron Energy-Loss Spectroscopy.- 1.1 Interaction of Fast Electrons with a Solid.- 1.2. The Electron Energy-Loss Spectrum.- 1.3. The Development of Experimental Techniques.- 1.4. Comparison of Analytical Methods.- 1.4.1. Ion-Beam Methods.- 1.4.2. Incident Photons.- 1.4.3. Electron-Beam Techniques.- 1.5. Further Reading.- 2. Instrumentation for Energy-Loss Spectroscopy.- 2.1. Energy-Analyzing and Energy-Selecting Systems.- 2.1.1. The Magnetic-Prism Spectrometer.- 2.1.2. Energy-Selecting Magnetic-Prism Devices.- 2.1.3. The Wien Filter.- 2.1.4. Cylindrical-Lens Analyzers.- 2.1.5. Retarding-Field Analyzers.- 2.1.6. Electron Monochromators.- 2.2. The Magnetic-Prism Spectrometer.- 2.2.1. First-Order Properties.- 2.2.2. Higher-Order Focusing.- 2.2.3. Design of an Aberration-Corrected Spectrometer.- 2.2.4. Practical Considerations.- 2.2.5. Alignment and Adjustment of the Spectrometer.- 2.3. The Use of Prespectrometer Lenses.- 2.3.1. Basic Principles.- 2.3.2. CTEM with Projector Lens On.- 2.3.3. CTEM with Projector Lens Off.- 2.3.4. Spectrometer-Specimen Coupling in a High-Resolution STEM.- 2.4. Recording the Energy-Loss Spectrum.- 2.4.1. Serial Acquisition.- 2.4.2. Electron Detectors for Serial Recording.- 2.4.3. Scanning the Energy-Loss Spectrum.- 2.4.4. Signal Processing and Storage.- 2.4.5. Noise Performance of a Serial Detector.- 2.4.6. Parallel-Recording Detectors.- 2.4.7. Direct Exposure of a Diode-Array Detector.- 2.4.8. Indirect Exposure of a Diode Array.- 2.4.9. Removal of Diode-Array Artifacts.- 2.5. Energy-Filtered Imaging.- 2.5.1. Elemental Mapping.- 2.5.2. Z-Contrast Imaging.- 3. Electron Scattering Theory.- 3.1. Elastic Scattering.- 3.1.1. General Formulas.- 3.1.2. Atomic Models.- 3.1.3. Diffraction Effects.- 3.1.4. Electron Channeling.- 3.1.5. Phonon Scattering.- 3.2. Inelastic Scattering.- 3.2.1. Atomic Models.- 3.2.2. Bethe Theory.- 3.2.3. Dielectric Formulation.- 3.2.4. Solid-State Effects.- 3.3. Excitation of Outer-Shell Electrons.- 3.3.1. Volume Plasmons.- 3.3.2. Single-Electron Excitation.- 3.3.3. Excitons.- 3.3.4. Radiation Losses.- 3.3.5. Surface Plasmons.- 3.3.6. Single, Plural, and Multiple Scattering.- 3.4. Inner-Shell Excitation.- 3.4.1. Generalized Oscillator Strength.- 3.4.2. Kinematics of Scattering.- 3.4.3. Ionization Cross Sections.- 3.5. The Spectral Background to Inner-Shell Edges.- 3.6. The Structure of Inner-Shell Edges.- 3.6.1. Basic Edge Shapes.- 3.6.2. Chemical Shifts in Threshold Energy.- 3.6.3. Near-Edge Fine Structure (ELNES).- 3.6.4. Extended Energy-Loss Fine Structure (EXELFS).- 4. Quantitative Analysis of the Energy-Loss Spectrum.