[Ifeffit] Ifeffit Digest, Vol 207, Issue 10

Wed May 6 02:05:50 CDT 2020

Dear All,

there is another publication about high precision measurements of X-Ray Absorption Edges from the early 90ies from the same group:
Measurement of the Energy of X‐Ray Absorption Edges
J. Stümpel P. Becker St. Joksch R. Frahm G. Materlik
First published: 16 April 1991
https://doi.org/10.1002/pssa.2211240224

The trick is that the authors don't use the monochromator in order to determine the energy of the edge, but instead an analyzer-crystal on a rotary stage with an encoder. This analyzer-crystal can be rotated to two different angles, where it fulfills the Bragg condition. Instead of measuring an absolute angle, they measure a difference of two angles, which is twice the Bragg-angle. This is more precise. They also measure the temperature of the crystal, which is essential to determine the lattice constant and to calculate the correct energy. 

More than 15 years ago, Syed Khalid used the values from the PTB-publications to calibrate the monochromator at X18b, and he used these values to determine the temperature of the monochromator crystal. All energies of the edges of the 3d-metals could be nicely reproduced, and the deviations from the PTB-values were rather small and not jumping by several eV as the Bearden-values from the yellow book. Unfortunately, the web-page got lost after NSLS was shut-down (at least, I cannot find it again). Syed tried to convince his users to use the PTB-values for calibration, but they all refused since they had used the old values for decades and did not want to switch. There might have been also users from UOP. 

Those, who listen to Jerry Seidler's Global XAFS Journal Club might remember the lecture by Joe Fowler: The tabulated values for the energies of fluorescence lines are ancient, some were still measured with photographic paper, and it is not much different for absorption edges.

I agree with Matt that new values are required, and we should compare our results!

With best regards,
Wolfgang

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Subject: Ifeffit Digest, Vol 207, Issue 10

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Today's Topics:

   1. Re: Reported W L3-edge and L2-edge energy (Matt Newville)

----------------------------------------------------------------------

Message: 1
Date: Tue, 5 May 2020 22:31:27 -0500
From: Matt Newville <newville at cars.uchicago.edu>
To: XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>
Subject: Re: [Ifeffit] Reported W L3-edge and L2-edge energy
Message-ID:
	<CA+7ESbpcjj0QNkTNFTax9W2e=SB4JeezJ2Lmfb-KMEKsQotCAw at mail.gmail.com>
Content-Type: text/plain; charset="utf-8"

Hi Simon,

This is definitely a timely discussion for me, as I've been spending part
of the quartine working on collating data and expanding datasets for an
XAFS spectral database.  I'm hoping to have something ready for public
comment and to start asking for contributions of data in a few weeks, but
I'll be happy to have more discussion about that sooner too.

I generally believe that the monochromator I use at GSECARS is both
well-calibrated and reasonably accurate.  That is, with 2 angular encoders
with a resolution of >130,000 lines per degree and an air-bearing, I
believe the angular accuracy and repeatability are very good.  I believe
there are equally good moons in existence.   As Matthew Marcus pointed to
the Kraft paper (which used an older source but 4-bounce mono to improve
resolution), we find that Fe foil is definitely better defined as 7110.75
and Cu foil is between 8980.0 and 8980.5 eV.  That is, we've measured
multiple foils, found their first derivatives, and refined the d-spacing
and angular offset.  We do this about once per run, and the offsets tend to
be very consistent.   For sure, there is some question about whether the
Kraft numbers are perfect.   For sure, putting Fe foil at 7110.75 +/- 0.25
eV appears to be "most right" to us.

I also believe that we should probably re-measure these metal foils (and
other compounds) with a single calibration set for both Si(111) and
Si(311).  We will probably have time to do that this summer in the time
between "beamline staff can get back to the beamline" and "open for outside
users".

What I can tell you now is:  I have some data on W metal, WO2, and WO3
measured all at the same time on our bending magnet line, with Si(111).  An
Athena project for this is attached (W.prj).   I cannot vouch for the
absolute calibration.

I also attach a set of foils (V, Fe, Cu, Mo) measured with the same
calibration (and Si(111) on our ID line), after adjusting d-space and
offset to be close to the Kraft values (CalibratedFoils2013.prj).

I also attach a set of foils (Fe, Cu, Au L3, Au L2, Au L1, Pb L3, Pb L2, Pb
L1 edges) measured in 2016 (again, using Si(111) on our ID line), also with
the same calibration values (FeCu_Au_Pb.prj).  I'm pretty certain these use
the same d-spacing as the 2013 Foils to at least 5 digits.   For
completeness, all of the raw data files are also under
https://github.com/XraySpectroscopy/XASDataLibrary/tree/master/data

In my experience, the Pb L3 edge value has the biggest variation in the
literature, with values ranging from 13035 to 13055 eV (possibly a typo
somewhere along the line).  Fortunately, the Kraft-based calibration splits
the difference and puts the value at 13040 eV.

For W in particular, I will look if I have measured this recently on our ID
line.  I can tell you that I use CdWO4 as a phosphor and use that to focus
our X-ray beam.   I use this trick all the time: any tail from the beam
penetrating the phosphor is shortest at the peak of the white-line and for
CdWO4 that is always between 10210 and 10215 eV.

I hope that helps.  I am interested in trying to get all these values as
accurately as possible, so any comments or suggestions would be most
welcome.

--Matt

On Tue, May 5, 2020 at 5:14 PM Bare, Simon R <srbare at slac.stanford.edu>
wrote:

> All:
>
>
>
> We are wondering if others agree that the reported values for the W L3 and
> W L2 edges are *incorrect*. We recently noticed the following:
>
>
>
> The ?Edge? ? defined by the inflection point of the absorption edge step
>
>
>
> When using the Ir L3 edge (11215.0 eV) as a calibration, the W L3- and L2-edges
> are *10203.4 eV* and *11542.4 eV*, respectively.
>
>
>
> When using the Pt L3 edge (11564.0 eV) as a calibration, the W L3- and L2-edges
> are *10203.3 eV* and *11542.4 eV*, respectively.
>
>
>
> These observations are thus different than the reported values of *10207.0
> eV* and *11544.0 eV* for the L3 and L2 edges, respectively.
>
>
>
> Thanks in advance for the discussion and feedback.
>
>
>
>
>
> Simon R Bare
>
> *Distinguished Scientist*
>
> *SSRL, MS69*
>
> *SLAC National Accelerator Lab*
>
> *2575 Sand Hill Road*
>
> *Menlo Park CA 94025*
>
>
>
> simon.bare at slac.stanford.edu
>
> Ph: 650-926-2629
>
>
>
> [image: co_access_logo_text]
>
>
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-- 
--Matt Newville <newville at cars.uchicago.edu> 630-252-0431
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