[Ifeffit] Ifeffit Digest, Vol 155, Issue 9

Adi Shivprasad a.shivprasad at gmail.com
Mon Jan 25 11:14:33 CST 2016


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-request at millenia.cars.aps.anl.gov
> wrote:

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> 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 <scalvin at sarahlawrence.edu>
> To: XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>
> Subject: Re: [Ifeffit] XANES pre-edge vs. edge features
> Message-ID: <9A201B66-AE18-4F10-A367-7E9A7851F705 at 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 <newville at cars.uchicago.edu
> <mailto:newville at 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 <aps202 at psu.edu<mailto:
> aps202 at 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
>
> aps202 at psu.edu<mailto:aps202 at psu.edu>
> Ph.D Candidate
> Nuclear Engineering Department
> Pennsylvania State University
>
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> ------------------------------
>
> Message: 2
> Date: Mon, 25 Jan 2016 09:15:41 -0600
> From: "Guanghui Zhang" <brighton30 at 126.com>
> To: "XAFS Analysis using Ifeffit"<ifeffit at millenia.cars.aps.anl.gov>
> Subject: Re: [Ifeffit] XANES pre-edge vs. edge features
> Message-ID: <4b95e618.1ba7.1527958edcb.Coremail.brighton30 at 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: gzhang21 at iit.edu; brighton30 at 126.com
>
>
>
>
> ????pushkar shejwalkar <pshejwalkar2004 at gmail.com>
> ?????2016-01-25 08:55
> ???Re: [Ifeffit] XANES pre-edge vs. edge features
> ????"XAFS Analysis using Ifeffit"<ifeffit at 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 <
> newville at 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 <aps202 at 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
>
>
> aps202 at psu.edu
> Ph.D Candidate
> Nuclear Engineering Department
> Pennsylvania State University
>
>
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>
>
>
>
>
>
> --
>
> Best Regards,
> Pushkar Shejwalkar.
> Post-doctoral -Researcher,
> Tokyo Engineering University,
> Tokyo-to
> Japan
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