Hello everyone, In removing background, most literature suggests Rbkg value of 1.0 because below this represents mostly noise and low freq components not part of the scattering effect. In using Feff and viewing the individual paths calculated a number of them have paths near 1.5 A, therefore meaning that they have some contributions very close to 1.0 A and below in R space due to phase shift. Has anyone modeled materials which contain these rather short Reff and how did you decide what was scattering and noise? Thank you all, Chris Patridge UB Graduate Student Department of Chemistry Cell: 315-529-0501
Chris,
If you look at Shelly Kelly's tutorial on xafs.org, she suggests Rbkg be
"about half the R value for the first peak." This setting worked well in my
case with vanadyl-silicates where my first feature was around 1.6 A. Her
tutorial also discusses setting Rbkg and what to look for when doing so.
-Richard
On Fri, Oct 23, 2009 at 3:51 PM, Chris Patridge
Hello everyone,
In removing background, most literature suggests Rbkg value of 1.0 because below this represents mostly noise and low freq components not part of the scattering effect. In using Feff and viewing the individual paths calculated a number of them have paths near 1.5 A, therefore meaning that they have some contributions very close to 1.0 A and below in R space due to phase shift. Has anyone modeled materials which contain these rather short Reff and how did you decide what was scattering and noise?
Thank you all,
Chris Patridge
UB Graduate Student
Department of Chemistry
Cell: 315-529-0501
_______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Hi Chris, One thing to keep in mind is that it is not wrong to eliminate part of your signal; it just means you're losing a little bit of data. This is similar to a common filtering mistake that beginners make when trying to choose the maximum end of their fitting range: they look at the paths they are including, and try to set Rmax high enough to include most of the contribution from the paths they are including. What they should be doing, however, is to look at the paths they are not including, and set Rmax low enough so that the contributions from those paths are tolerably small. Looking at correlations between background parameters and fitted parameters when the "fit background" option is selected also helps provide you with information. I'll admit that the "fit background" button confuses me a bit, though (I always have to spend ten minutes convincing myself again as to what exactly it is doing), so someone else should explain how that can be used to help address your question. --Scott Calvin Sarah Lawrence College On Oct 23, 2009, at 3:51 PM, Chris Patridge wrote:
Hello everyone,
In removing background, most literature suggests Rbkg value of 1.0 because below this represents mostly noise and low freq components not part of the scattering effect. In using Feff and viewing the individual paths calculated a number of them have paths near 1.5 A, therefore meaning that they have some contributions very close to 1.0 A and below in R space due to phase shift. Has anyone modeled materials which contain these rather short Reff and how did you decide what was scattering and noise?
Thank you all,
Hi Chris, There is nothing sacred about Rbkg=1.0 Ang. It was chosen as a reasonable default. If you know you have atoms much closer than 2Ang, lowering Rbkg to half the first neighbor distance is a fine place to start. You may have to play with this value (and the k-weight used in the background) a bit more than otherwise, but it should work out. Scott wrote:
... I'll admit that the "fit background" button confuses me a bit, though (I always have to spend ten minutes convincing myself again as to what exactly it is doing), so someone else should explain how that can be used to help address your question.
In Artemis, when you check "refine the background", the following things happen:
1. Your R-space fitting range [Rmin,Rmax] becomes [0,Rmax]
2. A spline is added to the model chi(k): model = spline(k) +
sum_of_paths(chi(k))
3. The spline is chosen with breakpoints equally spaced in k, using
Rmin as Rbkg to select the number of breakpoints.
--Matt
On Fri, Oct 23, 2009 at 3:24 PM, Scott Calvin
Hi Chris, One thing to keep in mind is that it is not wrong to eliminate part of your signal; it just means you're losing a little bit of data. This is similar to a common filtering mistake that beginners make when trying to choose the maximum end of their fitting range: they look at the paths they are including, and try to set Rmax high enough to include most of the contribution from the paths they are including. What they should be doing, however, is to look at the paths they are not including, and set Rmax low enough so that the contributions from those paths are tolerably small. Looking at correlations between background parameters and fitted parameters when the "fit background" option is selected also helps provide you with information. I --Scott Calvin Sarah Lawrence College On Oct 23, 2009, at 3:51 PM, Chris Patridge wrote:
Hello everyone,
In removing background, most literature suggests Rbkg value of 1.0 because below this represents mostly noise and low freq components not part of the scattering effect. In using Feff and viewing the individual paths calculated a number of them have paths near 1.5 A, therefore meaning that they have some contributions very close to 1.0 A and below in R space due to phase shift. Has anyone modeled materials which contain these rather short Reff and how did you decide what was scattering and noise?
Thank you all,
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participants (4)
-
Chris Patridge -
Matt Newville -
Richard Mayes -
Scott Calvin