Photoelectron Diffraction

Photoelectron diffraction is a technique for the quantitative determination of surface and near-surface structure in an atom-specific fashion. It exploits the coherent interference of a photoelectron wavefield emitted from a core level of a near-surface atom with components of the same wavefield elastically scattered by nearby atoms. There are two variants of the method. One of these, exploiting (near-zero angle) forward scattering and using higher photoelectron kinetic energies (>∼ 500 eV), is typically based on X-rays from Mg and Al K α sources that are also used for standard X-ray photoelectron spectroscopy (XPS). This version of the technique is mostly used to determine interatomic directions in surface molecules and thin epitaxial films. An alternative version of the technique uses lower energy (< 500 eV) photoelectrons to exploit the stronger backscattering that occurs at these energies but typically requires the use of the widely variable photon energies provided by synchrotron radiation, particularly in its scanned-energy mode of detection. This backscattering variant provides quantitative information of the local site and associated bond lengths of adsorbed atoms and molecules on the surface. The capabilities of these two modes are illustrated by several examples.