[Ifeffit] [IFEFFIT] Polarized FEFF calculation

Tue Feb 27 18:49:35 CST 2024

Dear Matthew,

Thank you so much for your input. I followed your advice, and indeed the
ratios of integrated chi(k) values between the polarized and isotropic
cases matched reasonably well with the 3*cos^2(θ) values I calculated. To
be exact, I integrated the scattering amplitude f(k) (i.e., the mag[feff]
column in the path file) and took their ratios.

In the process, I noticed that the f(k) is slightly different for the same
kind of paths. For example, there are two Ru-Ru single scattering paths at
3.10 and 3.54 angstroms, and below is the f(k) of the two paths (from a
non-polarized FEFF calculation):
[image: image.png]
Is this discrepancy just an artifact of the FEFF calculation? While I'm not
worried about such a small discrepancy for my project, this makes me wonder
how exactly the FEFF calculates the effective scattering amplitudes and
phases. While I will try to digest the FEFF user guide more, if you have
any quick words of advice, I would appreciate it.

Thank you very much,
Soyoung

On Mon, Feb 19, 2024 at 12:28 PM Matthew Newville <
newville at cars.uchicago.edu> wrote:

> Hi Soyoung, I would not place too much importance to the value of the
> "Importance" factor ;) That is, it is a quick estimate of the importance of
> a Path so that it might be filtered out. As you'll notice, the first path
> always has
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> Hi Soyoung,
>
> I would not place too much importance to the value of the "Importance"
> factor ;)    That is, it is a quick estimate of the importance of a Path so
> that it might be filtered out.    As you'll notice, the first path always
> has an Importance factor of 100, making comparisons between calculations
> difficult.
>
> If you want to check how the scattering amplitude varies with angle, say
> to check how close it is to cosine-squared, I recommend reading in the Path
> files and integrating the magnitude of chi(k).
>
>
>
> ------------------------------
> *From:* Ifeffit <ifeffit-bounces at millenia.cars.aps.anl.gov> on behalf of
> Soyoung Kim <soykim at lbl.gov>
> *Sent:* Thursday, February 15, 2024 3:01 PM
> *To:* XAFS Analysis using Ifeffit <Ifeffit at millenia.cars.aps.anl.gov>
> *Subject:* [Ifeffit] [IFEFFIT] Polarized FEFF calculation
>
> This Message Is From an External Sender
> This message came from outside your organization.
>
> Dear XAFS community,
>
> I have a series of Ru K-edge data on a single-crystal RuO2 sample measured
> at different incident X-ray angles. Because of the linear polarization of
> X-rays, the EXAFS of this single-crystal sample depends on the X-ray angle.
> I am trying to fit the four EXAFS spectra simultaneously with one set of
> parameters to extract the deltaR and MSRD parameters of this sample.
>
> For oriented samples, N for a certain single scattering path is supposed
> to be proportional to 3*cos^2(θ), where θ is the angle btwn the X-ray's
> e-vector and the absorber-scatterer vector. Instead of manually calculating
> the 3cos^2(θ) for all of the scattering paths in RuO2 (there are 7 single
> and 3 multiple scattering paths that need to be included to achieve a good
> fit with a reference RuO2 powder sample), I decided to use the polarization
> card in my FEFF input. For example, for one of the X-ray angles, the
> e-field vector would be (1, 0, 0), so I added the line "POLARIZATION 1 0 0"
> in the FEFF input file.
>
> Running this input file gave me a list of paths that had different
> importance values for each path compared to the non-polarized (isotropic)
> calculation. *Since "importance" is the relative magnitude of each path's
> scattering contribution (integration of the chi(k)), I thought that it
> would be proportional to 3cos^2(θ).* That is, for path i from a FEFF
> calculation for angle a, the "importance" (abbreviated as "Imp") of that
> path would be:
> Imp(a)_i = C(a) * Imp(iso)_i * 3cos^2(θ_i)
> where C(a) is a constant for each angle that accounts for the fact that
> importance values are relative (because it is scaled so that the first path
> in the list has an importance of 100).
>
> Now, I wanted to check whether the polarized FEFF calculations really
> follow this relationship underlined above. So, I tried manually calculating
> the values of 3cos^2(θ) for two of the paths from the same polarized FEFF
> calculation, then plug them into the above equation to get C(a). But, as
> the table below shows, the C(a) values are not the same. FEFF 1 has a
> disagreement that is small enough to ignore, but the disgareement of C(a)
> values for The disagreement is small enough to ignore for FEFF 1, but for
> FEFF 2 and 3 the disagreements are quite large.
> [image: image.png]
> *FEFF 1, 2, 3 designate individual polarized FEFF calculations. FEFF 1 was
> with polarization vector = (1, 0, 0). FEFF 2 and 3 were with polarization
> vector = (0.85, 1.13, 0). For this polarization, the two Ru sites gave
> different lists of paths.
>
> I do think the qualitative trend of the "importance" values in the
> polarized FEFF calculations is correct, and I can get a bad but
> not-disastrous fit from simultaneously fitting the data from different
> angles. However, the above analysis makes me wonder whether the
> "importance" values from polarized FEFF are truly proportional to
> 3cos^2(θ), and whether my fitting models for different angles, which I
> derived from the polarized FEFF calculations, are correct. An alternative
> would be to manually calculate 3cos^2(θ) for all the paths, but I'm not
> sure how to calculate it for multiple scattering paths.
>
> One note, I am running the FEFF calculation with Larix to get the
> importance values (labeled as "amp ratio" in the list.dat file, in the FEFF
> output folder). I wonder if the default settings for getting these
> amp ratios are not accurate enough for my purposes. It would be nice if I
> could just pull out the 3cos^2(θ) terms from the FEFF calculations...
>
> Anyways, thank you very much for reading this rather lengthy question.
> Hope it makes sense, and I would appreciate any help regarding this.
>
> Best,
> Soyoung
>
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