distinguishing between actual shells and artifacts
Dear all, I am currently fitting first and second shells with Zn-O and Zn-Zn paths,respectively. I get second shell features that could be fit quite well, but I am not certain if they are actually second shell features or possibly artifacts. For some samples I get second shell features that I know should not be present. What criteria would you use to distinguish between artifacts and actual shells? Your assistance is appreciated. Hashem Stietiya Louisiana State University
Hi Hashem, I doubt they're "artifacts." Much more likely is that they are either due to multiple-scattering paths, or to sidebands of the single- scatterers. During analysis, you don't want to think of the Fourier transform as being literally a radial distribution function; i.e., features in the Fourier transform don't have a one to one correspondence with coordination shells. --Scott On Jan 14, 2009, at 8:11 AM, mstiet2@lsu.edu wrote:
Dear all,
I am currently fitting first and second shells with Zn-O and Zn-Zn paths,respectively. I get second shell features that could be fit quite well, but I am not certain if they are actually second shell features or possibly artifacts. For some samples I get second shell features that I know should not be present. What criteria would you use to distinguish between artifacts and actual shells?
Your assistance is appreciated.
Hashem Stietiya
Louisiana State University
Hi Hashem You can copy-paste the respective EXAFS data in Athena and compare the backtransforms over the first shell, the first+second shell, the second shell. By comparison of the backtransforms with each other and with the measured EXAFS, you can see where the second-shell contribution has its origin in k-space (note that the backtransform is not corrected for the window function). For instance, if the second-shell signal adds to the spectrum in the low-k range (where noise should not be a problem), it is likely real and might result, e.g., from multiple scattering among first-shell O atoms. If the second-shell signal in the backtransform mainly contributes in the high-k range (where noise or glitches may be a problem), you may be able to see whether it falls together with obvious artefacts (such as glitches), or whether the backtransform matches spectral features that you consider to be real. Andreas ----------------------------------------------- Andreas Voegelin, PhD Soil Chemistry Group Inst. of Biogeochemistry and Pollutant Dynamics Dept. of Environmental Sciences ETH Zurich Universitätstrasse 16, CHN F24 CH-8092 Zürich, Switzerland Phone: +41 44 633 61 47 Fax: +41 44 633 11 18 E-Mail: voegelin@env.ethz.ch www: http://www.ibp.ethz.ch/people/andreavo
-----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of mstiet2@lsu.edu Sent: Mittwoch, 14. Januar 2009 14:12 To: ifeffit@millenia.cars.aps.anl.gov Subject: [Ifeffit] distinguishing between actual shells and artifacts
Dear all,
I am currently fitting first and second shells with Zn-O and Zn-Zn paths,respectively. I get second shell features that could be fit quite well, but I am not certain if they are actually second shell features or possibly artifacts. For some samples I get second shell features that I know should not be present. What criteria would you use to distinguish between artifacts and actual shells?
Your assistance is appreciated.
Hashem Stietiya
Louisiana State University
Hi Hashem, I show one method for doing this in my book chapter Figure 14-23 on page 431. (I feel a bit like a broken record). I will send you a copy if you want. Shelly
-----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit- bounces@millenia.cars.aps.anl.gov] On Behalf Of Voegelin Andreas Sent: Wednesday, January 14, 2009 9:57 AM To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] distinguishing between actual shells and artifacts
Hi Hashem
You can copy-paste the respective EXAFS data in Athena and compare the backtransforms over the first shell, the first+second shell, the second shell.
By comparison of the backtransforms with each other and with the measured EXAFS, you can see where the second-shell contribution has its origin in k-space (note that the backtransform is not corrected for the window function).
For instance, if the second-shell signal adds to the spectrum in the low-k range (where noise should not be a problem), it is likely real and might result, e.g., from multiple scattering among first-shell O atoms. If the second-shell signal in the backtransform mainly contributes in the high-k range (where noise or glitches may be a problem), you may be able to see whether it falls together with obvious artefacts (such as glitches), or whether the backtransform matches spectral features that you consider to be real.
Andreas
----------------------------------------------- Andreas Voegelin, PhD Soil Chemistry Group Inst. of Biogeochemistry and Pollutant Dynamics Dept. of Environmental Sciences ETH Zurich Universitätstrasse 16, CHN F24 CH-8092 Zürich, Switzerland Phone: +41 44 633 61 47 Fax: +41 44 633 11 18 E-Mail: voegelin@env.ethz.ch www: http://www.ibp.ethz.ch/people/andreavo
-----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of mstiet2@lsu.edu Sent: Mittwoch, 14. Januar 2009 14:12 To: ifeffit@millenia.cars.aps.anl.gov Subject: [Ifeffit] distinguishing between actual shells and artifacts
Dear all,
I am currently fitting first and second shells with Zn-O and Zn-Zn paths,respectively. I get second shell features that could be fit quite well, but I am not certain if they are actually second shell features or possibly artifacts. For some samples I get second shell features that I know should not be present. What criteria would you use to distinguish between artifacts and actual shells?
Your assistance is appreciated.
Hashem Stietiya
Louisiana State University
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participants (4)
-
Kelly, Shelly -
mstiet2＠lsu.edu -
Scott Calvin -
Voegelin Andreas