Re: [Ifeffit] Silent Oxygen
Hi Fuxiang, Further to Bruce's note, I looked at your file. Your model splits the nearest oxygens by only 0.01A. Is that the magnitude of splitting you anticipate? Are you familiar with the resolution criteria for EXAFS? (k-range [deltak]) * (R-resolution [deltaR]) ~ pi/2 for identical near neighbours This is also discussed in the archives, e.g.: http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2007-November/003414.html Looking at your chi(k), it seems usable, charitably, up to about 8 A-1, not to 12 A-1 as your window suggests you have used. Claims of being able to resolve small splittings with this data will be viewed with skepticism. -R. On 4/18/2017 11:34 AM, Bruce Ravel wrote: On 04/18/2017 11:10 AM, Fuxiang Zhang wrote: Hi, all, I am in studying the EXAFS of perovskite and clearly see the split of the first peak after ion irradiation. It should come from a symmetry change, so I use a tetragonal model (instead the original cubic) which contains two independent O sites. However, after running Atoms and Feff, all paths are contributed by only one O atom and nothing related to another Oxygen atoms (see attached file). As a result it may be difficult to get a good fit of the first split Ta-O peaks. I am not sure if it is true or something wrong. Thank you very much A fairly common question here on the mailing list. This is a *feature* of Artemis, not a bug. http://bruceravel.github.io/demeter/documents/Artemis/extended/fuzzy.html If you want to /model/ a split in distance, you can drag that path into the path list twice and adjust the delta R and amplitude parameters accordingly. B ___ 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
Re: [Ifeffit] Silent Oxygen
On 04/18/2017 11:10 AM, Fuxiang Zhang wrote: Hi, all, I am in studying the EXAFS of perovskite and clearly see the split of the first peak after ion irradiation. It should come from a symmetry change, so I use a tetragonal model (instead the original cubic) which contains two independent O sites. However, after running Atoms and Feff, all paths are contributed by only one O atom and nothing related to another Oxygen atoms (see attached file). As a result it may be difficult to get a good fit of the first split Ta-O peaks. I am not sure if it is true or something wrong. Thank you very much A fairly common question here on the mailing list. This is a *feature* of Artemis, not a bug. http://bruceravel.github.io/demeter/documents/Artemis/extended/fuzzy.html If you want to /model/ a split in distance, you can drag that path into the path list twice and adjust the delta R and amplitude parameters accordingly. B -- Bruce Ravel bra...@bnl.gov National Institute of Standards and Technology Synchrotron Science Group at NSLS-II Building 743, Room 114 Upton NY, 11973 Homepage:http://bruceravel.github.io/home/ Software:https://github.com/bruceravel Demeter: http://bruceravel.github.io/demeter/ ___ 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
Re: [Ifeffit] the shortest possible K-range ,
Hi Mahmoud, On Tue, Apr 18, 2017 at 5:11 AM, Al Humaidi, Mahmoud < mahmoud.alhuma...@student.uni-siegen.de> wrote: > Hi , > > I want to ask please what is the shortest k-range one can use for "Forward > Fourier transform" > > in my projects I used: > > Spline range k 0.0 - 9.0 > k range in Forward Fourier transform 1.0 - 9.0 A^-1 > That should be OK, though it will limit the resolution and number of parameters you can use to model the data. Starting at k=1 Ang^-1 is often considered to be a bit too low for EXAFS analysis, as this is generally still in the XANES region where multiple scattering contributions dominate and where the background subtraction is more uncertain. That is, starting a k=2 or 3 Ang^-1 might be a better choice. > > I am trying to reduce the contribution of noise in FT .. > > Well, limiting the k-range does not always reduce the noise in the part of the signal you care about (R= 1 to 6 Ang). It can, but just because k^2*chi(k) looks noisier at high k does not necessarily mean the data is only noisy in that range. Mostly the noisiness at high-k implies that the signal-to-noise ratio is reduced, but that's at least partially because the signal goes down and may not mean the noise is going up. OTOH, the measurements sometimes do get noisier at higher energy. > the attached project shows nice fit ,,,but high phase shift toward lower R > value ,, > any recommendation please I will appreciate .. > and thank you in advance , > > I don't understand why your "9%" merge data gets weird when making the spline go past 10 Ang^-1, but the "3%" ad "5%" merge data looks sort of OK to me going out to 11 or 12 Ang^-1. I am not sure the signal is real If these are merged data, I'd suggest looking at the scan to scan variations to decide if the data is real. Hope that helps, --Matt > best regards > Mahmoud AL Humaidi > > - > *Mahmoud Al Humaidi * > > Department of Physics, University of Siegen > Emmy-Noether-Campus > Walter-Flex-Str. 3, 57072 Siegen > Room : B-015 > Tel : +49 (0) 271 740 3727 <+49%20271%207403727> > - > > > ___ > 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
