Re: [Ifeffit] Help in background removal with Athena
-----Original Message-----
From: Ifeffit [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of ifeffit-request@millenia.cars.aps.anl.gov
Sent: 06 September 2016 14:57
To: ifeffit@millenia.cars.aps.anl.gov
Subject: Ifeffit Digest, Vol 163, Issue 6
Dear Irina,
Your efforts are probably sadly in vain. C K-edge NEXAFS or XANES is common, but the data range is rarely long enough for exafs analysis. You have ca 160 eV of data that is not enough for EXAFS analysis. Hence I don't think there is a way to get a good background and then FT for your data. Certainly looking at your data the noise level and thus uncertainty in which features are real and which may not be will probably prevent anything but a XANES analysis. There probably is an oscillation there but with the limited data range getting meaningful numbers from a fit is most unlikely.
Best wishes
Fred
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Today's Topics:
1. Help in background removal with Athena (Irina Pi)
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Message: 1
Date: Tue, 6 Sep 2016 13:56:19 +0000
From: Irina Pi
Dear Fred,
Thank you for the quick answer. Which range should I have for the EXAFS analysis to be meaningful? Like 200eV above the edge?
Thanks again,
Irina.
________________________________
From: Ifeffit
On 09/06/2016 10:55 AM, Irina Pi wrote:
Thank you for the quick answer. Which range should I have for the EXAFS analysis to be meaningful? Like 200eV above the edge?
I am always uncomfortable when newcomers ask for "rules". Having a rule without understanding when and why it might apply seems counter-productive. Firstly, lets discuss the relation ship between "energy above the edge" and photoelectron wave number. The conversion constant between energy and wavenumber is k = sqrt( ETOK * (E-E0) ) where ETOK is the appropriate combination of fundamental constants (2m_e / hbar^2, where m_e is the mass of the electron and hbar is Planck's constant) about equal to 1 / 3.81. So, 10 inverse Angstroms is about 381 volts above the edge. The extent of the signal in k determines how well you can interpret an EXAFS signal in a quantitative sense. See this page in the Athena manual: http://bruceravel.github.io/demeter/documents/Athena/plot/krange.html Scott Calvin discusses this in his book. There is more information about EXAFS data reduction among the tutorials at http://xafs.org/Tutorials. As Fred and Robert have said, you need "enough" data range to make a useful interpretation of the EXAFS. 200 eV is not a lot -- that will leave you with only a small handful of independent measurements in your data, possibly not even enough to reliably determine a first shell bond length. B -- Bruce Ravel ------------------------------------ bravel@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/
Alright thanks to all of you. I already knew that there is a big relationship between the Fourier transform k-range and spectral resolution, but we couldn't get more energy range in the lab and wanted to see if EXAFS analysis was trustworthy this way or not.
A last question then: in the project I'm attaching 1_data.prj (other data, where I know I don't have enough energy range, but I wanted to try it), I get a FT plot with two well resolved peaks at around the bond lengths of the graphite sample (1.42Å in the same layer and 3.35Å between layers). Then as there is not enough k-range, we cannot extract any conclusions on that? (I guess we can but without much confidence, because then with another background removal, 2_data.prj, we obtain something different). Sorry for all the emails but it seemed so confusing to me..
Thank you again a lot,
Irina.
________________________________
From: Ifeffit
Thank you for the quick answer. Which range should I have for the EXAFS analysis to be meaningful? Like 200eV above the edge?
I am always uncomfortable when newcomers ask for "rules". Having a rule without understanding when and why it might apply seems counter-productive. Firstly, lets discuss the relation ship between "energy above the edge" and photoelectron wave number. The conversion constant between energy and wavenumber is k = sqrt( ETOK * (E-E0) ) where ETOK is the appropriate combination of fundamental constants (2m_e / hbar^2, where m_e is the mass of the electron and hbar is Planck's constant) about equal to 1 / 3.81. So, 10 inverse Angstroms is about 381 volts above the edge. The extent of the signal in k determines how well you can interpret an EXAFS signal in a quantitative sense. See this page in the Athena manual: http://bruceravel.github.io/demeter/documents/Athena/plot/krange.html Scott Calvin discusses this in his book. There is more information about EXAFS data reduction among the tutorials at http://xafs.org/Tutorials. As Fred and Robert have said, you need "enough" data range to make a useful interpretation of the EXAFS. 200 eV is not a lot -- that will leave you with only a small handful of independent measurements in your data, possibly not even enough to reliably determine a first shell bond length. B -- Bruce Ravel ------------------------------------ bravel@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
participants (3)
-
Bruce Ravel -
fred.mosselmans＠diamond.ac.uk -
Irina Pi