Dear all, I´m trying to deal with exafs data of a Ni-phosphate compound, using Athena/Artemis. This is a simple structure (I think), with the hexacoordinated ion surrounded by water molecules on the first shell. The question I have concerns the negative value for e0 I´m having. Is this a bad thing? Maybe I´m doing something wrong? I´ve tried some issues, as those proposed on the Shelly Kelly´s tutorial, but without success. Can you help me, please? Attached are an example of the Athena and Artemis projects i´m using. Thanks a lot. Mauricio
Dear all, I´m trying to deal with exafs data of a Ni-phosphate compound, using Athena/Artemis. This is a simple structure (I think), with the hexacoordinated ion surrounded by water molecules on the first shell. The question I have concerns the negative value for e0 I´m having. Is this a bad thing? Maybe I´m doing something wrong? I´ve tried some issues, as those proposed on the Shelly Kelly´s tutorial, but without success. Can you help me, please? Attached are an example of the Athena and Artemis projects i´m using. Thanks a lot. Mauricio
Hi Mauricio, I think you might not be understanding the purpose of the e0 shift in EXAFS analysis. When we process data, we choose an edge energy which sets the zero of wavenumber. When we make a Feff calculation, it somehow chooses its interstitial level, setting its zero of wavenumber. There is no reason to expect that these two wavenumber grids will be aligned. The purpose of the e0 shift in EXAFS analysis is to align the k-grid of the data with the k-grid of the theory. Just as you cannot expect those k-grids to be aligned a priori, you don't know which way they will be misaligned. It is just as likely that you will need a negative value of e0 as a positive one. As long as you have not made any mistakes processing the data (such as a very poor choice of the edge energy) or in making the theory (such as computing for the K edge when you actually measured the L3 edge), then the absolute value of the e0 parameter should be small -- a few volts or less. There is nothing obviously wrong with the e0 shift in the artemis project you attached -- it was only about -2 volts. HTH, B PS. Sometimes you see fits with more than one e0 parameter. That might be justified if you have some reason to think that one component of your fit is electronically dissimilar from the rest of your fit. Typically, we use a single e0 parameter which then has the interpretation I gave above. -- Bruce Ravel ----------------------------------- bravel@bnl.gov My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/
Hello Bruce, thanks so much for your kind response... Now i'm okay :-)
Cheers
mauricio
2007/10/9, Ravel, Bruce
Dear all, I´m trying to deal with exafs data of a Ni-phosphate compound, using Athena/Artemis. This is a simple structure (I think), with the hexacoordinated ion surrounded by water molecules on the first shell. The question I have concerns the negative value for e0 I´m having. Is this a bad thing? Maybe I´m doing something wrong? I´ve tried some issues, as those proposed on the Shelly Kelly´s tutorial, but without success. Can you help me, please? Attached are an example of the Athena and Artemis projects i´m using. Thanks a lot. Mauricio
Hi Mauricio,
I think you might not be understanding the purpose of the e0 shift in EXAFS analysis. When we process data, we choose an edge energy which sets the zero of wavenumber. When we make a Feff calculation, it somehow chooses its interstitial level, setting its zero of wavenumber. There is no reason to expect that these two wavenumber grids will be aligned. The purpose of the e0 shift in EXAFS analysis is to align the k-grid of the data with the k-grid of the theory. Just as you cannot expect those k-grids to be aligned a priori, you don't know which way they will be misaligned. It is just as likely that you will need a negative value of e0 as a positive one. As long as you have not made any mistakes processing the data (such as a very poor choice of the edge energy) or in making the theory (such as computing for the K edge when you actually measured the L3 edge), then the absolute value of the e0 parameter should be small -- a few volts or less.
There is nothing obviously wrong with the e0 shift in the artemis project you attached -- it was only about -2 volts.
HTH, B
PS. Sometimes you see fits with more than one e0 parameter. That might be justified if you have some reason to think that one component of your fit is electronically dissimilar from the rest of your fit. Typically, we use a single e0 parameter which then has the interpretation I gave above.
-- Bruce Ravel ----------------------------------- bravel@bnl.gov
My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/
_______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Dear all I´m trying to deal with exafs data of a Fe-Co alloy sample, using Artemis. The bcc structure is simple to modeling, Fe at 0,0,0 and Co at 1/2,1/2,1/2 (at Fe-edge, N1= 8Co, N2= 6Fe, theorically), But in my fits N1=5.4 and N2=4.9, that is correct? amp=0.811293 value I used from Iron calibration data, that is correct too? A possibility (I think), is that the sample is inhomogeneous, because, by Mossbauer spectroscopy, show two Fe magnetic sites, one attributed to ordered phase, and the other to disordered phase. Can you help me? Attached the Artemis projects for this fit (Fe-egde). Thanks a lot. Ch. ------------------------- Chachi Rojas Ayala GMC FCF UNMSM Claro:01-97489347 email:crojasa@unmsm.edu.pe
participants (3)
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Chachi Rojas -
mauricio.maps -
Ravel, Bruce