Aditya,
The distinction between "edge" and "pre-edge" is not very clear, either
when looking at a single spectrum or even conceptually.
In broad terms, the main edge is at the energy where the unoccupied
electron levels start - the Fermi energy. For 1s levels, the transition
is to p levels (and for Fe K edge, the 4p level). So, the main edge is at
the energy of the empty 4p levels. This the transition as being to an
atomic level. In a solid (or liquid), the energy levels above the Fermi
level are highly delocalized and spread over many (if not all) atoms in the
systems. Once you get much above the main edge, it's not very easy to
assign transitions to identifiable atomic transitions, or even assign a
good quantum number to them.
Pre-edge features are generally considered to be unoccupied atomic levels
(that is, still assignable to a particular atom, or at least almost so)
below the main edge. For the transition metal K edges (such as Fe), the
main edge is 1s -> 4p. But Fe has many unoccupied 3d levels. For a K
edge to get to transition to these levels, either you need a quadrupole
transition (unlikely, but not impossible), or (more likely) for
bonding/anti-bonding with ligands (typically oxygen) to mix their
p-orbitals with the metal d-orbitals. This hybridization is often called
a ligand field or crystal field. It often gives very identifiable (and
at very predictable energies) peaks below the main edge. Two and
sometimes even three peaks can be seen and assigned with ligand field
terminology. There's sort of a whole industry built up around these
peaks for transition metal oxides.
These peaks can "leak" into the main edge, and in some cases (say, Cu1+)
the classification of "sharp features at the edge" is not very clear. For
Fe metal, it's pretty clear that the main edge (derivative at 7110.75 eV, a
small peak on the main edge around 7112.5 eV) is the 4p level, and the rest
of the features are actually explainable as EXAFS.
Hopefully, someone will correct anything I got wrong!
--Matt
On Mon, Jan 25, 2016 at 8:13 AM, Aditya Shivprasad
Dear list,
I was looking at the XANES standard for Fe foil from Hephaestus and I noticed that there was a small, curved feature at the edge (7112 eV), another inflection point at 7116.4 eV, and then the edge step at around 7131 eV. My question is: why is the feature at 7112 eV considered as the edge and not as a pre-edge feature? Are they due to fundamentally different phenomena? I would like to understand where this type of feature comes from so as to be consistent in the current paper that I am writing. I have attached the standard, just in case.
Thanks -- Aditya Shivprasad
aps202@psu.edu Ph.D Candidate Nuclear Engineering Department Pennsylvania State University
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