I agree Matthew, I also tend to use the primary K-edge peak for P calibration, but one issue to be wary of is attenuation/flattening of the primary peak (if one is using a concentrated sample).
Gypsum sounds like a good material to use for S, since it is commonly available and probably not too variable. My material of choice for P (lazulite) might fail on both counts, so it might be a poor choice. Mike > On May 8, 2020, at 10:19 AM, Matthew Marcus <mamar...@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 <newvi...@cars.uchicago.edu> >>>> wrote: >>> >>> >>> Hi Mike, >>> >>> >>>> On Tue, May 5, 2020 at 10:56 PM Mike Massey <mmas...@gmail.com >>>> <mailto:mmas...@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 >>>> <newvi...@cars.uchicago.edu <mailto:newvi...@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 >>>> <srb...@slac.stanford.edu <mailto:srb...@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.b...@slac.stanford.edu >>>> <mailto:simon.b...@slac.stanford.edu>____ >>>> >>>> Ph: 650-926-2629____ >>>> >>>> __ __ >>>> >>>> <image001.png> >>>> ____ >>>> >>>> __ __ >>>> >>>> _______________________________________________ >>>> Ifeffit mailing list >>>> Ifeffit@millenia.cars.aps.anl.gov >>>> <mailto:Ifeffit@millenia.cars.aps.anl.gov> >>>> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit >>>> Unsubscribe: >>>> http://millenia.cars.aps.anl.gov/mailman/options/ifeffit >>>> >>>> >>>> >>>> -- --Matt Newville <newville at cars.uchicago.edu >>>> <http://cars.uchicago.edu>> 630-252-0431 >>>> <W.prj> >>>> <CalibratedFoils2013.prj> >>>> <FeCu_AuPb.prj> >>>> _______________________________________________ >>>> Ifeffit mailing list >>>> Ifeffit@millenia.cars.aps.anl.gov >>>> <mailto:Ifeffit@millenia.cars.aps.anl.gov> >>>> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit >>>> Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit >>> _______________________________________________ >>> Ifeffit mailing list >>> Ifeffit@millenia.cars.aps.anl.gov >>> <mailto:Ifeffit@millenia.cars.aps.anl.gov> >>> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit >>> Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit >>> >>> >>> >>> -- >>> --Matt Newville <newville at cars.uchicago.edu <http://cars.uchicago.edu>> >>> 630-252-0431 >>> _______________________________________________ >>> Ifeffit mailing list >>> Ifeffit@millenia.cars.aps.anl.gov >>> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit >>> Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit >> _______________________________________________ >> Ifeffit mailing list >> Ifeffit@millenia.cars.aps.anl.gov >> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit >> Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit > _______________________________________________ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit
