Description
Ryszard Stekla$^{1}$,$^{2}$, Alexander Prange$^{3}$, Josef Hormes$^{4}$, Henning Lichtenberg$^{3}$,
Alexey Maximenko$^{1}$
$^{1}$ NSRC SOLARIS 30-392 Krakow, Poland,
$^{2}$ AGH University of Krakow, 30-059 Krakow, Poland,
$^{3}$ Niederrhein University of Applied Sciences, 47805 Krefeld, Germany,
$^{4}$ University of Bonn, 53113 Bonn, Germany
correspondence E-mail:stekla@student.agh.edu.pl
ASTRA is an X-ray absorption spectroscopy beamline specially constructed for measurements in the tender energy range of X-rays. One major field of activity is XANES and EXAFS spectroscopy at the sulfur K-absorption edge. Investigating the atomic environment of sulfur atoms in different materials, especially the local bond structure, plays an important role in petrochemistry and polymer science. Sulfur can form up to six bonds by donating, and up to two bonds by accepting electrons due to vacant 3p orbitals. The strength of these chemical bonds is influenced not only by its electronegativity difference but also by surrounding ligands$^i$ . The complexity of sulfur bonds is associated with shifts in energy levels caused by orbital hybridization. Hybridization modifies the electronic structure of a molecule, impacting the lowest unoccupied molecular orbital (LUMO), and consequently, the molecule's chemical behavior . A bond's impact on orbital occupation depends on its ionic character — the more ionic the bond towards the target atom, lower the energy required to transfer the electron. The more occupied the orbitals, lower the photon energy required to temporarily transfer electrons from the 1s orbital to the Rydberg states. Therefore, the local bond structure of sulfur in different organic compounds can be easily investigated using X-ray absorption fine structure (XAFS) spectroscopy.
Several years ago, researchers from the University of Bonn measured a comprehensive set of XANES spectra at the S K-edge of rare organic sulfur compounds with methyl and phenyl groups, sometimes bonded with extra oxygen bonds, at different synchrotrons (in Germany and the USA) before ASTRA was built. Unfortunately these spectra were no longer available as data files, but only printed on paper. Scientists at ASTRA are now digitizing the data and adding it to ASTRA’s XAS spectra library. Moreover, they performed an analysis of the spectra combined with the FEFF simulations. During the presentation, the main features of the spectra will be related to sulfur bonds with carbon, oxygen and other sulfur atoms and further discussed.
This work is a stepping stone for ASTRA’s beamline users conducting similar studies about organic compounds, providing the beamline with a XANES library of unique sulfur-containing compounds.
Acknowledgements: The further development of the ASTRA beamline for measuring at low photon energies was supported within the EU Horizon2020 programme (952148-Sylinda).
$^i$ Sulfur X-ray Absorption and Vibrational Spectroscopic Study of Sulfur Dioxide, Sulfite, and Sulfonate Solutions and of the Substituted Sulfonate Ions X3CSO3 - (X ) H, Cl, F) E. D. Risberg, L. Eriksson, J. Mink, L. G. M. Pettersson, M. Y. Skripkin, M Sandstrom
$^{ii}$ Functional Groups and Sulfur K-Edge XANES Spectra: Divalent Sulfur and Disulfides A. Mijovilovich,L. G. M. Pettersson, F. M. F. de Groot,B. M. Weckhuysen
