Fourier inversion of LEED data

Low-energy electron diffraction intensity profiles are Fourier inverted. The optical transform is more suitable for LEED than the usual cosine transform and yields a quantity related to the autocorrelation function. Transforms of individual LEED profiles taken at different diffraction geometry are not the same, but transforms of averaged, kinematic data give expected autocorrelation functions.     

Smoothing effect in the broadband laser through a dispersive wedge

The propagation expression of a broadband laser passing through a dispersive wedge is derived on the basis of the Huygens-Fresnel diffraction integral. Smoothing effects caused by the phase perturbation of the dispersive wedge on the intensity profiles are investigated in detail. The phase perturbation of the dispersive wedge induces a relative transverse position shift between the diffraction patterns of different frequency components. The relative transverse position shift is of great benefit to the fill of the intensity peaks of some patterns in the valleys of others when these patterns are overlapped and thus the smoothing effect is achieved.     

Computations of relativistic LEED intensities and spin-polarisations for tungsten (001)

The effects of relativistic corrections in low-energy electron diffraction intensity calculations are assessed by reference to relativistic and non-relativistic computations for tungsten (001). Small changes are observed in the calculated intensities and band structures and a significant degree of spin-polarisation is predicted for the diffracted beams. The spin-polarisation profiles are more complicated than those observed in gas-phase experiments because of the influence of multiple scattering. The results suggest that LEED could provide a strong source of polarised electrons for use in scattering experiments. It is suggested that in future LEED experiments both spin-polarisation and intensity measurements should be performed on the diffracted electron beams.     

Diffraction effects in low-energy electron emission

The origin of diffraction peaks in the energy distribution of intensity of low-energy (< 1000 eV) electron emission from crystals is discussed from the standpoint of the dynamical theory of diffraction. The emitted electrons are considered to originate at relatively incoherent point sources in the crystal. The two-beam approximation of dynamical theory is used. The theory accounts for the chief regularities of diffraction peaks: temperature-dependence of peak intensities like that for low-energy electron diffraction (LEED) peaks, correlation of peak energies with X-ray absorption fine structure, and correlation of peak energies with the energies of normal-incidence LEED peaks in specular reflection. It is shown that the conditions for diffraction peaks coincide with the conditions for emergence of Kikuchi lines. It is predicted that for energies just above those of diffraction peaks, such emergences should be observable in the angular distribution of emission as intensity minima for emission along low-index crystal axes. Theory of Kikuchi band profiles is developed in an Appendix.     

钆钴非晶合金薄膜的电子衍射研究

用电子衍射方法研究了一些非晶钆钴合金磁性薄膜的结构,用Lorentz电子显微技术观察了其相应的磁结构。在所有薄膜的电子衍射图上均可清楚地看见七个弥散衍射环,其中两个最小的环分布在s<0.6?^-1(s=2sinθ/λ)的范围内。在不同的薄膜中观察到磁泡或平面畴结构。本文就这种由两种原子半径相差较大的元素,如稀土和过渡元素构成的非晶合金,讨论了如何只根据s<0.6?^-1范围内的衍射强度峰形,直接、定性地了解稀土与稀土原子、稀土与过渡族原子以及过渡族与过渡族原子之间最近邻配位数相对大小的问题。也讨论了钆钴薄膜相分离对磁各向异性的影响。 关键词：     

Quantitative analysis of periodic antiphase boundaries by lattice image simulations

Periodic antiphase boundaries in the intermetallic compound Cu₃Pd have been studied by high resolution electron microscopy and image simulation. The intensity profiles across the boundary were recorded and the thickness of the boundary compared to the values deduced from X-ray and neutron diffraction data. It is shown that electron microscopy always minimizes the thickness of the boundary. The possible factors for this discrepancy are analysed and discussed.     

Structure in LEED Bragg reflexes of spherical Ag crystals

LEED at room temperature is performed on spherically-shaped silver crystals grown by solidification of a drop of melt. The domains of intensity in the reciprocal lattice, which are usually rodlike, change to double cones caused by the crystal sphere. The size and the structure of Bragg reflexes were studied as a function of primary electron energy and diffraction geometry. The influence of impurities on the diffraction patterns is examined.     

