Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/X-ray absorption spectroscopy

Figure 1: Transitions that contribute to XAS edges

Figure 2: Three regions of XAS data

X-ray absorption spectroscopy (XAS) is a widely-used technique for determining the local geometric and/or electronic structure of matter. The experiment is usually performed at synchrotron radiation sources, which provide intense and tunable X-ray beams. Samples can be in the gas-phase, solution, or condensed matter (i.e. solids).

XAS data are obtained by tuning the photon energy using a crystalline monochromator to a range where core electrons can be excited (0.1-100 keV photon energy). The "name" of the edge depends upon the core electron which is excited: the principal quantum numbers n=1, 2, and 3, correspond to the K-, L-, and M-edges, respectively. For instance, excitation of a 1s electron occurs at the K-edge, while excitation of a 2p electron occurs at an L-edge (Figure 1).

There are three main regions found on a spectra generated by XAS data (Figure 2). The dominant feature is called the "rising edge", and is sometimes referred to as XANES (X-ray Absorption Near-Edge Structure) or NEXAFS (Near-edge X-ray Absorption Fine Structure). The pre-edge region is at energies lower than the rising edge. The EXAFS (Extended X-ray Absorption Fine Structure) region is at energies above the rising edge, and corresponds to the scattering of the ejected photoelectron off neighboring atoms. The combination of XANES and EXAFS is referred to as XAFS.

Since XAS is a type of absorption spectroscopy, it follows the same quantum mechanical selection rules. The most intense features are due to electric-dipole allowed transitions (i.e. Δ l = ± 1) to unfilled orbitals. For example, the most intense features of a K-edge are due to 1s → np transitions, while the most intense features of the L₃-edge are due to 2p → nd transitions.

XAS methodology can be broadly divided into four experimental categories that can give complementary results to each other: Metal K-edge, metal L-edge, ligand K-edge, and EXAFS.

Applications

XAS is an experimental technique used in different scientific fields including molecular and condensed matter physics, materials science and engineering, chemistry, earth science, and biology. In particular, its unique sensitivity to the local structure, as compared to x-ray diffraction, have been exploited for studying:

• Amorphous solids and liquid systems
• Solid solutions
• Doping and ion implantation materials for electronics
• Local distortions of crystal lattices
• Organometallic compounds
• Metalloproteins
• Metal clusters
• Catalysis
• Vibrational dynamics
• Ions in solutions
• Speciation of elements
• Liquid water and aqueous solutions

History

An informative account about the history of XAS and EXAFS (originally called Kossel's structures) is given in the paper "A History of the X-ray Absorption Fine Structure} by R. Stumm von Bordwehr", Ann. Phys. Fr. vol. 14, 377-466 (1989) (author's name is C. Brouder).