Dear all,
Thanks for the quick and informative responses! When I mentioned
consistency, I should have clarified that, since I am in the Nuclear
Engineering field, the audience for my paper may not have any prior
experience with XAS, so I wanted to understand how to explain spectrum
features in a way that is accurate and easy to understand from their point
of view. To that end, I wanted to make sure I had a better understanding
before writing anything incorrect.
Thanks,
Aditya
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than "Re: Contents of Ifeffit digest..." Today's Topics: 1. Re: XANES pre-edge vs. edge features (Scott Calvin)
2. Re: XANES pre-edge vs. edge features (Guanghui Zhang) ---------------------------------------------------------------------- Message: 1
Date: Mon, 25 Jan 2016 10:06:17 -0500
From: Scott Calvin Hi Aditya, Matt gave a good explanation of what?s responsible for the various
features, which was part of your question. Another aspect of your question
has to do with the way we use terminology. (Bruce?s answer, and your response, came in while I was writing this. So
it may be somewhat superfluous, but since it?s already mostly written, I
figure I?ll pass it along.) As Matt said, ?edge? is a poorly defined term. One meaning of it is that
it?s the big rise in absorption that occurs at the start of a XAFS
spectrum. That, for example, is the meaning that?s in use when we try to
quantify an ?edge step? or an ?edge jump? so that we can normalize the
data. In that sense, if a feature occurs before most of that rise, it can
be called ?pre-edge.? But another use of the word ?edge? is as a synonym for ?E0,? which is
itself a concept that is not unambiguously defined. One definition of E0
would be ?the energy origin needed to make the EXAFS equation accurate.?
But the EXAFS equation is itself an approximation which is only useful at
energies starting many eV above E0. So there?s no theoretical reason that
the E0 used in EXAFS has to correspond to an energy with a simple
theoretical meaning or to any particular feature in the spectrum. The best
we can say is that it should be within, or sometimes a bit above, the
rising portion of the absorption spectrum. But when we?re trying to align and process data we need some working
definition of E0, both for XANES analysis and to get a preliminary chi(k)
for EXAFS analysis. So we come up with other definitions, such as ?the
maximum of the first significant peak in the first derivative spectrum.? As
Bruce said, the exact definition used is not important, but it is important
that the same definition be used with all spectra being compared. Depending on the definition chosen, the preliminary value of E0 may very
well be chosen well below the bulk of the rise in the spectrum. If we call
the energy of E0 the ?edge,? but also call features that come before the
big rise "pre-edge features,? we end up with the confusing terminology that
the edge energy may well be below some of the pre-edge features! That terminology is unfortunate for people new to the field, but rarely
causes any actual ambiguity. ?Scott Calvin
Sarah Lawrence College On Jan 25, 2016, at 9:40 AM, Matt Newville 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 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.edumailto:aps202@psu.edu
Ph.D Candidate
Nuclear Engineering Department
Pennsylvania State University _______________________________________________
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