G.R. Harp

An electrostatic microscope for synchrotron radiation x-ray absorption microspectroscopy (invited)

An x‐ray photoemission microscope is described that is designed for x‐ray absorption imaging and spectroscopy with 1‐μm spatial resolution. The energy resolution of x‐ray absorption near‐edge structures microspectroscopy is determined only by the bandpass of the monochromator, and not by the photoemission microscope detector.

Structural effects in single-crystal photoelectron, Auger-electron, and Kikuchi-electron angular diffraction patterns

Abstract The full-hemisphere diffraction patterns of primary photoelectrons, photoemission Auger electrons, and Kikuchi electrons are reported for single-crystal surfaces of Cu(100), Cu(111), Cu(110), Ir(111), and Ag(100), to test models for direct structure determinations from angle-dependent final-state diffraction patterns. Our measurements show a simple correlation between the low-index crystallographic directions of the substrate and local intensity maxima in the electron angular distributions. We find that the angular anisotropy can be qualitatively explained in all cases studied by final-state elastic forward scattering. The strong forward scattering features in photoemission diffraction patterns are used to measure bond angles and determine the structure of Cu ultrathin films on Ir(111). In addition, a radial image function based on a holographic Fourier-transform algorithm is evaluated for the determination of bond lengths from three-dimensional images reconstructed from the two-dimensional diffraction pattern.

Surface science lettersX-ray photoelectron holography of ultrathin film and single crystal Cu(111): improving the accuracy of bond-length determination

Abstract Forward-scattering diffraction patterns from single-crystal Cu(111) substrates and an epitaxial film of FCC Cu/Ir(111) have been analyzed to produce real-space images of the atomic lattice by Fourier-transform holography. The accuracy of atomic positions is found to be better in the single-crystal holograms than in the ultrathin film of copper. The angular dependence of the atomic scattering factor produces a shift in the position of atoms in the holographic image, and causes the atom image to be asymmetric. Model calculations reproduce the atom shift and asymmetry. A modified Fourier-transform algorithm is shown to correct for both the atom-shift and the image asymmetry.

The effects of various experimental parameters on the accuracy of photoemission holography

Abstract Experimental and theoretically simulated Auger electron diffraction patterns are studied to determine the effects of several experimental parameters on the holographic images generated from these patterns. Among the parameters examined are the experimental angular resolution, electron kinetic energy, and total collection solid angle of the diffraction pattern. Several techniques of combining the images from diffraction patterns taken at different kinetic energies are critically analyzed and compared. The origin of small radius “forward scattering artifacts” in the holographic images is discussed. A technique for removing these artifacts is introduced which requires no prior knowledge of the structure being studied.

Computation of photoelectron and Auger-electron diffraction III. Evaluation of angle-resolved intensities PAD3

In this paper, we describe the third part of a suite of computer programs for the computation of Auger- and photoelectron diffraction intensities, using the so-called concentric shell algorithm (CSA). The function of the present program is to calculate the diffraction intensities capable of being measured in a variety of different experimental configurations. The present program takes as its input the cluster transmission matrix calculated in the second part of this sequence of programs, as well as the spherical wave amplitudes of the atomic photoemission process, and other input specifying the type of experiment. The output of this program is a file containing the diffraction intensities in a form for direct comparison with experimental results.

Computation of photo-electron and Auger-electron diffraction II. Multiple scattering cluster calculation PAD2

In this paper, we describe the second (PAD2) of the suite of computer programs for the calculation of angle-resolved photo- or Auger-electron diffraction intensities, using a concentric-shell algorithm (CSA). The function of the present program is to evaluate the scattering matrix that relates the wavefunction of an electron immediately after its emission from an atomic core to the wavefield that may be detected outside a sample, using an angular-momentum expansion centered on the emitter atom. This program allows that matrix to be evaluated by any of the following schemes (in ascending order of accuracy, and computer requirements): single scattering (SS), outward multiple scattering (OS) and full multiple scattering (MS).