- 4.1. Removal of Plural Scattering from the Low-Loss Region.- 4.1.1. Fourier-Log Deconvolution.- 4.1.2. Approximate Methods.- 4.1.3. Angular-Dependent Deconvolution.- 4.2. Kramers-Kronig Analysis.- 4.3. Removal of Plural Scattering from Inner-Shell Edges.- 4.3.1. Fourier-Log Deconvolution.- 4.3.2. Fourier-Ratio Method.- 4.3.3. Van Cittert's Method.- 4.3.4. Effect of a Collection Aperture.- 4.4. Background Fitting to Ionization Edges.- 4.4.1. Energy Dependence of the Background.- 4.4.2. Background-Fitting Procedures.- 4.4.3. Background-Subtraction Errors.- 4.5. Elemental Analysis Using Inner-Shell Edges.- 4.5.1. Basic Formulas.- 4.5.2. Correction for Incident-Beam Convergence.- 4.5.3. Effect of Sample Orientation.- 4.5.4. Effect of Specimen Thickness.- 4.5.5. Choice of Collection Angle.- 4.5.6. Choice of Integration and Fitting Regions.- 4.5.7. Microanalysis Software.- 4.5.8. Calculation of Partial Cross Sections.- 4.6. Analysis of Extended Energy-Loss Fine Structure.- 4.6.1. Spectrum Acquisition.- 4.6.2. Fourier-Transform Method of Data Analysis.- 4.6.3. Curve-Fitting Procedure.- 5. Applications of Energy-Loss Spectroscopy.- 5.1. Measurement of Specimen Thickness.- 5.1.1. Measurement of Absolute Thickness.- 5.1.2. Sum-Rule Methods.- 5.2. Low-Loss Spectroscopy.- 5.2.1. Phase Identification.- 5.2.2. Measurement of Alloy Composition.- 5.2.3. Detection of Hydrogen and Helium.- 5.2.4. Zero-Loss Images.- 5.2.5. Z-contrast Images.- 5.2.6. Plasmon-Loss Images.- 5.3. Core-Loss Microanalysis.- 5.3.1. Choice of Specimen Thickness and Incident Energy.- 5.3.2. Specimen Preparation.- 5.3.3. Elemental Detection and Mapping.- 5.3.4. Quantitative Microanalysis.- 5.3.5. Measurement and Control of Radiation Damage.- 5.4. Spatial Resolution and Elemental Detection Limits.- 5.4.1. Electron-Optical Considerations.- 5.4.2. Loss of Resolution due to Electron Scattering.- 5.4.3. Statistical Limitations.- 5.4.4. Localization of Inelastic Scattering.- 5.5. Structural Information from EELS.- 5.5.1. Low-Loss Fine Structure.- 5.5.2. Orientation Dependence of Core-Loss Edges.- 5.5.3. Core-Loss Diffraction Patterns.- 5.5.4. Near-Edge Fine Structure.- 5.5.5. Extended Fine Structure.- 5.5.6. Electron-Compton Measurements.- Appendix A. Relativistic Bethe Theory.- Appendix B. FORTRAN Programs.- B.3. Incident-Convergence Correction.- B.4. Fourier-Log Deconvolution.- B.5. Kramers-Kronig Transformation.- Appendix C. Plasmon Energies of Some Elements and Compounds.- Appendix D. Inner-Shell Binding Energies and Edge Shapes.- Appendix E. Electron Wavelengths and Relativistic Factors Fundamental Constants.- References.

