[Ifeffit] question about S02 (passive electron reduction factor)
Grant Bunker
bunker at biocat1.phys.iit.edu
Thu Feb 26 12:52:59 CST 2004
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 at millenia.cars.aps.anl.gov wrote:
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> Today's Topics:
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> 1. Re: question about S02 (passive electron reduction factor)
> (Scott Calvin)
> 2. Re: question about S02 (passive electron reduction factor)
> (Bruce Ravel)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Wed, 25 Feb 2004 13:18:29 -0500
> From: Scott Calvin <scalvin at slc.edu>
> Subject: Re: [Ifeffit] question about S02 (passive electron reduction
> factor)
> To: XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>
> Message-ID: <3.0.1.32.20040225131829.00acff58 at 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.
> >
>
>
> ------------------------------
>
> Message: 2
> Date: Thu, 26 Feb 2004 10:43:48 -0500
> From: Bruce Ravel <ravel at phys.washington.edu>
> Subject: Re: [Ifeffit] question about S02 (passive electron reduction
> factor)
> To: XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>
> Message-ID: <200402261043.48841.ravel at 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 at 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)
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> 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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