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 On Mon, Jan 25, 2016 at 10:14 AM, <ifeffit-requ...@millenia.cars.aps.anl.gov > wrote: > Send Ifeffit mailing list submissions to > ifeffit@millenia.cars.aps.anl.gov > > To subscribe or unsubscribe via the World Wide Web, visit > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > or, via email, send a message with subject or body 'help' to > ifeffit-requ...@millenia.cars.aps.anl.gov > > You can reach the person managing the list at > ifeffit-ow...@millenia.cars.aps.anl.gov > > When replying, please edit your Subject line so it is more specific > 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 <scal...@sarahlawrence.edu> > To: XAFS Analysis using Ifeffit <ifeffit@millenia.cars.aps.anl.gov> > Subject: Re: [Ifeffit] XANES pre-edge vs. edge features > Message-ID: <9a201b66-ae18-4f10-a367-7e9a7851f...@slc.edu> > Content-Type: text/plain; charset="utf-8" > > 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 <newvi...@cars.uchicago.edu > <mailto:newvi...@cars.uchicago.edu>> wrote: > > 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 <aps...@psu.edu<mailto: > aps...@psu.edu>> wrote: > 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 > > aps...@psu.edu<mailto:aps...@psu.edu> > Ph.D Candidate > Nuclear Engineering Department > Pennsylvania State University > > _______________________________________________ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov<mailto:Ifeffit@millenia.cars.aps.anl.gov > > > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit > > > > _______________________________________________ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov<mailto:Ifeffit@millenia.cars.aps.anl.gov > > > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit > > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: < > http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20160125/891a1a92/attachment-0001.html > > > > ------------------------------ > > Message: 2 > Date: Mon, 25 Jan 2016 09:15:41 -0600 > From: "Guanghui Zhang" <brighto...@126.com> > To: "XAFS Analysis using Ifeffit"<ifeffit@millenia.cars.aps.anl.gov> > Subject: Re: [Ifeffit] XANES pre-edge vs. edge features > Message-ID: <4b95e618.1ba7.1527958edcb.coremail.brighto...@126.com> > Content-Type: text/plain; charset="utf-8" > > Aditya, > Pushkar is correct, and there are several pretty good papers talking about > the geometry and pre-edge features of Fe. The following paper is one of > them which I found very useful. > http://pubs.acs.org/doi/abs/10.1021/ja964352a > > Guanghui > > > > > 2016-01-25 > > > > Guanghui Zhang, PhD > Senior Research Associate > Department of Chemistry > Illinois Institute of Technology > 3300 South Federal Street, Chicago, IL 60616 > E-mail: gzhan...@iit.edu; brighto...@126.com > > > > > ????pushkar shejwalkar <pshejwalkar2...@gmail.com> > ?????2016-01-25 08:55 > ???Re: [Ifeffit] XANES pre-edge vs. edge features > ????"XAFS Analysis using Ifeffit"<ifeffit@millenia.cars.aps.anl.gov> > ??? > > Dear Aditya, > Matt has, I think sufficiently explained in details about the > pre-edge and its distinction with main energy jump. If my understanding is > correct, and it is possible that I may be wrong as well, the pre-edge peak, > its appearance shape, intensity and energy value can therefore be used by > organometallic researchers to identify and characterize the specific > geometry as well. e.g. tetrahedral geometry in Fe (II). By theoretical > calculations it is possible to validate such values and thus it is possible > to predict the possible geometry of the metal centre (especially with > metals like Fe, I used it earlier). > I hope this will be helpful as well. > Pushkar > > > On Mon, Jan 25, 2016 at 11:40 PM, Matt Newville < > newvi...@cars.uchicago.edu> wrote: > > 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 <aps...@psu.edu> wrote: > > 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 > > > aps...@psu.edu > Ph.D Candidate > Nuclear Engineering Department > Pennsylvania State University > > > _______________________________________________ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit > > > > > > > _______________________________________________ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit > > > > > > > > -- > > Best Regards, > Pushkar Shejwalkar. > Post-doctoral -Researcher, > Tokyo Engineering University, > Tokyo-to > Japan > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: < > http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20160125/ca4fffd5/attachment.html > > > > ------------------------------ > > Subject: Digest Footer > > _______________________________________________ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit > > > ------------------------------ > > End of Ifeffit Digest, Vol 155, Issue 9 > *************************************** >
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