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
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
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