CRYSOL : a program to evaluate X-ray solution scattering of biological macromolecules from atomic coordinates

Abstract: A program for evaluating the solution scattering from macromolecules with known atomic structure is presented. The program uses multipole expansion for fast calculation of the spherically averaged scattering pattern and takes into account the hydration shell. Given the atomic coordinates (e.g. from the Brookhaven Protein Data Bank) it can either predict the solution scattering curve or fit the experimental scattering curve using only two free parameters, the average displaced solvent volume per atomic group and the contrast of the hydration layer. The program runs on IBM PCs and on the major UNIX platforms.

Multiple-scattering calculations of x-ray-absorption spectra

Abstract: A high-order multiple-scattering (MS) approach to the calculation of polarized x-ray-absorption spectra, which includes both x-ray-absorption fine structure and x-ray-absorption near-edge structure, is presented. Efficient calculations in arbitrary systems are carried out by using a curved-wave MS path formalism that ignores negligible paths, and has an energy-dependent self-energy and MS Debye-Waller factors. Embedded-atom background absorption calculations on an absolute energy scale are included. The theory is illustrated for metallic Cu, Cd, and Pt. For these cases the MS expansion is found to converge to within typical experimental accuracy, both to experiment and to full MS theories (e.g., band structure), by using only a few dozen important paths, which are primarily single-scattering, focusing, linear, and triangular.

Approximations Leading to a Unified Exponential/Power-Law Approach to Small-Angle Scattering

Abstract: A new approach to the analysis of small-angle scattering is presented that describes scattering from complex systems that contain multiple levels of related structural features. For example, a mass fractal such as a polymer coil contains two structural levels, the overall radius of gyration and the substructural persistence length. One structural level is described by a Guinier and an associated power-law regime. A function is derived that models both the Guinier exponential and structurally limited power-law regimes without introducing new parameters beyond those used in local fits. Account is made for both a low-q and a high-q limit to power-law scattering regimes. The unified approach can distinguish Guinier regimes buried between two power-law regimes. It is applicable to a wide variety of systems. Fits to data containing multiple power-law and exponential regimes using this approach have previously been reported. Here, arguments leading to the unified approach are given. The usefulness of this approach is demonstrated through comparison with model calculations using the Debye equation for polymer coils (mass fractal), equations for polydisperse spheres (Porod scattering) and randomly oriented ellipsoids of revolution with diffuse interfaces, as well as randomly oriented rod and disc-shaped particles.

Introduction to Wave Scattering, Localization, and Mesoscopic Phenomena

Abstract: Quantum and Classical Waves.- Wave Scattering and the Coherent Potential Approximation.- Coherent Waves and Effective Media.- Diffusive Waves.- The Coherent Backscattering Effect.- Renormalized Diffusion.- The Scaling Theory of Localization.- Localized States and the Approach to Localization.- Localization Phenomena in Electronic Systems.- Mesoscopic Phenomena.

Optical models for direct volume rendering

Abstract: This tutorial survey paper reviews several different models for light interaction with volume densities of absorbing, glowing, reflecting, and/or scattering material. They are, in order of increasing realism, absorption only, emission only, emission and absorption combined, single scattering of external illumination without shadows, single scattering with shadows, and multiple scattering. For each model the paper provides the physical assumptions, describes the applications for which it is appropriate, derives the differential or integral equations for light transport, presents calculation methods for solving them, and shows output images for a data set representing a cloud. Special attention is given to calculation methods for the multiple scattering model. >

Resonance light scattering : a new technique for studying chromophore aggregation

Abstract: Light scattering experiments are usually performed at wavelengths away from absorption bands, but for species that aggregate, enhancements in light scattering of several orders of magnitude can be observed at wavelengths characteristic of these species. Resonance light scattering is shown to be a sensitive and selective method for studying electronically coupled chromophore arrays. The approach is illustrated with several examples drawn from porphyrin and chlorin chemistry. The physical principles underlying resonance light scattering are discussed, and the advantages and limitations of the technique are reviewed.

PENELOPE: An algorithm for Monte Carlo simulation of the penetration and energy loss of electrons and positrons in matter

Abstract: A mixed algorithm for Monte Carlo simulation of relativistic electron and positron transport in matter is described. Cross sections for the different interaction mechanisms are approximated by expressions that permit the generation of random tracks by using purely analytical methods. Hard elastic collisions, with scattering angle greater than a preselected cutoff value, and hard inelastic collisions and radiative events, with energy loss larger than given cutoff values, are simulated in detail. Soft interactions, with scattering angle or energy loss less than the corresponding cutoffs, are simulated by means of multiple scattering approaches. This algorithm handles lateral displacements correctly and completely avoids difficulties related with interface crossing. The simulation is shown to be stable under variations of the adopted cutoffs; these can be made quite large, thus speeding up the simulation considerably, without altering the results. The reliability of the algorithm is demonstrated through a comparison of simulation results with experimental data. Good agreement is found for electrons and positrons with kinetic energies down to a few keV.

