Hello Users, I was looking for an opinion about the chi(k) signal in a set of data I am analyzing. Brief background, this is a set of in-situ XAS data collected at the Fe K edge from a working electrochemical cell at a range of potentials during charge; I did not collect the data. I suspect the feature at ~ 8 angstroms-1, although present in all the spectra is noise or glitch and wondered if I am being overly cautious? My conservative range ( k = 2-7 and R = 1-2) really constrains the model Nidp = 3.31. Luckily, multiple datasets ( 8 ) to the rescue to give me some flexibility. In a multiple dataset fitting, is the R-factor of the whole set just the average or total mismatch across all the datasets or it calculated another way? Working towards happiness, Chris Patridge -- ******************************** Christopher J. Patridge, PhD NRC Post Doctoral Research Associate Naval Research Laboratory Washington, DC 20375 Cell: 315-529-0501
Hi Chris, I don't see a reason to think that data is a glitch. For one thing, it's not consistent across datasets. The features also look smooth, and not so glitch-like. The spike around 8.2 inverse angstroms in some of the datasets looks a bit more like a glitch, but it's fairly modest and narrow enough not to mess you up too much. The spacing of those features look OK--there's a double feature in some of the datasets around 6-7 inverse angstroms; it's plausible there would be another reature like that above it. In fact, I can make an argument that there's some kind of beating going on that gives a shoulder at 3.5-5, a double peak at 5-7, and two peaks at 7-8 inverse angstroms. So I would recommend including that data and seeing what it does to your fits. If that range is garbage, your fits will probably reject it. As for your second question, R-factors are always a kind of average across the data, by definition. So "total" mismatch doesn't really make sense. Off-hand, though, I don't recall how ifeffit weights the data for the purposes of calculating R-factors for multiple datasets, and that may be your question. --Scott Calvin Sarah Lawrence College On Jan 15, 2013, at 9:21 AM, Christopher Patridge wrote:
Hello Users,
I was looking for an opinion about the chi(k) signal in a set of data I am analyzing. Brief background, this is a set of in-situ XAS data collected at the Fe K edge from a working electrochemical cell at a range of potentials during charge; I did not collect the data. I suspect the feature at ~ 8 angstroms-1, although present in all the spectra is noise or glitch and wondered if I am being overly cautious?
My conservative range ( k = 2-7 and R = 1-2) really constrains the model Nidp = 3.31. Luckily, multiple datasets ( 8 ) to the rescue to give me some flexibility. In a multiple dataset fitting, is the R-factor of the whole set just the average or total mismatch across all the datasets or it calculated another way?
Working towards happiness,
Chris Patridge
-- ******************************** Christopher J. Patridge, PhD NRC Post Doctoral Research Associate Naval Research Laboratory Washington, DC 20375 Cell: 315-529-0501
_______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Thank you Scott, I guess that is a refinement of my question concerning R-factor.' Chris ******************************** Christopher J. Patridge, PhD NRC Post Doctoral Research Associate Naval Research Laboratory Washington, DC 20375 Cell: 315-529-0501 On 1/15/2013 9:39 AM, Scott Calvin wrote:
Hi Chris,
I don't see a reason to think that data is a glitch. For one thing, it's not consistent across datasets. The features also look smooth, and not so glitch-like. The spike around 8.2 inverse angstroms in some of the datasets looks a bit more like a glitch, but it's fairly modest and narrow enough not to mess you up too much.
The spacing of those features look OK--there's a double feature in some of the datasets around 6-7 inverse angstroms; it's plausible there would be another reature like that above it. In fact, I can make an argument that there's some kind of beating going on that gives a shoulder at 3.5-5, a double peak at 5-7, and two peaks at 7-8 inverse angstroms.
So I would recommend including that data and seeing what it does to your fits. If that range is garbage, your fits will probably reject it.
As for your second question, R-factors are always a kind of average across the data, by definition. So "total" mismatch doesn't really make sense. Off-hand, though, I don't recall how ifeffit weights the data for the purposes of calculating R-factors for multiple datasets, and that may be your question.
--Scott Calvin Sarah Lawrence College
On Jan 15, 2013, at 9:21 AM, Christopher Patridge wrote:
Hello Users,
I was looking for an opinion about the chi(k) signal in a set of data I am analyzing. Brief background, this is a set of in-situ XAS data collected at the Fe K edge from a working electrochemical cell at a range of potentials during charge; I did not collect the data. I suspect the feature at ~ 8 angstroms-1, although present in all the spectra is noise or glitch and wondered if I am being overly cautious?
My conservative range ( k = 2-7 and R = 1-2) really constrains the model Nidp = 3.31. Luckily, multiple datasets ( 8 ) to the rescue to give me some flexibility. In a multiple dataset fitting, is the R-factor of the whole set just the average or total mismatch across all the datasets or it calculated another way?
Working towards happiness,
Chris Patridge
-- ******************************** Christopher J. Patridge, PhD NRC Post Doctoral Research Associate Naval Research Laboratory Washington, DC 20375 Cell: 315-529-0501
_______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Chris, That double peak around 8 is normal for Fe (oxyhydr)oxides; see the attached for an example. Also see Toner et al. 2009 (GCA 73 p 388, Fig 6) for a nice discussion of what structures can make it show up. I think perhaps you are seeing an oxidation state shift too. Best of luck with the happiness! -Leslie -----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of Christopher Patridge Sent: Tuesday, January 15, 2013 6:22 AM To: XAFS Analysis using Ifeffit Subject: [Ifeffit] k-range question & R-factor Hello Users, I was looking for an opinion about the chi(k) signal in a set of data I am analyzing. Brief background, this is a set of in-situ XAS data collected at the Fe K edge from a working electrochemical cell at a range of potentials during charge; I did not collect the data. I suspect the feature at ~ 8 angstroms-1, although present in all the spectra is noise or glitch and wondered if I am being overly cautious? My conservative range ( k = 2-7 and R = 1-2) really constrains the model Nidp = 3.31. Luckily, multiple datasets ( 8 ) to the rescue to give me some flexibility. In a multiple dataset fitting, is the R-factor of the whole set just the average or total mismatch across all the datasets or it calculated another way? Working towards happiness, Chris Patridge -- ******************************** Christopher J. Patridge, PhD NRC Post Doctoral Research Associate Naval Research Laboratory Washington, DC 20375 Cell: 315-529-0501 ** Leslie L. Baker, Ph.D. Department of Plant, Soil and Entomological Sciences and Department of Geological Sciences University of Idaho 875 Perimeter Drive MS 2339 Moscow, ID 83844-2339 208-885-9239 http://scholar.google.com/citations?user=ejpH5p0AAAAJ http://scholar.google.com/citations?user=ejpH5p0AAAAJ
participants (3)
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Baker, Leslie -
Christopher Patridge -
Scott Calvin