Method for spatially resolved imaging of energy-dependent photoelectron diffraction

Surface atomic arrangement visualization via reference-atom-specific holography

We demonstrate the direct reconstruction of 3D atomic images from measured low-energy electron diffraction (LEED) intensity spectra. A multiple-incident angle and multiple-energy integral are first applied to the spectra to obtain a map of interatomic vectors. From this map, a nonbulk interatomic vector is chosen that points to a desired reference atom. A second integral transformation, using the chosen interatomic vector as a filter, is applied to the LEED spectra to produce images of individual atoms in the vicinity of the selected reference atom. This two-step method overcomes the problem of multiple, nonequivalent reference atoms and is applicable to elemental or compound materials.

Surface Patterson function by inversion of low-energy electron diffraction I-V spectra at multiple incident angles

An accurate Patterson function, free of artifacts, is obtained by transforming low-energy electron diffraction I-V spectra at multiple incident angles. The demonstration is carried out using normal incidence measured spectra and calculated spectra at three angles of incidence. The errors between intensity spots in the Patterson function and known interatomic distances are less than 0.01 A in the horizontal direction and 0.09 A in the vertical direction. The reason for the high accuracy in the horizontal direction is given.

Atomically resolved images from near node photoelectron holography experiments on Al(111)

Szöke's concept for electron holography is hampered by forward scattering that dominates electron diffraction from electron point sources below the surface top layer. Forward scattering was proposed to be suppressed if the anisotropic nature of the electron source wave is exploited [T. Greber and J. Osterwalder, Chem. Phys. Lett. 256, 653 (1996)]. Experiments show a strong suppression of forward scattering in Al(111) if Al 2s photoelectrons (E(kin) = 952 eV) are measured near the nodal plane of the outgoing p wave. The holographic reconstruction from such diffraction data provides three dimensional images of atomic sites in unit cells with a size of more than 10 A.

Multiple scattering in low-energy electron holography

The theory of the low-energy electron point source (LEEPS) microscope is reformulated in matrix form to readily account for multiple scattering. An algorithm is developed for the storage of the structure matrix and an iterative method is used to solve the matrix equation for the structure factor. Examples of small and large clusters of atoms are given to compare single and multiple scattering. A Kirchhoff-Helmholtz transform is used for the reconstruction. We find that in some cases the multiple scattering is too strong and reconstruction is not possible. We give examples which show that, even when multiple scattering is important, one can still obtain reconstructions that reveal the atomic structure both along and lateral to the optical axis. We also compare our results with those found in LEED.

Photoelectron diffraction imaging for C2H2 and C2H4 chemisorbed on Si(100) reveals a new bonding configuration

A new adsorption site for adsorbed acetylene on Si(100) is observed by photoelectron imaging based on the holographic principle. The diffraction effects in the carbon 1s angle-resolved photoemission are inverted (including the small-cone method) to obtain an image of the atom's neighboring carbon. The chemisorbed acetylene molecule is bonded to four silicon surface atoms. In contrast to the C2H2 case, the image for adsorbed C2H4 shows it bonded to two Si surface atoms.