EXAFS as a tool for catalyst characterization: a review of the ... - INT

The X-ray absorption coefficient for an atom decreases as the X-ray
energy increases. It displays discontinuities (absorption edges) as an
incident photon is absorbed by the atom and electronic transitions
from a core atomic level to unoccupied conduction states above the
Fermi level take place.
The photoelectron emitted in this process can be represented as a
wave. If the excited atom is surrounded by other atoms, the outgoing
wave scatters from the surrounding atoms, producing ingoing waves.
These ingoing waves can constructively or destructively interfere with
the outgoing waves. This interference produces the oscillatory
behavior of the fine structure.
The extended X-ray absorption fine structure (EXAFS) is the
oscillation in the absorption coefficient on the high-energy side of X-
ray absorption edges, ranging from 30 to about 1000eV above the
edge. Stern, Sayers and Lytle related these fluctuations of the
absorption coefficient to the atomic arrangement surrounding the
absorbing atom (Stern, 1974; Stern et al., 1975; Lytle et al., 1975).
In this paper, we will be concerned with neither the theory of EXAFS
(Stern, 1974) nor the experimental techniques used to measure
EXAFS (Lytle et al., 1975; Meitzner, 1998). The aim of this work is to
review the mathematical procedure for treating the EXAFS spectrum
in order to calculate the physical parameters. A final section presents
one example of EXAFS application to catalysis.
Data Analysis
An X-ray absorption experiment involves the measurement of the
total linear absorption coefficient, µ, as the photon energy is varied.
In the case of a transmission experiment,
ln I 0 /I = µ x (1)
where I 0 and I are the photon intensities before and after the
absorber of thickness x.
A typical X-ray absorption spectrum can be separated into four parts
(Bart and Vlaic, 1987) (Figure 1):
a- pre-edge region;
b- edge region;
c- X-ray absorption near edge structure (XANES) region;
d- extended X-ray absorption fine structure (EXAFS) region.