3.2 Characterization techniques 53
Figure 3.6: Decay phenomena for excited states in an atom. a) Fluorescence radiation,
which occurs when an electron lls an empty lower state and the dierence in
energy is emitted as a photon. b) Auger process, where the energy dierence from the
decay is used to emit a photoelectron with a characteristic energy. Figure from 197.
The analysis of XAS spectra is often divided into two main categories, the X-ray
Absorption Near Edge Spectroscopy region, XANES, and the Extended X-ray
Absorption Fine Structure, EXAFS. These two regions are shown for a spectrum
in gure 3.7.
The analyses included in this thesis encompass XANES, so EXAFS will not
be treated any further. In XANES the near edge features of XAS spectra are
analysed with the aim of obtaining knowledge about the valance state and coordination
environment. The near edge region can be further split into pre-edge
and rising edge. A common application of the XANES region is to determine
valance states from the shift of the rising edge. A shift towards higher photon
energies can be attributed to higher oxidation states since the core electrons are
stronger bound when valence electrons are missing. However, there are multiple
ways of measuring the shift and currently various experimental groups use
dierent conventions. By comparing to measurements of reference compounds
it is possible to evaluate valence states given that a robust method is used to
evaluate the shift in edge energy. In this thesis, a method called "the moment
method" will be used to evaluate the shift. This method is based on calculating
an energy weighted quantity, Ew, with the following equation:
Ew =
R Ef
Ei
E I(E)dE
R Ef
I(E)dE
Ei
(3.5)
where E is the photon energy, Ei the energy right before the pre-edge, Ef the
energy at which features are not expected to shift signicantly and I(E) the
normalised absorbance after background has been subtracted. Ew is therefore