Research with synchrotron radiation
Synchrotron radiation is used by us as a highly brilliant source for investigations of crystal structure, electronic, and magnetic properties by means of X-ray absorption spectroscopy. In addition to our application-driven basic research, we are also active in further development of various analytical methods. Most of the experiments were carried out at the electron storage ring BESSY II of the Helmholtz-Zentrum Berlin (HZB) in Berlin-Adlershof - just 4 S-Bahn stations away from our campus.
This website is still under construction. Soon there will be more information about synchrotron light and examples of our research activities.
EXAFS: Extended X-ray absorption fine structureOpen areaClose area
Analysis of the extended X-ray absorption fine structure (EXAFS) is a method to investigate the local environment around an absorbing element. In a simple picture, the EXAFS signal is an interference effect between the outgoing photo-electron excited by X-ray absorption and its backscattered waves from neighbouring atoms. The frequency of the EXAFS oscillations depends mainly on the distance of nearest neighbour atoms, while the envelope contains information about the atomic species and coordination number.
The analysis of EXAFS data can be performed using a standard Fourier analysis e.g. using the Artemis software based on the FEFFIT programs. After definition of the structure, the EXAFS signal is calculated for various scattering paths and summed up to fit the measured data.
It can be complemented by an analysis of the wavelet transform of EXAFS oscillations. The wavelet transform can have high resolution in both real space and reciprocal space and visualises contributions to the EXAFS signal from different backscattering elements without further data treatment. We adopted the AGU-Vallen Wavelet package which has been developed by Vallen Systeme GmbH, Aoyama Gakuin University (AGU), Tokyo, and University of Denver for analysis of acoustic emission.
C. Antoniak, Beilstein J. Nanotechn. 2, 237 (2011)
C. Antoniak et al., Phys. Rev. B 78, 0401406(R) (2008)
XANES: X-ray absorption near-edge structureOpen areaClose area
In X-ray absorption spectroscopy, a core-level electron is excited into a higher unoccupied state, if the photon energy matches the energy needed for the allowed transition. These so-called absorption edges are element-specific and depend also on the chemical environment. The absorption intensity is a measure of the number of unoccupied final states and the shape of the absorption signal in the near-edge region contains information about the energy-dependent density of unoccupied states. The absorption can be measured either in transmission for the case of thin samples, by measuring the fluorescence yield or the total electron yield by detecting the sample drain current. We could show that it is even possible to obtain soft-xray XANES signals from nanoparticles in an organic solvent.
It may be useful to compare experimental data to simulated absorption spectra, e.g. to unveil the origin of spectral faetures. For the case of metal ions, the CTM4XAS charge-transfer multiplet software based on a Hartree-Fock method is known to give reliable results. For a more detailed analysis, the Quanty software is employed. For other (metallic) systems, band structure calculations are performed e.g. using the spin polarised relativistic Korringa-Kohn-Rostoker (SPR-KKR) package. While calculations for bulk materials with a high symmetry can be performed on a common office PC, support is appreciated for calculations e.g. for clusters or other low-symmetry systems.
C. Schmitz-Antoniak, Rep. Progr. Phys. 78, 062501 (2015)
XMCD: X-ray magnetic circular dichroismOpen areaClose area
In the case of absorption of circularly polarized X-rays, the excited core-level electrons are polarised regarding both their angular momentum and spin. This polarisation is reversed for reversed polarisation of X-rays. In a simple two-step model, the spin-polarised electrons probe the spin polarisation of the unoccupied final states yielding a large absorption intensity for the case of matching polarisation and a small absorption intensity otherwise, i.e. the so-called circular dichroism. The intensity of this X-ray magnetic circular dichroism (XMCD) is proportional to the spin polarisation of unoccupied final states (magnetisation) and can be used to deduce spin and orbital magnetic moments.
Furthermore, we showed that XMCD is a useful tool to monitor phase transitions connected to changes in the asphericity of the spin-density distribution. Like for XANES, a comparision of exrimental data with simulated absorption spectra is useful to understand the reason behind the occurence of particular spectral faetures.
D. Schmitz et al., Sci. Rep. 4, 5760 (2014)
X(M)LD: X-ray (magnetic) linear dichroismOpen areaClose area
By measuring the X-ray absorption of linearly polarised light for different angles of incidence, one can use the electric field vector of the X-rays as a search light for the maximum and minimum unoccupied states. Thus, with XLD one can probe anisotropies in the charge density that may result from the crystal structure (x-ray natural linear dichroism) or magnetic ordering (x-ray magnetic linear dichroism).