Charge density determination in icosahedral AlPdMn quasicrystal using quantitative convergent beam electron diffraction

Charge density distribution in icosahedral AlPdMn quasicrystal has been studied on a single-crystal specimen by using quantitative convergent beam electron diffraction (QCBED) technique. The QCBED systematic row method was used in the refinement of structure factors. To refine the low-order structure factors, the wave-mechanical formulation of electron diffraction dynamical theory was used in the calculation of electron diffraction intensities for the quasicrystal in fitting the experimental intensity line scan profiles. The shapes of atomic surfaces (occupation domains) were described with symmetry-adapted series of surface harmonics. An iterative procedure was used in determination of structure factors of the quasicrystal. The structure factors of nine strongest symmetry inequivalent reflections according to X-ray diffraction experiment were refined with QCBED technique. The average of refinement results for a given reflection performed on several CBED patterns, which were slightly different in orientation and sample thickness, and on different line scans, was taken as the value of structure factor for the reflection. The obtained structure factors for electrons were transformed into X-ray structure factors with Mott formula. The bonding charge density map for the quasicrystal was constructed with the obtained nine structure factors. Assuming that the atoms are spheres, the gain or loss of electrons for different atoms were calculated. It shows that identical atoms can have different valences at different kinds of positions. The bonding charge is localized along certain directions.     

Effects of adsorbates on electron spin polarization in low energy electron diffraction from tungsten (001): I. Disordered CO and O2 overlayers

Electron spin polarization and intensity profiles have been measured in low energy electron diffraction (LEED) for the (00) beam at θ = 13δ and ø = 0δ from W(001) as a function of surface exposure to CO and O₂. Significant changes have been observed in the profiles upon exposure to both adsorbate gases, and the implications of these results are discussed.     

Atomic structures by direct transform of diffraction patterns

We propose all the diffraction patterns can be directly transformed to provide three-dimensional atomic structures for the system studied. Depending on the scattering process, either the holography or Patterson transform scheme is used. For diffraction patterns which are generated from a localized emitter source or dominated by an inelastic-scattering feature like core-level photoelectron or low-energy Kikuchi electron, holography transform is needed. On the other hand, for diffraction patterns which were dominated by elastic-scattering, like grazing-incidence X-ray diffraction, electron correlated thermal diffuse scattering or low-energy electron diffraction curves, Patterson transform is needed. To prove our point, high-fidelity and artifact-free three-dimensional atomic structures obtained by transform of low-energy Kikuchi electron patterns and low-energy electron diffraction curves are presented. The future of these direct methods by transforming diffraction patterns will be discussed.     

Characterization and estimation of refractive index profile of laser-written photopolymer optical waveguides

In this study, channel waveguides fabricated in photopolymer films by direct-writing using a low-power CW laser, are used as phase objects in a simple plane-wave diffraction setup, and the refractive index modulation profiles of the waveguides are characterized using the recorded diffraction patterns. Index profiles are modeled by piece-wisely combining two Gaussian functions representing the central and the tail regions. Measured diffraction patterns are matched with patterns generated using the model. This simple model makes it possible to design various channel waveguides embedded into polymer substrates. The proposed model is tested on three distinctive waveguide profiles written on the same Acrylamide/Polyvinyl Alcohol based photopolymer with different exposures.     

Electron channelling contrast imaging of dislocations in a conventional SEM

Abstract

Dislocations in shock loaded tantalum single crystals were imaged using both transmission electron microscope (TEM) and electron channelling contrast image (ECCI) in a scanning electron microscope with a conventional backscattered electron detector. The results were compared with backscattered electron intensity profiles across dislocations calculated via the dynamic theory of electron diffraction. A one-to-one correspondence between ECCI and TEM is established. High voltage and low index reflections should be used to obtain the highest dislocation contrast and greatest imaging depth.     

Formation of holographic diffraction gratings in photoresist

The exposure-development process for the fabrication of holographic diffraction gratings for integrated optics in the positive photoresist Shipley AZ-1350 is theoretically and experimentally investigated. An analysis of the light intensity distribution is carried out, taking into account the reflectivity at the photoresist-substrate interface and the attenuation in the photoresist for two holographic exposure arrangements. The influence of the exposure energy, the development time for a fixed concentration of the developer and the initial photoresist thickness are described. Various grating profiles are calculated for photoresist films, coated on matched and reflective substrates. Scanning electron micrographs demonstrate very good agreement between the actual and calculated gratings profiles.     

