X-ray absorption near-edge structure (XANES) spectroscopy

Abstract

The previous Reviews in Mineralogy volume on spectroscopic methods (Vol. 18 Spectroscopic Methods in Mineralogy and Geology, Frank C. Hawthorne, ed. 1988), contained a single chapter on X-ray absorption spectroscopy which reviewed aspects of both EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-ray Absorption Near-Edge Structure) (Brown et al. 1988, Chapter 11) However, since publication of that review there have been considerable advances in our understanding of XANES theory and applications. Hence EXAFS and XANES have been separated into their own individual chapters in the current volume. In this chapter we endeavor to bring the reader up to date with regard to current XANES theories, as well as, introducing them to the common applications of the technique in mineralogy, geochemistry and materials science. There have been several reviews of XANES (cf., Brown et al. 1988, Brown and Parks 1989, Manceau et al. 2002, Brown and Sturchio 2002, Mottana 2004, Rehr and Ankudinov 2005, de Groot 2001, 2005, and papers therein). In this chapter on XANES it is not our intention to provide a comprehensive review of all the XANES studies since 1988 but to summarize what X-ray edges are commonly investigated and what one can expect to be able to extract from the data. The reader is also advised to read the chapters in this volume on analytical transmission electron microscopy by Brydson et al. (2014, this volume) where (core level) electron energy loss (EELS) spectroscopy is discussed, and by Lee et al. (2014, this volume) on X-ray Raman spectroscopy (XRS), as these techniques provide element specific information similar to XANES. X-ray absorption near-edge structure (XANES) spectroscopy using synchrotron radiation is a well-established technique providing information on the electronic, structural and magnetic properties of matter. In XANES, a photon is absorbed and an electron is excited from a core state to an empty state. To excite an electron in a given core-level, the photon energy has to be equal or higher than the binding energy of this core-level. This gives rise to the opening of a new absorption channel when the photon energy is scanned. The energy of an absorption edge therefore corresponds to the core-level energy, which is characteristic for each element, making XANES an element-selective technique. In addition to the XANES region, at higher energies the extended X-ray absorption fine structure (EXAFS) region is found. The spectral shape in the near-edge region is determined by electronic density of states effects and gives mainly information about the electronic properties and the local geometry of the absorbing atom. The EXAFS region is dominated by single scattering events of the outgoing electron on the neighboring atoms, providing mainly information about the local geometric structure around the absorbing site. In this chapter we will focus on XANES.