Electron emission holography at keV energies: Estimates of accuracy and limitations

Auger Electron and Photoelectron Diffraction in Magnetic Thin Films

The role of the final-state character of the emitted electron in Auger electron diffraction (AED) and x-ray photoelectron diffraction (XPD) is examined with respect to magnetic materials. Single scattering cluster calculations with the inclusion of the spherical wave character and the final-state character of the emitted electron (both angular momentum quantum number and magnetic quantum number) show that selective emission from different M-levels, generated by a non-statistical distribution of initial M-levels or by an M-selective excitation process, results in distinctly different emission patterns.

3D atomic imaging by internal-detector electron holography

A method of internal-detector electron holography is the time-reversed version of photoelectron holography. Using an energy-dispersive x-ray detector, an electron gun, and a computer-controllable sample stage, we measured a multiple-energy hologram of the atomic arrangement around the Ti atom in SrTiO3 by recording the characteristic Ti Kα x-ray spectra for different electron beam angles and wavelengths. A real-space image was obtained by using a fitting-based reconstruction algorithm. 3D atomic images of the elements Sr, Ti, and O in SrTiO3 were clearly visualized. The present work reveals that internal-detector electron holography has great potential for reproducing 3D atomic arrangements, even for light elements.

Surface crystallography by low energy electron diffraction

The present status of the methodology of full dynamical surface structure determination by low energy electron diffraction (LEED) is reviewed with respect to both experiment and theory. Restriction is to today widely used experimental and computational techniques including the powerful approach by Tensor LEED on the theoretical side. Special emphasis is on more recent developments to tackle increasingly complex surface structures. So, we describe new structural search procedures which aim to arrive at the global rather than only a local R-factor minimum in parameter space as the best fit between experiment and theory. Also, we illuminate the application of LEED to disordered adsorbates and the related development of holographic image reconstruction from diffuse diffraction patterns. The most recent extension of this direct method to ordered structures is included as well, showing that the resulting structural information is most valuable if not essential for finding the correct atomic model of the surface. Examples are given in each case and a selection of particularly demanding structure determinations is presented as well.

Inverse electronic scattering by singular values decomposition within the Fresnel-Kirchhoff formalism

The inverse scattering technique we presented previously to enable a sample reconstruction from the diffraction figures obtained by electronic projection microscopy is reforrmulated within the Fresnel-Kirchhoff formalism, which describes the sample as a two-dimensional mask. The method relies on the use of singular values decomposition techniques, thus providing the best least-squares solutions and enabling a reduction of noise. The technique is applied to the analysis of a two-dimensional nanometric sample that is observed in Fresnel conditions with an electronic energy of 40 eV. The algorithm turns out to provide results with a mean relative error around 1% and to be very stable against random noise.