P. Korecki

X-ray imaging inside the focal spot of polycapillary optics using the coded aperture concept

We present a method for x-ray imaging of objects inside the focal spot of polycapillary optics that resolves details smaller than the focal spot dimensions. This method employs coded aperture imaging, in which the micro-structure of polycapillary optics is treated as the coding pattern. Projection of the object is decoded from a magnified x-ray image of the polycapillary structure which is specifically sharpened by the object. Field of view can be extended by scanning the object across the focal spot.

Structure and electronic properties of ionic nano-layers MBE-grown on III–V semiconductors

Thin epitaxial NaCl and KBr layers of various thickness (from 3 to 100 monolayers) have been deposited on (100) surfaces of GaAs and InSb semiconductors by means of molecular beam epitaxy (MBE). Electronic and structural properties of the freshly prepared films were subsequently investigated in the attached UHV analytical chamber by means of low energy electron diffraction, low electron energy loss spectroscopy, Auger electron spectroscopy and an electron holography. Alkali halide growth mode was found to be a two-dimensional layer-by-layer type (Franck van der Merve growth mode). It was possible to demonstrate that the first monolayer of alkali halide on the AIIIBV semiconductor is arranged by the strong bond formed between the halogen ion and the AIII metallic element. In case of NaCl/GaAs(100) system a local atomic configuration was found for an early stage of epitaxy (three to five monolayers) by means of the electron holography. A detailed analysis of the reconstructed diffraction patterns revealed that an initial formation of the Cl-Ga bond occurred in the system, and after deposition of five monolayers NaCl, the substrate was uniformly covered by a layer at least three monolayers thick. q 2000 Elsevier Science S.A. All rights reserved.

Fluorescence detection of white-beam X-ray absorption anisotropy: towards element-sensitive projections of local atomic structure

A method for a direct measurement of X-ray projections of the atomic structure is described. Projections of the atomic structure around Nb atoms in a LiNbO3 single crystal were obtained from a white-beam X-ray absorption anisotropy pattern detected using Nb K fluorescence.

X-ray microlaminography with polycapillary optics

We demonstrate layer-by-layer x-ray microimaging using polycapillary optics. The depth resolution is achieved without sample or source rotation and in a way similar to classical tomography or laminography. The method takes advantage from large angular apertures of polycapillary optics and from their specific microstructure, which is treated as a coded aperture. The imaging geometry is compatible with polychromatic x-ray sources and with scanning and confocal x-ray fluorescence setups.

γ-Ray Holography — Three-Dimensional Imaging of a Local Atomic Structure

γ-ray holography is a novel method for three-dimensional (3D) imaging of a local atomic structure, which utilizes the internal reference idea of holography. The principles of this method will be presented with emphasis on the phase information gained by means of complex holograms.

Simulation of image formation in x-ray coded aperture microscopy with polycapillary optics

In x-ray coded aperture microscopy with polycapillary optics (XCAMPO), the microstructure of focusing polycapillary optics is used as a coded aperture and enables depth-resolved x-ray imaging at a resolution better than the focal spot dimensions. Improvements in the resolution and development of 3D encoding procedures require a simulation model that can predict the outcome of XCAMPO experiments. In this work we introduce a model of image formation in XCAMPO which enables calculation of XCAMPO datasets for arbitrary positions of the object relative to the focal plane as well as to incorporate optics imperfections. In the model, the exit surface of the optics is treated as a micro-structured x-ray source that illuminates a periodic object. This makes it possible to express the intensity of XCAMPO images as a convolution series and to perform simulations by means of fast Fourier transforms. For non-periodic objects, the model can be applied by enforcing artificial periodicity and setting the spatial period larger then the field-of-view. Simulations are verified by comparison with experimental data.

Wavelet analysis of white beam x-ray fluorescence holograms: determination of lattice sites and imaging of local atomic structure

The prospects of atomic structure imaging with the continuous spherical wavelet transform (CSWT) as applied to white beam x-ray fluorescence holograms (XFH) are discussed. Recording of XFH with a white x-ray beam eliminates holographic twin images and minimizes extinction effects. However, the lack of these parasitic effects is accompanied by a limited radial resolution. In this work, by introducing an approximation of the white x-ray spectrum based on the Gumbel distribution, we propose an improvement both in generation of white beam XFH and in data analysis. Using approximate analytical models and realistic numerical simulations, we give a detailed description of the properties and resolution of local structure projections directly obtained from XFH by using wavelet analysis. It is demonstrated that the CSWT and, in particular, its windowed inversion can be effectively used to enhance and speed up reliability factor (R-factor) analysis of the data, which enables precise fully three-dimensional localization of multiple lattice sites of dopants. For this, an exact analytic formula for the inversion is given, enabling its fast calculation in a single step. As an example system, we consider magnetic ions in wurtzite GaN.

Photon stimulated desorption from alkali halide surfaces at near threshold energies

Abstract Desorption processes induced in alkali halides by photons of synchrotron radiation (of energy 5–20 eV) have been investigated by means of quadrupole mass spectroscopy. The relative yields and angular characteristics of alkali and halogen atoms emitted from (100) surfaces of KI and RbI crystals were measured as a function of the photon energy and the sample temperature. For comparison, analogous measurements were performed with a 1 keV electron beam. In the latter case the relevant desorption processes are already better understood. We have found that both the fundamental exciton and the fundamental gap excitations result in alkali and halogen desorption with the same angular distributions. This leads to the conclusion that dynamic desorption processes, pronounced for electron excitation, are inefficient during desorption stimulated by photons with near threshold energies. It is argued that such a difference can be understood by taking into account different selection rules, and differences in excitation depth and excitation density distribution for photon- and electron-induced processes.

Thickness dependent electron stimulated desorption of thin epitaxial films of alkali halides

Abstract Thin epitaxial NaCl layers of various thicknesses have been deposited on (100) GaAs substrates and subsequently desorbed with a 1 keV electron beam. The film thickness was monitored with a quartz crystal microbalance and verified by recording the substrate Ga2p1/2(111 eV) photoelectron peak attenuation due to overlayer growth. Electronic and structural properties of the freshly prepared films were subsequently investigated in the attached analytical chamber by means of LEED, LEELS, and AES. The signal intensities of desorbed alkali and halogen atoms were detected with a quadrupole mass spectrometer. For the first time the thickness dependence of the alkali halide electron stimulated desorption (ESD) yield was determined in absolute units. The experimental results are well described by a simple diffusion equation indicating that range dependent transport processes are involved in ESD of alkali halides.

Grid-enhanced X-ray coded aperture microscopy with polycapillary optics

Polycapillary devices focus X-rays by means of multiple reflections of X-rays in arrays of bent glass capillaries. The size of the focal spot (typically 10–100 μm) limits the resolution of scanning, absorption and phase-contrast X-ray imaging using these devices. At the expense of a moderate resolution, polycapillary elements provide high intensity and are frequently used for X-ray micro-imaging with both synchrotrons and X-ray tubes. Recent studies have shown that the internal microstructure of such an optics can be used as a coded aperture that encodes high-resolution information about objects located inside the focal spot. However, further improvements to this variant of X-ray microscopy will require the challenging fabrication of tailored devices with a well-defined capillary microstructure. Here, we show that submicron coded aperture microscopy can be realized using a periodic grid that is placed at the output surface of a polycapillary optics. Grid-enhanced X-ray coded aperture microscopy with polycapillary optics does not rely on the specific microstructure of the optics but rather takes advantage only of its focusing properties. Hence, submicron X-ray imaging can be realized with standard polycapillary devices and existing set-ups for micro X-ray fluorescence spectroscopy.