able for proper diffraction (see Section 3.3 ). X - ray absorption spectroscopy
(XAS) can yield limited molecular structural information on noncrystalline
(amorphous) solid samples, frozen solutions, solutions, and gases. A tunable
X - ray source is required because of the energy - dependent absorption coeffi -
cient of the material under study. Synchrotron radiation has provided a high -
intensity tunable source of X - rays, and its use has been increasingly responsible
for the popularity of this technique over the last three decades.^4 The following
discussion summarizes the material found in reference 4. Another good source
for basic information is the following: Matthew Newville at the Consortium
for Advanced Radiation Sources, University of Chicago has posted a pdf fi le
“ Fundamentals of XAFS ” at the website http://cars9.uchicago.edu/xafs/xas_
fun/xas_fundamentals.pdf.
Figure 1 of reference 4 shows a typical X - ray absorption spectrum. A sharp
rise, called the X - ray absorption edge, occurs at a well - defi ned X - ray photon
energy, which is unique to the absorbing element. The absorption edge is
due to electron dissociation from a core level of the absorbing atom to
valence energy levels. Spectral features in the edge region, sometimes
called the near edge, are related to the electronic structure of the absorbing
atom and can often be used to identify the geometric arrangement of its
ligands. For instance, one might be able to distinguish between octahedral and
tetrahedral geometric arrangements about the absorbing atom through analy-
sis of XAS edge spectral features. For the same absorbing atom, differences
in absorption energy at the edge are related to its valence or oxidation state.
Usually, higher oxidation states will result in absorption at higher energies.
This analysis is often given the acronym XANES (X - ray absorption near - edge
structure).
In the region above (to the right of) the edge, variation in the X - ray absorp-
tion coeffi cient known as extended X - ray absorption fi ne structure (EXAFS)
may be analyzed to yield structural information about atoms in the ligand
sphere (within 4 - to 5 - Å radius) of the absorbing atom. The EXAFS phenom-
enon arises through interaction of photoelectrons from the absorbing atom,
symbolized by an “ a, ” with electron density of surrounding atoms. Scattering
atoms or scatterers are symbolized by an “ s. ” Scattering atoms contribute
damped sine waves of measurable frequency (related to distance between a
and s), amplitude (related to coordination number about a and s atom types),
and phase (related to s atom type). Analysis of EXAFS data can answer the
question, How many of what type of atom are at what distance from the
absorbing atom?
The XAS spectrometer is similar to a UV – visible system in that it consists
of a source, a monochromator, and a detector. The most favorable XAS source,
synchrotron radiation, is tunable to different wavelengths of desirable high
intensity. A laboratory instrument for analysis of solids and concentrated solu-
tions may use a rotating anode source (further described in Section 3.3 ).
The monochromator for X - ray radiation usually consists of silicon single crys-
tals. The crystals can be rotated so that the wavelength ( λ ) of the X - rays pro-
duced depends of the angle of incidence ( θ ) with a Bragg lattice plane of
XAS AND EXAFS 79