Ab-Initio Simulation Laboratory

We invert high-energy (E greater-than-or-equal-to 400 eV) photoelectron and Auger-electron interference patterns to construct 3D images of surface and interface atoms. A new scheme is introduced to correct the phase shift of the image. Image reconstruction is demonstrated for Si(111) square-root 3 x square-root 3-B, a system in which multiple-scattering effects are small and all source waves are equivalent. Using diffraction results from multiple-scattering slab calculations, we achieve a spatial resolution of 1.0-1.3 angstrom, thus qualifying the technique as a direct structural tool.

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We present a method for spatially resolved imaging of energy-dependent photoelectron diffraction. Energy-dependent photoelectron-diffraction spectra are individually Fourier transformed to three-dimensional vector space. The complex transformed amplitudes are summed over a span of phi angles or over a span of polar angles. The images are, respectively, well resolved in the radial and azimuthal directions, or in the radial and polar directions. The intersections of these real-space maps fix the atomic coordinates. We show how the intensity loci from single and multiple scattering paths are separately resolved and how most multiple scattering contributions are eliminated. By varying the collection angles, atoms in different regions relative to the emitter, e.g., surface or bulk atoms, are imaged. One can also use the photon's A vector to enhance the near-pi backscattering geometry. We compare this method with another direct method: extended x-ray-absorption fine structure.

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