Hello everyone, In my experience, it is best to stay away from sulfate which has a hugely intense peak subject to self absorption. At SSRL, we advice users to choose Sodium Thiosulfate with the low lying and sharp S-S sigma star peak at 2472.02 eV and Tetraphenyl phosphonium bromide at 2146.96 eV for Phosphorus (see Scott's paper at: J. Synchrotron Rad. <http://journals.iucr.org/s> (2018). *25* <http://journals.iucr.org/s/contents/backissues.html>, 529-536) Thanks, Riti
On Thu, May 7, 2020 at 3:19 PM 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 >
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