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Near Edge X-ray Absorption Spectroscopy (NEXAFS)

Exposing samples to x-ray radiation can lead to absorption. If the photon energy is higher than the vacuum energy a core electron (e.g. 1s) can leave the sample and, for example, be measured via XPS. Choosing, however, photon energies near the vacuum energy (therefore the name “near edge”), especially closely below this level, the electron cannot leave the sample. Nevertheless, an excitation is possible if there are unoccupied states in this area. Such excitations are the subject NEXAFS measurements deal with. Contrary to XPS, by means of this method Auger electrons that are created during relaxation processes are detected instead of direct detection of the photoelectrons. Due to the short mean free path of these electrons NEXAFS is extremely surface-sensitive.

For NEXAFS measurements a continuous x-ray spectrum is necessary to scan the photon energy in the range of the edge, therefore this method can only be used at synchrotron facilities. For this reason a team from our research group travels to the Berlin Electron Storage Ring BESSY II several times a year.

Besides the spectroscopic information on unoccupied target states, another property of the synchrotron radiation can be used to determine molecular orientations. Synchrotron sources like BESSY II not only provide continuous but also linearly polarized x-rays. Excitations important for NEXAFS are described by a vector quantity, the transition dipole moment. Only if the transition dipole moment is parallel to the E-field vector of the radiation, the maximum intensity is obtained. In case of a preferred direction of the molecules on a surface, this orientation can be determined by angle-resolved measurements of the intensity, varying the angle of incidence between beam and surface. Such a change in intensity during variation of angles is called dichroism. The orientation of molecules in thin films can also be determined with diffraction methods (see XRD), however, NEXAFS is able to do this for non-crystalline samples and particularly for extremely thin samples. Actually, NEXAFS is capable of determining the orientation of molecules in a monolayer (only one layer of molecules on the surface), which is of great importance for surface science and the physics of interfaces.
 
In order to use the full potential of this method, an exact interpretation of the spectra is crucial. To this purpose simulations allowing an exact assignment of transitions and peaks in the spectra are necessary. For such simulations several theoretical approaches are known. In our research group a DFT-code called StoBe is used, which is specifically designed for excitations of core electrons.

Figure 1: Schematice of the NEXAFS method: Colored arrows denote allowed transitions leading to peaks in the spectrum, whereas grey arrows show forbidden or rarely visible transitions.

Typical questions concerning this method are the following:

- How are molecules oriented on surfaces?

 → Studying the dichroism

- Which unfilled final states are possible? How does the NEXAFS spectrum arise?

→ Comparison of experimental data to simulated spectra

Figure 3: Comparison of an experimental NEXAFS spectrum of anthracene to simulated data. The orange and blue bars denote the calculated energies for different transitions.

Some exemplary publications of our group where Near Edge X-ray Absorption Spectroscopy has been utilized are:

• Understanding the F1s NEXAFS Dichroism in Fluorinated Organic Semiconductors.
  Michael Klues, Paul Jerabek, Tobias Breuer, Martin Oehzelt, Klaus E. Hermann, Robert Berger, and Gregor Witte
  Journal of Physical Chemistry C 120 (23), 12693–12705 (2016) • DOI: 10.1021/acs.jpcc.6b04048
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• Effects of Molecular Orientation in Acceptor-Donor Interfaces between Pentacene and C60 and Diels-Alder Adduct Formation at the Molecular Interface.
  Tobias Breuer, Andrea Karthäuser, Gregor Witte
  Advanced Material Interfaces 3 (7), 1500452 (2016) • DOI: 10.1002/admi.201500452
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• Characterization of Orientational Order in π-conjugated Molecular Thin Films by NEXAFS.
  Tobias Breuer, Michael Klues, Gregor Witte
  Journal of Electron Spectroscopy and Related Phenomena 204 (A), 102-115 (2015), DOI: 10.1016/j.elspec.2015.07.011
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• Lattice Matching as the Determining Factor for Molecular Tilt and Multilayer Growth Mode of the Nanographene Hexa-peri-hexabenzocoronene.
  Paul Beyer, Tobias Breuer, Saliou Ndiaye, Anton Zykov, Andreas Viertel, Manuel Gensler, Jürgen P. Rabe, Stefan Hecht, Gregor Witte, and Stefan Kowarik
  ACS Appl. Mater. Interfaces 6 (23), 21484–21493 (2014)
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• Analysis of the near-edge X-ray-absorption fine-structure of anthracene: A combined theoretical and experimental study.
  Michael Klues, Klaus Hermann and Gregor Witte
  J. Chem. Phys. 140 (1), 14302 (2014)
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