X-ray photoelectron diffraction: Probing atom positions and molecular orientation at surfaces

With the growing need to gain control over atomic scale objects in nanosciences new tools have to be developed. Quantitative structural information is fundamental to the understanding of the physical properties, e.g. for the interpretation of spectroscopic results or as a starting point for theoretical calculations. This review will focus on the angle-resolved photoemission experiment to probe the geometrical structure on the atomic scale. Using soft X-rays shallow core levels can be excited for a chemical analysis of the surface. Photoelectrons emitted from a particular atom are mapped as a function of emission angle in the so-called photoelectron diffraction experiment. From the interference between the photoelectron wave directly reaching the detector and the waves scattered from the neighboring atoms we obtain very direct knowledge of the local real-space environment of the emitting atom. Photoelectron diffraction is able to determine atom positions at surfaces, to distinguish between different crystallographic surface structures, to identify impurity or dopant site positions or to unravel molecular orientation.