Extended X-ray Absorption Fine Structure (EXAFS)
Introduction to X-ray Absorption
An atom absorbs an X-ray when the photon energy is sufficient to eject
a photoelectron (see the XPS document for an energy
level diagram). Below this threshold energy there is no absorption. Photons
with energies greater than the threshold energy to produce a photoelectron
are absorbed because the excess energy is conserved by transferring it
to kinetic energy of the photoelectron. However, the probability of the
absorption occuring decreases as the photon energy increases above the
threshold.
Use of X-ray absorption as an analytical method is fairly uncommon because
other techniques such as X-ray fluorescence are more sensitive. The absorption
of X-rays by a certain element is often used in analytical instrumentation
as a filter to block some X-ray wavelengths. For example, the absorption
edge of Zr will block KB and most of the continuum radiation
of X-rays from a Mo target.
Extended X-ray Absorption Fine Structure (EXAFS)
Extended X-ray Absorption Fine Structure (EXAFS) occurs due to interference
effects as photoelectrons leave the surface of a material. The interference
depends on the interatomic distance between the atom that ejected the photoelectron
and the nearest neighbor atom, and to lesser extents the next nearest neighbor
and other atoms. EXAFS therefore provides a means to determine the structure
of the atoms on the surface of a material. The fine structure appears as
a modulation on the absorption edge, and requires sophisticated modeling
to extract the structural information about the sample surface. The most
common source for EXAFS is synchrotron radiation, which provides an intense
and tunable source of X-rays.