物構研談話会（15-15）

概要

Extended X-ray absorption fine structure (EXAFS) spectroscopy using synchrotron radiation has for more than 40 years been an invaluable tool to determine the structure around an absorbing atom. The theoretical description of the EXAFS region of the X-ray absorption spectrum has also for many years been established and a range of data treatment and analysis programs are available. On the other hand, the theoretical description the absorption edge region is still in progress even though very large steps have been taken in recent years. The reason to this fact is that the X-ray absorption near edge structure (XANES) region besides about distances around the absorbing atom, also obtained in the EXAFS region, contains information about geometry and electronic energy levels. Another difficulty is to obtain EXAFS data of sufficiently good quality at energies below 3.5-4.0 keV due to high absorption, while collection of good XANES data is a fairly easy task.

A method using internally calibrated sulfur K edge XANES spectra of model compounds has been developed to determine the relative speciation of sulfur compounds in natural matrices as soil and wood.¹ The speciation of different kind of sulfur compounds can with this technique be quantified with an accuracy of a couple of percent when the total sulfur content is 0.05% weight% or more. The method will be discussed in detail, and a number of examples presented.

EXAFS gives only information about the coordination geometry of a metal complex when significant multiple scattering from the inner coordination sphere is present, thus the complex must have high symmetry. However, to obtain geometric information from low symmetry complexes detailed XANES analyses must be applied. The structure and geometry determination using the MXAN program package² of some low symmetry tin(II) complexes in solution will be presented and discussed.

MAX IV is new synchrotron light source to be inaugurated in June 2016, and the first beam-lines will open for general users late 2016 or early 2017. One of the first beam-lines to open is Balder³ (the God of light in the Nordic mythology), a hard X-ray absorption spectroscopy beam-line. The design of the beam-line has focused on a high flux of photons in a wide energy range (2.4-40 keV), and an experimental set-up allowing several kinds of measurements to be performed simultaneously. The high brilliance from the MAX IV storage ring in combination with the beam-line design allows for highly diluted samples and samples sensitive to radiation damage to be measured, making this in-situ hard X-ray XAS beam-line highly competitive with the world leading XAS beam-lines.

References

1. G. Almkvist, K. Boye and I. Persson, J. Synchrotron Rad. 17 (2010) 683-688.
2. M. Benfatto, S. Della Longa and C. R. Natoli, J. Synchrotron Rad. 10 (2003) 51-57.
3. https://www.maxlab.lu.se/balder