TRENDS AND FUTURE PROSPECTS - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 467

Figure7.17. XAFS spectra of titanium and different titanium oxides. The K absorption edge of
titanium appears at 4.964 keV, the pure foil of titanium and three different oxides show different
peaks at the steep flank and oscillations at the sloping flank. The NEXAFS region is nearly 30 eV
above an absorption edge (see inset), while the EXAFS region reaches up to 300 eV above this
edge. Figure from Ref. [126], reproduced with permission. Copyright2009, American Chemical
Society.
mode , XAFS spectra are mainly recorded in this fluorescencemode . The
fluorescence intensity is plotted versus the primary photon energy, typically
in a range up to ∼ 500 eV above an absorption edge (EXAFS). If the range is
restricted to less than 50 eV, the method is called XANES or NEXAFS (NE
means near edge) (see Section 7.3.3). A typical example for EXAFS is given in
Figure 7.17 for a titanium foil (300 nm) and three different pellets of titanium
oxides [126]. TiO, Ti 2 O 3 , and TiO 2 with different valences (bi-, tri-, and tetra-
valent) can clearly be distinguished from metallic titanium (zero or nonvalent).
In principle, XAFS spectra of a metal show a first steep rise at their
respective K, L, and M edges. It is attributed to elevation of a K, L, or M
electron above the Fermi level. Monoatomic gases can show a single peak or
two peaks within 5 eV above the high-energy side of the respective edges. They
are due to electron transitions into unoccupied higher shells. Molecular gases
and chemical compounds of liquids and solids show several fluctuations at the
sloping flank due to transitions into a valence band of neighboring atoms.
These structures occur in the range of 5-50 eV. They are attributed to multiple
reflections of the emitted photons even at more distant atoms (short-range
order effects) and are called XANES [127]. Further and lower oscillations can
be found in the range of 50-500 eV above the absorption edge. They are related
to reflections by nearest neighbors of the central atom (long-range order) and
also depend on the crystal lattice, temperature, and pressure. These structures
are called EXAFS [125]. A theory based on the essential physics showed that