Rayleigh-scattering calculations for the terrestrial atmosphere

Abstract: Rayleigh-scattering cross sections and volume-scattering coefficients are computed for standard air; they incorporate the variation of the depolarization factor with wavelength. Rayleigh optical depths are then calculated for the 1962 U.S. Standard Atmosphere and for five supplementary models. Analytic formulas are derived for each of the parameters listed. The new optical depths can be 1.3% lower to 3% higher at midvisible wavelengths and up to 10% higher in the UV region compared with previous calculations, in which a constant or incorrect depolarization factor was used. The dispersion of the depolarization factor is also shown to affect the Rayleigh phase function slightly, by approximately 1% in the forward, backscattered, and 90° scattering-angle directions.

Analysis of multiple-scattering XAFS data using theoretical standards

Abstract: Theoretical standards for scattering amplitudes and phase shifts are often necessary for XAFS analysis, as for cases in which multiple scattering paths are important over the R-range of interest. Even when not necessary, they are often more convenient and reliable than experimental standards. We discuss several important considerations that must be taken into account to successfully compare ab initio theoretical calculations from FEFF to experimental XAFS spectra, and present a computer program, FEFFIT, to assist in using FEFF to get reliable information from experimental XAFS data.

Electromagnetic scattering by an aggregate of spheres

Abstract: We present a comprehensive solution to the classical problem of electromagnetic scattering by aggregates of an arbitrary number of arbitrarily configured spheres that are isotropic and homogeneous but may be of different size and composition. The profile of incident electromagnetic waves is arbitrary. The analysis is based on the framework of the Mie theory for a single sphere and the existing addition theorems for spherical vector wave functions. The classic Mie theory is generalized. Applying the extended Mie theory to all the spherical constituents in an aggregate simultaneously leads to a set of coupled linear equations in the unknown interactive coefficients. We propose an asymptotic iteration technique to solve for these coefficients. The total scattered field of the entire ensemble is constructed with the interactive scattering coefficients by the use of the translational addition theorem a second time. Rigorous analytical expressions are derived for the cross sections in a general case and for all the elements of the amplitude-scattering matrix in a special case of a plane-incident wave propagating along the z axis. As an illustration, we present some of our preliminary numerical results and compare them with previously published laboratory scattering measurements.

Geometric scattering theory

Abstract: List of illustrations Introduction 1. Euclidean Laplacian 2. Potential scattering on Rn 3. Inverse scattering 4. Trace formulae and scattering poles 5. Obstacle scattering 6. Scattering metrics 7. Cylindrical ends 8. Hyperbolic metrics.

Discrete dipole approximation for calculating extinction and Raman intensities for small particles with arbitrary shapes

Abstract: We present a discrete dipole approximation (DDA) method to determine extinction and Raman intensities for small metal particles of arbitrary shape. The Raman intensity calculation involves evaluation of surface electromagnetic fields, and thus is relevant to surface enhanced Raman scattering (SERS) intensities. We demonstrate convergence of the method by considering light absorption and scattering from an isolated spheroid, from an isolated tetrahedron, from two coupled spheroids, and from a spheroid on a flat surface. We also examine comparisons with traditional T‐matrix methods. Extensions and simplifications of the method in studies of clusters and arrays of particles are presented.

Robust acoustic time reversal with high-order multiple scattering

Abstract: We report the first experiments showing the reversibility of transient acoustic waves through high-order multiple scattering by means of an acoustic time-reversal mirror. A point source generates a pulse which scatters through 2000 steel rods immersed in water. The time-reversed waves are found to converge to their source and recover their original wave form, despite the high order of multiple scattering involved and the usual sensitivity to initial conditions of time-reversal processes. Surprisingly, the observed resolution was one-sixth of the theoretical limit for the mirror's aperture.