Structure determination of Pt‐coated Au dumbbells via fluctuation X‐ray scattering

A fluctuation X‐ray scattering experiment has been carried out on platinum‐coated gold nanoparticles randomly oriented on a substrate. A complete algorithm for determining the electron density of an individual particle from diffraction patterns of many particles randomly oriented about a single axis is demonstrated. This algorithm operates on angular correlations among the measured intensity distributions and recovers the angular correlation functions of a single particle from measured diffraction patterns. Taking advantage of the cylindrical symmetry of the nanoparticles, a cylindrical slice model is proposed to reconstruct the structure of the nanoparticles by fitting the experimental ring angular auto‐correlation and small‐angle scattering data obtained from many scattering patterns. The physical meaning of the refined structure is discussed in terms of their statistical distributions of the shape and electron density profile.     

Solution-combustion synthesis of Tb-doped Y3Al5O12 nanoparticles

Nano-sized YAG:Tb powder phosphors were prepared by a solution-combustion method, using the general inorganic salts as starting materials. The X-ray diffraction (XRD) measurements showed that the precursor can be well-crystallized at 900 °C. As-prepared particles have sizes mostly in the range between 30 and 100 nm as obtained by scanning electron microscope (SEM) and transition electron microscope (TEM). Selected area electron diffraction (SAED) patterns proved that the larger particles are monocrystalline. The effects of annealing temperature and Tb-doping concentration on the luminescence intensity were studied.     

Surface studies by electron diffraction

The problems in the analysis of surface structures by means of electron diffraction, particularly at low energy, are reviewed. A brief introduction is given to the basic scattering and diffraction phenomena occurring at a solid surface after which the nature of the experimental diffraction data is described. The theoretical interpretation of the diffracted intensity by various kinematical and dynamical methods is outlined and the present difficulties in obtaining a complete surface structure determination are examined. The different types of ordered and disordered layers which may be formed on a surface—usually by adsorption of foreign species—are discussed and the interpretation of the corresponding diffraction patterns is illustrated by some examples.     

X射线衍射多晶谱计算机深度层析技术探索

提出了一种可用于将来自不同真实深度处的X射线衍射谱信息各自分离出来的技术方案,可以得出不同深度处的衍射强度、峰位和线形.该方法是定量和无损的,并且其深度尺度是真实尺度.此外并提出了吸收深度的概念.这一技术可称为直接法X射线衍射计算机深度层析技术,其可行性用Ni/Mo.双层膜样品进行了初步验证.该方法可应用于定量无损地测量峰形、峰位和峰强的深度剖面,并有可能用于界面层分析. 关键词：     

Biprismainterferenzen mit langsamen Elektronen

Diffraction patterns of low energy electrons (150 eV) were observed with an electron interferometer. An interferometer of special design was developed for this purpose. The electron source is reduced in size by a retardation lens, which simultaneously decelerates the electrons down to about 150 eV. The electron beam is split up by an electrostatic biprism (filament diameter 10 μ ¦) in two coherent waves, which recombine behind the biprism giving a Fresnel diffraction pattern. Then the electrons are accelerated and the interference fringes are electron optically magnified. In comparison with an interferometer for high energy electrons the intensity on the viewing screen is higher.     

Digital processing of X-ray diffraction patterns from oriented fibers

Accurate measurement of intensity data from x-ray diffraction patterns of oriented fibers is one of the major difficulties in the determination of polymer structures. Accurate diffraction data are particularly important when discrimination between different, but similar, molecular models is required. Although digital processing of diffraction patterns has provided more accurate data than was obtainable by traditional measurements, many of the methods used have been partly subjective and restricted to well-resolved diffraction patterns. Background is a major source of error in intensity measurement. A general method of two-dimensional background removal has been developed. Objective methods of measuring the intensities of Bragg reflections from x-ray diffraction patterns of polycrystal-line fibers are described. A profile fitting method (which is an extension of the previously reported angular deconvolution procedure) for measuring diffraction data from noncrystalline oriented polymers is also presented. These methods have been used to collect data from diffraction patterns of a variety of nucleic acid polymers.     

Theory of the Kapitza-Dirac diffraction effect

We treat the Kapitza-Dirac diffraction effect observed recently by Batelaan et al. using a newly developed nonperturbative quantum-field scattering theory. Our theory shows that an electron beam passing perpendicularly through a focused standing light wave can produce diffraction patterns. Our theory predicts (1) the minimum value of the ponderomotive energy is (Planck's over 2 pi omega)(2)/m(e)c(2), (2) the critical laser intensity above which the first pair of electron diffraction peaks will occur, and (3) the existence of sidebands in the electron spectra separated far from the central band by a momentum of several hundred photons. Our theory provides a unified explanation of the experimental results of Bucksbaum et al. and Batelaan et al.