X-ray absorption and photoelectron spectroscopy investigation of selenite reduction by FeII-bearing minerals

Wavelet transform (WT) has been proven as a valuable tool for EXAFS data analysis for structures, where two types of backscattering atoms are at the same distance from the central atom [1-3]. WT provides not only radial distance resolution of the spectra like the Fourier transform, but resolves also the wave vector space permitting to probe the discrimination of atoms by their elemental nature.
A short introduction to the continuous WT will be given. The variation of the parameters of the Morlet mother wavelet will be used to build an adapted wavelet for the analysis of specific EXAFS spectra. Thereby the analysis of the (Heisenberg) uncertainty boxes of the Morlet wavelet is a central point to understand the resolution limitations of the WT as well as the form of the wavelet ridges.
Two examples for the use of WT for EXAFS data analysis will be given.
First, the method is applied to a structural problem of Zn-Al layered double hydroxides, demonstrating the homogeneity of the metal cation distribution in the hydroxide layers. Depending on the specific problem, either the well-known Morlet wavelet was used, or a newly developed FEFF-Morlet wavelet, based on theoretical EXAFS back scattering functions.
Second, the reduction from soluble selenium species to elemental Se and iron selenides by Fe containing minerals is examined in detail [4]. Here, the wavelet analysis is used to answer the question: Is Se coordinated to Se atoms/elemental Se or to the Fe (and Se) atoms? This analysis is performed by comparison to well–known reference spectra.
In the outlook two open questions will be discussed: the k weighting and the destructive interference by phase shifts.

References:

[1] H. Funke, A. C. Scheinost, and M. Chukalina, Phys. Rev B, 71, 094110, 2005.
[2] H. Funke, M. Chukalina, and A. C. Scheinost, J. Synchrotron Rad. 14, 426-432, 2007.
[3] M. Muñoz, P. Argoul, and F. Farges, Am. Mineral. 88, 694, 2003.
[4] A. C. Scheinost, L. Charlet , Environ. Sci. Technol. 42, 1984, 2008.