Nature of light scattering in dental enamel and dentin at visible and near-infrared wavelengths

Abstract: The light-scattering properties of dental enamel and dentin were measured at 543, 632, and 1053 nm. Angularly resolved scattering distributions for these materials were measured from 0° to 180° using a rotating goniometer. Surface scattering was minimized by immersing the samples in an index-matching bath. The scattering and absorption coefficients and the scattering phase function were deduced by comparing the measured scattering data with angularly resolved Monte Carlo light-scattering simulations. Enamel and dentin were best represented by a linear combination of a highly forward-peaked Henyey–Greenstein (HG) phase function and an isotropic phase function. Enamel weakly scatters light between 543 nm and 1.06 μm, with the scattering coefficient (μs) ranging from μs = 15 to 105 cm−1. The phase function is a combination of a HG function with g = 0.96 and a 30–60% isotropic phase function. For enamel, absorption is negligible. Dentin scatters strongly in the visible and near IR (μs ≅ 260 cm−1) and absorbs weakly (μa ≅ 4 cm−1). The scattering phase function for dentin is described by a HG function with g = 0.93 and a very weak isotropic scattering component (~2%).

Singular quasiparticle scattering in the proximity of charge instabilities.

Abstract: We analyze the dynamic scattering amplitude between quasiparticles at the Fermi surface in the proximity of a charge instability, which may occur in the high temperature superconductors. Within the infinite-$U$ Hubbard-Holstein model we find that in the absence of long-range Coulomb forces the scattering amplitude is strongly singular at zero momentum transfer close to the phase separation instability. In the presence of long-range Coulomb forces the singularity occurs at finite wave vectors. In both cases we show how normal state properties are largely affected by this scattering.

Spatially varying optical property reconstruction using a finite element diffusion equation approximation

Abstract: A finite element reconstruction algorithm for optical data based on a diffusion equation approximation is presented. A frequency domain approach is adopted and a unified formulation for three combinations of boundary observables and conditions is described. A multidetector, multisource measurement and excitation strategy is simulated, which includes a distributed model of the light source that illustrates the flexibility of the methodology to modeling adaptations. Simultaneous reconstruction of both absorption and scattering coefficients for a tissue-like medium is achieved for all three boundary data types. The algorithm is found to be computationally practical, and can be implemented without major difficulties in a workstation computing environment. Results using simulated data suggest that qualitative images can be produced that readily highlight the location of absorption and scattering heterogeneities within a circular background region of close to 4 cm in diameter over a range of contrast levels. Absorption images appear to more closely identify the true size of the heterogeneity; however, both the absorption and scattering reconstructions have difficulty with sharp transitions at increasing depth. Quantitatively, the reconstructions are not accurate, suggesting that absolute optical imaging involving simultaneous recovery of both absorption and scattering profiles in multicentimeter tissues geometries may prove to be extremely difficult.

Depolarization of light backscattered by randomly oriented nonspherical particles

Abstract: We derive theoretically and validate numerically general relationships for the elements of the backscattering matrix and for the linear, delta(L), and circular, delta(C), backscattering depolarization ratios for nonspherical particles in random orientation. For the practically important case of randomly oriented particles with a plane of symmetry or particles and their mirror particles occurring in equal numbers and in random orientation, delta(C) = 2delta(L)/(1 - delta(L)). Extensive T-matrix computations for randomly oriented spheroids demonstrate that, although both delta(L) and delta(C) are indicators of particle nonsphericity, they cannot be considered a universal measure of the departure of particle shape from that of a sphere and have no simple dependence on particle size and refractive index.

