Modern Spectroscopy

8.3 EXTENDED X-RAY ABSORPTION FINE STRUCTURE 333
Figure 8.38 Curve fitting of Mo extended X-ray absorption fine structure (EXAFS) for
Mo(SC 6H 4NH) 3, taking into account (a) sulphur and (b) sulphur and nitrogen atoms as near
neighbours. (Reproduced, with permission, from Winnick, H. and Doniach, S. (Eds), Synchrotron
Radiation Research, p. 436, Plenum, New York, 1980)
from four identical Fe–S distances in a rubredoxin. In the examples of plant and bacterial
ferredoxins in Figures 8.39(b) and 8.39(c) there are two types of waves indicating two types
of Fe–S linkage consistent with the structures shown.
The example in Figure 8.40 illustrates the use of EXAFS in indicating the degree of order
in a sample of a catalyst. In this figure is shown the k3wðkÞ EXAFS signal from osmium
atoms in the pure metal to be compared with that of a 1% osmium catalyst in small clusters
dispersed on a silica surface. The main peak in the Fourier transformed spectrum is due to
Os–Os nearest neighbours and is much more intense (ca 3.2) in the pure metal than in the
catalyst (ca 0.8), indicating many more Os–Os bonds. When the filter window is applied,
followed by retransformation, the main difference in the catalyst is that the single wave is
damped much more rapidly than in the pure metal. The damping is due to the expð 2s2 j k2Þ factor in Equation (8.20). The quantity sj is a measure of the disorder in the material which
is clearly greater in the catalyst resulting in more rapid damping. Analysis of the data
has shown that (a) N1 ¼ 8:3 2:0 for the catalyst, compared with 12 for pure osmium,
which has a face-centred cubic structure, and (b) Ds 2 1 ¼ s2 1 ðcatalystÞ s2 1
metalÞ ¼
0:0022 0:0002, showing the great disorder in the catalyst.
The EXAFS method can be used for different elements in the same material or for
different absorption edges, such as 1s, 2s or 2p, in the same element.
Auger electrons, produced in the way described in Section 8.2.1, may also produce
scattering interference in the same way as ordinary photoelectrons. The technique is useful
for studying surfaces and is known as surface EXAFS, or SEXAFS. An example of its use is
an investigation of Br 2 adsorbed onto carbon in the form of 50% carbon crystallites and 50%
graphite. The fact that the X-ray radiation from a storage ring is plane polarized was used to
show that, for a 20% Br2 monolayer coverage, the Br–Br bond is randomly orientated
relative to the surface whereas, for 60 to 90 per cent coverage, the alignment is parallel to the
surface.