X-ray Photoelectron Diffraction

X-Ray Photoelectron Diffraction (XPD) is a powerful crystallographic technique which combines information on distinct and complementary aspects of the system under study,  including its morphology, electronic structure and chemical composition. In a typical photoelectron diffraction experiment, a photon beam is shone on the sample, in such a way to bring the system to an excited state and induce the emission of an electron from the core levels of an atom (the emitter).

The difference between the excitation energy and the kinetic energy of the ejected electron corresponds to the measured binding energy of the photoelectron. In XPD, the intensity of a core level line is monitored as a function of the photoelectron kinetic energy and emission angle. Along its path to the detector, the emitted electron can undergo a series of scattering processes from the atoms surrounding the emitter (hence called the scatterers).
The final state is given by the interference between the direct and the coherently scattered components of the photoelectron signal. The resulting interference pattern is determined by the path-length differences and  the scattering phase shifts between the electron waves. These in turn depend on the electron kinetic energy, on the detector geometry and on the position of the emitter atom relative to the scatterers. By varying the position of the detector or the kinetic energy of the emitted electrons, the interference conditions are changed. The photoemission intensity modulations  observed on changing the electron energy and the photoemission angle can thus be used to extract geometrical information.
In this respect, XPD provides a direct structure determination tool, and is best suited for applications on periodic surfaces, like oriented single crystal surfaces, ordered adsorbate layers on crystalline substrates, or thin epitaxial films.
Unlike valence band electrons, which are involved in bond formation, core-level electrons are localized close to the atomic nuclei. For this reason, core level photoemission spectroscopy is an efficient probe of the local structural and chemical environment experienced by the electrons.
Due to its high sensitivity to the surface structural details on the local scale, an XPD analysis can also be performed on systems lacking long-range periodicity, e.g. small atomic surface complexes and supported nanoclusters.