Modern aspects of small-angle scattering

Abstract: Preface. 1. Some Fundamental Concepts and Techniques Useful in Small-Angle Scattering Studies of Disordered Solids P.W. Schmidt. 2. Instrumentation for Small-Angle Scattering J.S. Pedersen. 3. Reduction of Data from SANS Instruments A.R. Rennie. 4. Modern Methods of Data Analysis in Small-Angle Scattering and Light Scattering O. Glatter. 5. Grazing Incidence Small-Angle X-Ray Scattering: Application to Layers and Surface Layers A. Naudon. 6. Anomalous Small-Angle X-Ray Scattering (ASAXS) A. Naudon. 7. Contrast Variation H.B. Stuhrmann. 8. Metals and Alloys: Phase Separation and Defect Agglomeration G. Kostorz. 9. The Anisotropy of Metallic Systems -- Analysis of Small-Angle Scattering Data A.D. Sequeira, G. Kostorz. 10. Characterization of Porosity in Ceramic Materials by Small-Angle Scattering: VYCOR(R) Glass and Silica Aerogel D.W. Schaefer, R.K. Brow, B.J. Olivier, T. Rieker, G. Beaucage, L. Hrubesh, J.S. Lin. 11. Small-Angle Scattering of Catalysts H. Brumberger. 12. Thermodynamic and Scattering Properties of Dense Fluids of Monodisperse Isotropic Particles: an Information Theory Approach V. Luzzati. 13. Small-Angle Scattering from Complex Fluids E.W. Kaler. 14. Small-Angle Neutron Scattering of Biological Macromolecular Complexes Consisting of Proteins and Nucleic Acids R.P. May. 15. X-Ray and Neutron Small-Angle Scattering on Plasma Lipoproteins P. Laggner. 16. Time-Resolved X-Ray Small-Angle Diffraction with Synchrotron Radiation on Phospholipid Phase Transitions. Pathways,Intermediates and Kinetics P. Laggner, M. Kriechbaum. 17. Polymers in Solution-Flow Techniques P. Lindner. 18. Bulk Polymers A.R. Rennie. Index.

Generalized Field Propagator For Electromagnetic Scattering And Light Confinement

Abstract: We present a new theoretical and numerical framework for the study of the optical properties of micrometric and nanometric three-dimensional structures of arbitrary shape. We show that the field distribution induced inside and outside such a structure by different external monochromatic sources can be obtained from a unique generalized field propagator expressed in direct space. An application of the method to the confinement of optical fields due to the scattering of subwavelength objects is presented.

Tooth Color and Reflectance as Related to Light Scattering and Enamel Hardness

Abstract: Tooth color is determined by the paths of light inside the tooth and absorption along these paths. This paper tests the hypothesis that, since the paths are determined by scattering, a relation between color and scattering coefficients exists. One hundred and two extracted incisors were fixed in formalin, mounted in a standardized position in brass holders, and pumiced. A facet was prepared near the incisal edge on the labial plane to allow for Knoop hardness measurements with a 500-gram load. Light scattering by the enamel was measured in a 45°/0° geometry; light scattering by both enamel and dentin was measured in a 0°/0° geometry. The reflection spectrum of the tooth was measured from the labial plane with a spectroradiometer in a 45°/0° geometry, with standard illuminant A and standard illuminant D65. To include all volume-reflected light, we used entire-tooth illumination and small-area measurement. CIELAB color coordinates were calculated from the spectra. Neither spectra nor coordinates showed evid...

Monte Carlo Modeling of Light Transport in Tissues

Abstract: Monte Carlo simulations of photon propagation offer a flexible yet rigorous approach toward photon transport in turbid tissues. This method simulates the “random walk” of photons in a medium that contains absorption and scattering. The method is based on a set of rules that govern the movement of a photon in tissue. The two key decisions are (1) the mean free path for a scattering or absorption event, and (2) the scattering angle. Figure 4.1 illustrates a scattering event. At boundaries, a photon is reflected or moves across the boundary. The rules of photon propagation are expressed as probability distributions for the incremental steps of photon movement between sites of photon—tissue interaction, for the angles of deflection in a photon’s trajectory when a scattering event occurs, and for the probability of transmittance or reflectance at boundaries. Monte Carlo light propagation is rigorous yet very descriptive. However, this method is basically statistical in nature and requires a computer to calculate the propagation of a large number of photons. To illustrate how photons propagate inside tissues, a few photon paths are shown in Fig. 4.2.

