Chris, You'll answer this question when you post a project file; but since you mention the pre-edge feature, have you reset the Eo to approx. halfway up the absorption edge instead of on the pre-edge feature itself? -Richard On Tue, Aug 25, 2009 at 11:10 AM, Carlo Segre wrote:

Hi Chris:

If you could attach an Athena project file, it would help us to answer your question.

Carlo

On Tue, 25 Aug 2009, Chris Patridge wrote:

...

Hello all,

After calibration, alignment and merging V K edge data, there seems to be a large peak in R space below 1 A. From most examples and readings, this would seem to be noise since atoms do not reside that close to each other. Adding a background addresses this noise but then the number of variables becomes too large and meaningless to the first shell R space range. I believe the problem lies with the large pre edge feature seen with high valence state Vanadium compounds. Any suggestions on how to work through this and make realistic EXAFS analysis on these compounds.

Thanks everyone,

-- Carlo U. Segre -- Professor of Physics Associate Dean for Graduate Admissions, Graduate College Illinois Institute of Technology Voice: 312.567.3498 Fax: 312.567.3494 segre@iit.edu http://www.iit.edu/~segre segre@debian.org _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit

Hi Chris: I have taken a look at your project file and, indeed, the edge setting is more or less correct. In this case, the intensity below 1A is certainly background and you can see this by plotting the data in R-space with k-weight of 2 or 3 and then slowly increase the Rbkg (I find that 0.02 is a good increment). You will see that there is not much change until you reach Rbkg=1.08 and then the intensity below 1A basically all disappears without affecting the peak sizes or positions of everything above 1A. This indicates to me that it is really a background problem. If you now plot the XMU and the background you will see that the red background line is going through the data much more reasonably. There are certainly other methods to remove this background peak. Everyone has their own methods but I am pretty confident that the result will be the same. an Rbkg of 1.08 is not bad at all. Cheers, Carlo -- Carlo U. Segre -- Professor of Physics Associate Dean for Graduate Admissions, Graduate College Illinois Institute of Technology Voice: 312.567.3498 Fax: 312.567.3494 segre@iit.edu http://www.iit.edu/~segre segre@debian.org

Bruce Ravel wrote:

On Tuesday 25 August 2009 11:02:26 am Chris Patridge wrote:

...

Hello all,

After calibration, alignment and merging V K edge data, there seems to be a large peak in R space below 1 A. From most examples and readings, this would seem to be noise since atoms do not reside that close to each other. Adding a background addresses this noise but then the number of variables becomes too large and meaningless to the first shell R space range. I believe the problem lies with the large pre edge feature seen with high valence state Vanadium compounds. Any suggestions on how to work through this and make realistic EXAFS analysis on these compounds.

Thanks everyone,

Didn't Richard Mayes answer this same question from you last month?

http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2009-July/004365.html

In my experience, Richard's answer was spot on for V K edge data.

If Richard's good advice didn't help, then you need to be more explicit about what the problem is, perhaps by posting a small athena project file that demonstrates the problem.

B

Yes it is a very similar question. I did make this type of correction by setting E0 myself. When moving to R space the data shows significant intensity below 1 A which I thought was noise but it may not be. Are these low A signals data or noise is my main concern because EXAFS fitting does not fit all that well which then would point me to my model as the problem which I think i understand. Thank you all for your patience, -- Christopher J. Patridge Graduate Student SUNY Buffalo - Dept of Chemistry 716-645-6800 x 2110 315-529-0501