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> 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> 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> 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@slac.stanford.edu

Ph: 650-926-2629

 

<image001.png>

 

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<CalibratedFoils2013.prj>
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