Mie and Rayleigh modeling of visible-light scattering in neonatal skin

Abstract: Reduced-scattering coefficients of neonatal skin were deduced in the 450-750-nm range from integrating-sphere measurements of the total reflection and total transmission of 22 skin samples. The reduced-scattering coefficients increased linearly at each wavelength with gestational maturity. The distribution of diameters d and concentration ρ(A) of the skin-sample collagen fibers were measured in histological sections of nine neonatal skin samples of varying gestational ages. An algorithm that calculates Mie scattering by cylinders was used to model the scattering by the collagen fibers in the skin. The fraction of the reduced-scattering coefficient µ(s)' that was attributable to Mie scattering by collagen fibers, as deduced from wavelength-dependent analysis, increased with gestational age and approached that found for adult skin. An assignment of 1.017 for n(rel), the refractive index of the collagen fibers relative to that of the surrounding medium, allowed the values for Mie scattering by collagen fibers, as predicted by the model for each of the nine neonatal skin samples to match the values for Mie scattering by collagen fibers as expected from the measurements of µ(s)'. The Mie-scattering model predicted an increase in scattering with gestational age on the basis of changes in the collagen-fiber diameters, and this increase was proportional to that measured with the integrating-sphere method.

A GTD-based parametric model for radar scattering

Abstract: This paper presents a new approach to scattering center extraction based on a scattering model derived from the geometrical theory of diffraction (GTD). For stepped frequency measurements at high frequencies, the model is better matched to the physical scattering process than the damped exponential model and conventional Fourier analysis. In addition to determining downrange distance, energy, and polarization, the GTD-based model extracts frequency dependent scattering information, allowing partial identification of scattering center geometry. We derive expressions for the Cramer-Rao bound of this model; using these expressions, we analyze the behavior of the new model as a function of scatterer separation, bandwidth, number of data points, and noise level. Additionally, a maximum likelihood algorithm is developed for estimation of the model parameters. We present estimation results using data measured on a compact range to validate the proposed modeling procedure. >

The Microscopic Basis of the Glass Transition in Polymers from Neutron Scattering Studies

Abstract: Recent neutron scattering experiments on the microscopic dynamics of polymers below and above the glass transition temperature T g are reviewed. The results presented cover different dynamic processes appearing in glasses: local motions, vibrations, and different relaxation processes such as α- and β-relaxation. For the α-relaxation, which occurs above T g , it is possible to extend the time-temperature superposition principle, which is valid for polymers on a macroscopic scale, to the microscopic time scale. However, this principle is not applicable for temperatures approaching T g . Below T g , an inelastic excitation at a frequency of some hundred gigahertz (on the order of several wave numbers), the "boson peak," survives from a quasi-elastic overdamped scattering law at high temperatures. The connection between this boson peak and the fast dynamic process appearing near T g is discussed.

Laser Light Scattering

Abstract: This chapter provides introduction to both the classical and quasi-elastic forms of laser light scattering, which can serve as an introductory text for students and a reference for research workers. Classical light scattering studies are concerned with the measurement of the intensity of scattered light as a function of the scattering angle. Lasers produce collimated, quasi-monochromatic radiation having high intensity. In all but the least expensive lasers, the output is highly polarized. The chapter describes information that can be obtained from the various experiments rather than either presenting a comprehensive survey of the literature. The decrease in intensity may seem a nuisance, but it turns out to be a unique source of information pertaining to the size and shape of the particle. Inhomogeneity in either the size or shape of the particles leads to ambiguity in the interpretation of the angular dependence of the scattered intensity.