X-ray absorption spectroscopy

X-ray absorption spectroscopy (XAS) probes the interaction of monochromatic hard X-ray photons with core electrons of a material. Due to the high specificity of core electron binding energies, XAS is an element-specific technique.

Depending on the energy of the incident photons, core electrons can either be excited into the unoccupied states of the atom, or, if the photon energy is above the binding energy of the electron, form a photoelectron wave spherically propagating away from the atom.

The information content of these two processes is quite different. Excitation into unoccupied states (so-called X-ray absorption near edge structure, XANES) spectroscopy yields information about symmetry and orbital character of the unoccupied states. Due to the chemical shift of core electron binding energy, the XANES furthermore contains information about the oxidation state of the core atom.

In contrast, formation of the photoelectron wave gives rise to the so-called “extended X-ray absorption fine structure”, EXAFS. The photoelectron wave is elastically scattered at neighbor atoms; and interference of the original and scattered photoelectron waves leads to oscillations of the absorption coefficient. These contain information about local structure around the core atom, and permit modelling of structure in both amorphous and crystalline material.

Applications in physical solid state chemistry are many-fold:

1. The relatively high transmission of hard X-rays even in solid state materials renders in situ / in operando measurements possible. Hereby, the progress e.g. of solid state reactions [1,2] or solid/gas reactions (i.e. catalysis) [3] can be followed.
2. The sensitivity towards local order makes EXAFS a versatile method for structural elucidation even in amorphous solid state systems [4,5].
3. Determination of oxidation states of transition metals e.g. in materials with oxygen or lithium non-stoichiometry gives insight into the nature of charge compensation mechanisms [6,7].

Publications:

1. J. Brendt, D. Samuelis, T.E. Weirich, M. Martin, Phys. Chem. Chem. Phys. 2009, 11, 3127. DOI: 10.1039/b901819k
2. D. Röhrens, J. Brendt, D. Samuelis, M. Martin, J. Solid State Chem. 2010, 183, 532. DOI: 10.1016/j.jssc.2009.12.024
3. P. Kampe, L. Giebeler, D. Samuelis, et al., Phys. Chem. Chem. Phys. 9 (2007) 3577. DOI: 10.1039/b700098g
4. L. Nagarajan, R. A. De Souza, D. Samuelis, I. Valov, A. Börger, J. Janek, K.-D. Becker, P. C. Schmidt, M. Martin, Nature Materials, 2008, 7, 391. DOI: 10.1038/nmat2164
5. M. Soorholtz, D. Samuelis, et al., to be submitted.
6. D. N. Mueller, R. A. De Souza, J. Brendt, D. Samuelis, M. Martin, J. Mater. Chem., 2009, 2009, 19, 1960. DOI: 10.1039/b819415g
7. D. Samuelis, J.-Y. Shin, J. Maier, manuscript in preparation.