6.2.7 Glitches
Another source of artefacts comes from the crystal monochromator. As one scans the
monochromator with the primary reflection obeying Bragg’s law, there are usually a
few angles at which another crystallographic plane just happens to diffract
[217]. When this happens, there is a dip in the intensity of the primary reflection,
as some of the beam is directed into the other reflection. Any slight nonlinearity in
the measurement will result in a glitch in the observed spectrum. (A more sophisticated name is umweganregung or “detour radiation” [218]). Even without
nonlinearity, there can be changes in the spatial distribution of the beam intensity,
the harmonic composition, and even its polarization [219]. Facilities like SSRL keep
glitch spectra for different crystal orientations, so you can pick the one with the least
severe glitches across your EXAFS region. The expected location of glitches can
also be calculated with computer software [220].
6.3 Essential Physics of EXAFS
One of the strengths of X-ray absorption is that each element has an absorption edge
with a relatively constant energy and cross section. From the energy of the edge, you
immediately know which element you are looking at, and from the size of the edge
jump, you can quantitate the concentration of that element. The energies for these
edges are summarized in Fig. 6.2 and Appendix G. If X-ray absorption always
exactly followed the tables of mass absorption coefficients, it would be a nice
analytical tool for elemental composition, but not much else.
The exciting properties of XAS are the deviations from the norm—the fine
structure that reveals the electronic and molecular structure of the sample. As a
simple example, in Fig. 6.6 we compare the X-ray absorption for the monatomic
noble gas krypton with the neighboring diatomic Br 2 . Past the absorption edge, the
Kr spectrum shows a smooth, featureless falloff with increasing X-ray energy. In
contrast, Br 2 shows periodic oscillations past the edge called “EXAFS”—an acronym for “Extended X-ray Absorption Fine Structure,” which reflect the Br-Br
distance. It also has a sharp feature at the edge (part of the region called “X-ray
Absorption Near Edge Structure” (“XANES”); see Chap. 7) that reflects a transition
to a vacant π
à molecular orbital. A final acronym, “XAFS”, “X-ray Absorption Fine
Structure”, is sometimes used to define the overall fine structure patterns including
both XANES and EXAFS.
The EXAFS signal is defined as the fractional modulation of X-ray absorption for
an atom in a particular environment compared to the free atom X-ray absorption:
138
6 X-ray Absorption and EXAFS
Another source of artefacts comes from the crystal monochromator. As one scans the
monochromator with the primary reflection obeying Bragg’s law, there are usually a
few angles at which another crystallographic plane just happens to diffract
[217]. When this happens, there is a dip in the intensity of the primary reflection,
as some of the beam is directed into the other reflection. Any slight nonlinearity in
the measurement will result in a glitch in the observed spectrum. (A more sophisticated name is umweganregung or “detour radiation” [218]). Even without
nonlinearity, there can be changes in the spatial distribution of the beam intensity,
the harmonic composition, and even its polarization [219]. Facilities like SSRL keep
glitch spectra for different crystal orientations, so you can pick the one with the least
severe glitches across your EXAFS region. The expected location of glitches can
also be calculated with computer software [220].
6.3 Essential Physics of EXAFS
One of the strengths of X-ray absorption is that each element has an absorption edge
with a relatively constant energy and cross section. From the energy of the edge, you
immediately know which element you are looking at, and from the size of the edge
jump, you can quantitate the concentration of that element. The energies for these
edges are summarized in Fig. 6.2 and Appendix G. If X-ray absorption always
exactly followed the tables of mass absorption coefficients, it would be a nice
analytical tool for elemental composition, but not much else.
The exciting properties of XAS are the deviations from the norm—the fine
structure that reveals the electronic and molecular structure of the sample. As a
simple example, in Fig. 6.6 we compare the X-ray absorption for the monatomic
noble gas krypton with the neighboring diatomic Br 2 . Past the absorption edge, the
Kr spectrum shows a smooth, featureless falloff with increasing X-ray energy. In
contrast, Br 2 shows periodic oscillations past the edge called “EXAFS”—an acronym for “Extended X-ray Absorption Fine Structure,” which reflect the Br-Br
distance. It also has a sharp feature at the edge (part of the region called “X-ray
Absorption Near Edge Structure” (“XANES”); see Chap. 7) that reflects a transition
to a vacant π
à molecular orbital. A final acronym, “XAFS”, “X-ray Absorption Fine
Structure”, is sometimes used to define the overall fine structure patterns including
both XANES and EXAFS.
The EXAFS signal is defined as the fractional modulation of X-ray absorption for
an atom in a particular environment compared to the free atom X-ray absorption:
138
6 X-ray Absorption and EXAFS