Using x-ray absorption fine structure spectroscopy to characterize high-energy-density temperature and chemistry

X-ray absorption fine structure (XAFS) spectroscopy is a unique technique capable of constraining both the electronic structure and temperature of high-energy-density (HED) materials. Temperature measurements are historically difficult at low temperature HED conditions (above 100 GPa and below 5000 K) and extended x-ray absorption fine structure (EXAFS) spectroscopy is one of the few experimental techniques capable of constraining temperature at these conditions by measuring the variation in the distances between neighboring atoms. By studying a variety of synthetic and low-temperature EXAFS datasets, we have identified systematic issues with the EXAFS analysis and shown how the atomic potentials can be modified at HED conditions. On the other hand, characterizing chemical bonding and the behavior of valence electrons has historically been difficult at high-energy-density (HED) conditions. X-ray absorption fine structure (XAFS) spectroscopy was performed on Fe₂O₃ (hematite) to determine the interactions between iron and oxygen under compression. At the Omega-60 Laser Facility, Fe₂O₃ was ramp compressed to above 700 GPa and probed with an implosion x-ray source. Analysis of the XANES spectrum revealed that the iron 3d electron orbitals, which are bonded to the oxygen 2p electron orbitals, spread in energy as the oxygen atoms were compressed closer to the absorbing iron atom. The persistence of this 1s to 3d transition peak is an indication that the iron remains bonded to oxygen above 700 GPa demonstrating the ability for iron to capture oxygen deep into the cores of super-Earth planets. In this talk, these techniques for characterizing temperature and chemistry at HED conditions will be presented and discussed.