MULTIPLE-SCATTERING EFFECTS IN ELECTRON HOLOGRAPHIC IMAGES

An exact multiple-scattering formalism is used to simulate a wave multiply scattered from a cluster, and this is used to provide a direct quantitative analysis of the influence of multiple scattering on holographic imaging. Although multiple scattering may help in identifying atomic positions in real space, we show that it does cause a loss of resolution. We also show that a filter function can considerably reduce the multiple-scattering contribution to holographic images.

HOLOGRAPHIC IMAGES OF IODINE ATOMS ON A SILVER SURFACE FROM ELECTRON-EMISSION PATTERNS

BARTON 1 has suggested that photoelectron interference patterns may be used directly as holograms to obtain atomic-resolution images of surface structures. Bulk structures have been obtained previously by this means from experimental patterns of high-energy Kikuchi(quasi-elastically scattered) and Auger electrons 2,3. Here we test the feasibility of this technique for determination of surface structures using Auger intensity patterns obtained 4,5 from iodine chemisorbed on a pseudomorphic silver monolayer on Pt{111}. By direct numerical holographic inversion, we obtain three-dimensional images which show that iodine adatoms are located in hollows of 3-fold symmetry on the surface. The images yield the site symmetry with good atomic resolution in the surface plane, but suffer from poor resolution along the Ag-I axis. We anticipate that data with better angular resolution obtained at low temperatures would improve the spatial resolution of such images.

A STUDY ON THE HOLOGRAPHIC TRANSFORM FOR ELECTRON-DIFFRACTION FROM SURFACES

We break down photoelectron diffraction intensities into four terms in analogy to optical holography and discuss the effect of each term on reconstructed images. The second term involving products of scattered waves SIGMA-SIGMA-O(i)O(j)*, is in this case not structure-less. Theoretical analysis and simulations demonstrate that this term may lead to spurious features in real space images in holographic transforms of medium energy electron diffraction patterns. If it is small enough the problem may be overcome by an iterative correction process.