On Thu, May 7, 2020 at 3:19 PM Matthew Marcus <mamarcus@lbl.gov> wrote:

For elements like P and S, people often use the energies of peaks.
These are more immune to noise, energy-resolution effects and
overabsorption than inflection points are. For instance, on ALS 10.3.2,
I used the sulfate peak of gypsum set at 2482.74eV. I forget where I
got that number. Going down to soft X-rays, a common convention for the
carbon edge is to use a pair of sharp peaks in CO2 gas at 292.74 and
294.96eV.
mam

On 5/7/2020 3:09 PM, Mike Massey wrote:
> Hi Matt,
>
>
> Indeed, in my experience (which is limited to one beamline at one
> synchrotron facility for P XAS), once it is calibrated, the energy
> selection tends to be quite stable, so I think you're on-target there.
>
> The trouble I still run into, though, is comparability of data between
> studies. The difficulty is magnified by the fact that people tend to
> identify certain near-edge features by the energy range at which they
> occur. I do the same, of course, but I also try to carefully document
> the material and energy I used to calibrate the monochromator.
>
> For the P K-edge, it doesn't really seem like people have settled on a
> convention for calibrating the monochromator, unlike in the case of
> iron, for example (where one just uses a foil and sets some feature of
> that spectrum to their preferred value). If everyone was using the same
> thing all would be happy, but most people use different materials and
> different values. So datasets for P at the K-edge really aren't too
> comparable just yet.
>
> Sorry to hijack the conversation, it's just an issue I've been mulling
> over for a few years. The discussion of energy calibration values made
> me think of it again.
>
>
> Best,
>
>
>
> Mike
>
>
>
>
>> On May 8, 2020, at 8:51 AM, Matt Newville <newville@cars.uchicago.edu>
>> wrote:
>>
>>
>> Hi Mike,
>>
>>
>> On Tue, May 5, 2020 at 10:56 PM Mike Massey <mmassey@gmail.com
>> <mailto:mmassey@gmail.com>> wrote:
>>
>> On a tangentially related topic, I find that phosphorus K-edge XAS
>> energy calibration conventions are still in a bit of a "Wild West"
>> state, with a wide variety of materials and values in use for
>> energy calibration. As an extreme example, one or two frequently
>> cited papers in my field from the 2000s don't even report the
>> material or value used for energy calibration, and only show
>> portions of the spectra on an energy axis with values relative to
>> an unknown E0.
>>
>>
>> I have never measured a P K edge, or indeed any edge lower in energy
>> than the S K edge (ignoring some X-ray raman work). But if one is
>> using a Si(111) double-crystal monochromator where P or S is
>> approximately the low-energy (high-angle) limit, then it really should
>> be that the calibration does not drift much and cannot be too wrong at
>> low energies.
>>
>> That is, a mono calibration is controlled by a d-spacing and angular
>> offset. Normally (or perhaps, in my experience), "re-calibrating" is
>> done by changing the angular offset, leaving the d-spacing alone.
>> That is, the d-spacing is presumably known, at least to within some
>> thermal drift.
>> If that is the case that the d-spacing really is not changing and what
>> needs to be refined is the angular offset, then setting the offset at
>> relatively high energy edges will be much more sensitive, and changing
>> the angular offset to that a high-energy edge is correct should move
>> lower energy edges by a smaller amount. The corollary is that you
>> have to move the offset a lot to move the P K edge around, and that
>> would have a larger (and ever-increasing) impact on higher energy
>> edges such as Ca, Fe, Cu or Mo.
>>
>> The counter-argument is also true: d-spacing has a bigger effect on
>> the high-angle / low-energy edges.
>>
>> So, if you believe the mono d-spacing (or you believe the beamline
>> scientist who believes it ;)) then calibrate at the highest energy you
>> can. The Kraft values don't go very low in energy.
>>
>> All that said, if using a different mono crystal such as InSb or more
>> exotic crystals, I have no idea how stable those are.
>>
>>
>> I too have picked my own material and value, and will be the first
>> to acknowledge that I did so out of necessity and ease of
>> comparison to other available data, rather than because I thought
>> it was correct.
>>
>> The issue of calibration conventions and values definitely seems
>> to be one that merits continued discussion. It has been
>> interesting to watch things evolve over time in the case of iron,
>> for example (it's good to know that 7110.75 is a candidate
>> calibration value...) I appreciate Matt's detailed thoughts, and
>> the data that he's been working with. Thanks Matt!
>>
>>
>> Cheers,
>>
>>
>>
>> Mike
>>
>>
>>
>>
>>
>>> On May 6, 2020, at 3:32 PM, Matt Newville
>>> <newville@cars.uchicago.edu <mailto:newville@cars.uchicago.edu>>
>>> wrote:
>>>
>>>
>>> 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@slac.stanford.edu <mailto:srbare@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 L_3 edge (11215.0 eV) as a calibration, the
>>> W L_3 - and L_2 -edges are *10203.4 eV* and *11542.4 eV*,
>>> respectively. ____
>>>
>>> __ __
>>>
>>> When using the Pt L_3 edge (11564.0 eV) as a calibration, the
>>> W L_3 - and L_2 -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 L_3 and L_2
>>> 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@slac.stanford.edu
>>> <mailto:simon.bare@slac.stanford.edu>____
>>>
>>> Ph: 650-926-2629____
>>>
>>> __ __
>>>
>>> <image001.png>
>>> ____
>>>
>>> __ __
>>>
>>> _______________________________________________
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>>>
>>> --
>>> --Matt Newville <newville at cars.uchicago.edu
>>> <http://cars.uchicago.edu>> 630-252-0431
>>> <W.prj>
>>> <CalibratedFoils2013.prj>
>>> <FeCu_AuPb.prj>
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>> --
>> --Matt Newville <newville at cars.uchicago.edu
>> <http://cars.uchicago.edu>> 630-252-0431
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