Re: [Ifeffit] question about S02 (passive electron reduction factor)

As a follow-on to Scott's and Bruce's responses, if the samples were measured in fluorescence, the sample self-absorption effects would be a likely explanation for the problem. At the Vanadium edge the absorption length of the sample would probably be short (microns) and you would have to go to some trouble to avoid those problems. There are good correction procedures for that problem (e.g. see papers by Booth and Bridges, Haskell...). grant bunker On Thu, 26 Feb 2004 ifeffit-request@millenia.cars.aps.anl.gov wrote:

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Today's Topics:

1. Re: question about S02 (passive electron reduction factor) (Scott Calvin) 2. Re: question about S02 (passive electron reduction factor) (Bruce Ravel)

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Message: 1 Date: Wed, 25 Feb 2004 13:18:29 -0500 From: Scott Calvin Subject: Re: [Ifeffit] question about S02 (passive electron reduction factor) To: XAFS Analysis using Ifeffit Message-ID: <3.0.1.32.20040225131829.00acff58@mail.slc.edu> Content-Type: text/plain; charset="us-ascii"

Hi Yu-Chuan,

There are several possibilities. One is sample prep. If you have an extremely uneven sample (lots of pinholes, etc.) in transmission it can lead to results like you're describing. Another is normalization. If you're using Athena for background subtraction make sure you look at the pre-edge line and post-edge curve and see that they look reasonable. Sometimes if the background has a funny shape to it Athena can create a post-edge curve that shoots way up at the edge energy; this could also lead to the effect you're describing. Finally, look at the correlation between S02 and sigma2...if it is very high (say 0.95 or above) it may simply be coming up with low estimates for each. There are, of course, other possibilities as well, but in my experience those are the most common.

--Scott Calvin Sarah Lawrence College

...

This is Yu-Chuan. I am now trying to fit my sample, magnesium orthovanadate. I try to follow Bruce's Cu example and Scott's ZnO example steps and I found a problem come to my fitting. At the beginning of the fitting, the S02(amp) are far away from 0.9 (just about 0.3). It's much different than Bruce's and Scott's examples. For Cu and ZnO examples, the fitting results of amp are close to 0.9 at the beginning even if you set just four parameters (guess:amp, e0, delr, and ss). Also, John have said that the S02 value should be around 0.9. That's why I am wondering if there is anything wrong with my data processing? Does anyone happen to have this kind of problem? Thank you for your help.

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Message: 2 Date: Thu, 26 Feb 2004 10:43:48 -0500 From: Bruce Ravel Subject: Re: [Ifeffit] question about S02 (passive electron reduction factor) To: XAFS Analysis using Ifeffit Message-ID: <200402261043.48841.ravel@phys.washington.edu> Content-Type: text/plain; charset="iso-8859-1"

Hi,

Yesterday we had a question about oddly small S02 values. In his thorough summary of possible explanations, Scott mentioned this:

On Wednesday 25 February 2004 01:18 pm, Scott Calvin wrote:

...

Another is normalization. If you're using Athena for background subtraction make sure you look at the pre-edge line and post-edge curve and see that they look reasonable. Sometimes if the background has a funny shape to it Athena can create a post-edge curve that shoots way up at the edge energy; this could also lead to the effect you're describing.

I thought that a word of explanation about why Athena does what she does might be helpful to folks. In this email I am going to refer to data taken on an iron foil. To follow along, fire up Athena and select "Import a demo project" from the Help menu. Select the `calibrate.prj' file. This contains an iron foil spectrum.

Set the plotting range in energy to [-200:1500]. Set the upper bound of the normalization range to 400. Click on the "post-edge line" button in the plotting options section. This will plot the data, the background, and the post-edge line. Note that the post-edge line is U-saped, diverging from the data a bit at low energy and significantly at high energy. Note also that the edge step is about 3.05.

Now set the upper bound of the normalization range to 1700 and click the red E plotting button. Now the post-edge line goes through the data and the edge step is about 2.86.

So what's going on? Well, the post-edge line is determined by regressing a quadratic polynomial to the data in the user-specified normalization range. When the upper bound was set to 400, the square term in the polynomial was quite large because *that* was the regression to the data over that short data range. When the upper bound was set to 1700, the square term was a lot smaller, the post-edge line was closer to linear and it was constrained by the regression to follow the data over the entire data range.

Setting the upper bound of the normalization range had the effect of increasing the edge step by about 6%. Remember that the edge step is the value of the post-edge line extrapolated back to E0 after the pre-edge line is removed from the data. With the short energy range, the edge step is too large and the chi(k) is attenuated. In the subsequent fit, S02 will have to be similarly smaller to compensate. In this case it was a 6% effect, but if the post-edge line is really screwy, the attenuation could be much larger.

So this was the long-winded way of repeating what Scott said -- plot the post-edge line and make sure the regression was done such that the post-edge line goes through the entire data in a way that seems sensible. It is also, as Scott said, prudent to check the pre-edge line as well.

B

-- ********* PLEASE NOTE MY NEW PHONE, FAX, & ROOM NUMBERS ******************

Bruce Ravel ----------------------------------- ravel@phys.washington.edu Code 6134, Building 3, Room 405 Naval Research Laboratory phone: (1) 202 767 2268 Washington DC 20375, USA fax: (1) 202 767 4642

NRL Synchrotron Radiation Consortium (NRL-SRC) Beamlines X11a, X11b, X23b National Synchrotron Light Source Brookhaven National Laboratory, Upton, NY 11973

My homepage: http://feff.phys.washington.edu/~ravel EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/

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