The photoelectron diffraction , often with PED, PhD or XPD ( English X-ray photoelectron diffraction for short), is a method from the group of photoelectron spectroscopy (PES). It is used, among other things, to determine the structure of crystalline surfaces or to investigate the spatial position of adsorbates on surfaces.
Photoelectron spectroscopy (PES) is the basis of the measuring process . It measures how the intensity of the photoelectrons depends on the angle of the electron emission . However, here, as with angle-dependent photoelectron spectroscopy, the focus is not on the impulse of the photoelectron, but rather the interference of the wave function of the photoelectron. Depending on the direction of emission and the kinetic energy of the photoelectron, one finds differences in intensity, called modulations. These intensity modulations arise from constructive and destructive interference between the electron wave that reaches the detector directly (reference wave ) and those that arise from waves elastically scattered one or more times around the emitting atom (object waves). The path differences and intensities of the individual waves depend on the geometric arrangement and the type of neighboring atoms. Given a sufficient number of measured intensities, the geometric structure can be determined from the modulations by comparing the experimentally measured modulations with corresponding simulations.
In the case of inelastically scattered waves, there is no fixed phase relationship to the reference wave, so they do not contribute to the interference. Since the scattering of the electrons is strongest especially at high energies in the forward direction, in the simplest case the scattering of the electrons on the atoms below the ionized atom can be neglected. When scattering at the atoms further up, the photoelectrons are focused in the direction of these atoms ("forward focusing").
The simplest applications are based on forward focusing by atoms above the photoionized atom. This can be used to determine whether certain atoms are located directly on the surface or deeper, and in the case of adsorbed molecules, whether there are other atoms (and in which direction) above a type of atom. The XPD can be used to determine the crystallographic structure of metal and semiconductor surfaces. In addition, information is obtained about the spatial position of molecules on surfaces, the bond lengths and bond angles.
Comparison with complementary techniques
Compared to other techniques for the determination of structures on surfaces is photoelectron diffraction
- independent of long-range order, so there does not have to be a perfectly ordered surface (in contrast to diffraction methods with waves that come in from outside, such as LEED),
- chemically specific, d. That is, the environment of an atomic species can be specifically examined; unlike SEXAFS also atoms of the same element, but individually with different binding partners can often be detected (by utilizing the chemical shift (engl. chemical shift ), thus smaller energy differences of photoelectrons)
- Experimentally complex for complete structural investigations, because the X-ray energy has to be varied and monochromatized synchrotron radiation from an electron storage ring is required for this; a large number of spectra also have to be measured (even if only in a small energy range, which corresponds to the examined torso level). The simulation calculations are also time-consuming because the electrons can be scattered several times and therefore a great number of different possible “paths” of the electrons must be taken into account.
- Brent D. Hermsmeier: XPD and AED. X-Ray Photoelectron and Auger Electron Diffraction . In: CR Brundle, Charles A. Evans, Shaun Wilson (Eds.): Encyclopedia of materials characterization: surfaces, interfaces, thin films . Gulf Professional Publishing, 1992, ISBN 978-0-7506-9168-0 , pp. 240–252 ( limited preview in Google Book search).
- Karl ‐ Michael Schindler: Photoelectron Diffraction . In: Chemistry in Our Time . tape 30 , no. 1 , 1996, p. 32-38 , doi : 10.1002 / ciuz.19960300106 .