[Ifeffit] Re: Athena Suggestions

[Bruce Ravel]

Dave,

I hope you do not mind that I am answering via the mailing list.  The
answer I have planned will be something that I think others will find
useful.


On Saturday 20 March 2004 03:30 pm, Barton, David (DG) wrote:
> LOCATION OF Eo
>   -Attached is two Ga XAFS spectra in an Athena file project.  Look
> closely at the chosen Eo positions, I think you would agree that the
> choice is not near the maximum of the first derivative.  These data
> sets both have a fairly reasonable signal/noise so that shouldn't be
> the source of the error.
>
> Athena Report of Eo:
> 	--Ga_035 (10372.6 eV) and Ga_32-8 (10374.1 eV)
>       ==Eo difference = 1.5 eV
> Using another plotting program:
> 	--Ga_035 (10373.3 eV) and Ga_32-8 (10373.5 eV)
>       ==Eo difference = 0.2 eV
>
> I understand that using the maximum in the first derivative is just
> an approximation of the Fermi Energy so the absolute value may not
> be so meaningful.  However, we often use relative changes in the
> position of Eo as an indication of structural changes in our
> catalytic materials, so it would be helpful if Ifeffit was slightly
> more robust and precise in choosing the Eo value.  Maybe you could
> consider incorporating a "smoothing" parameter which would help
> reduce the noise in the first derivative or some other algorithm
> which makes a better choice in the presence of some noise in the
> data.

For the sake of everyone else, I'll mention that the data Dave sent me
had a slightly ambiguous e0 because the peaks in the first derivative
were split into two peaks about the same height and about 1/2 volt
apart.  In one case, Ifeffit's algorithm for identifying the peak came
up with a point a bit to the left of the two peaks and the other time
a bit to the right.  In neither case did Ifeffit (and hence Athena)
pick one of the points at the peak.  These choices to the left and
right of the peaks result in the unphysically large energy separation
that Dave is asking about.

My initial response to questions like this is that it is rarely wise
to blindly trust a program -- ours or anyone else's -- to do the right
thing all the time.  Just because we get one spectrum right or someone
else gets another spectrum right is not sufficient reason to always
trust any program.  If you need it to be right, check and make sure it
is right.

That was fine advice -- possibly even tautological.  But it wasn't
very practical.  The software really should work hard to get it right
as often as possible.  You suggest smoothing.  That would probably
work pretty well in this case.  Indeed, it woul probably result in
choosing a value between the two peaks.  That's closer to correct and
would probably at least be more reproducible between spectra than what
happened with your data.  But it would not be a panacea.  Look at Zr
0+ some time.  The maximum in the first derivative is more than 10
volts above the Fermi energy.  Smoothing would not fix the "maximum in
the first derivative" algorithm in that situation.  So even with a
more robust algorithm, you would be well served to be sceptical.

I think the solution to your problem is to be a bit more hands-on and
to make use of more of Athena's features.  I typically use Athena's
preprocessing features to handle ensembles of related data.  The
details of what you might do vary from situation to situation, but
here is the outline:

1.  Import one scan.  This is going to be your "standard" against
    which the rest will be compared, so it should be a trustworthy
    one.  Calibrate it by hand using the Calibration option in the
    Data menu.  Play around with the background removal and FT
    parameters until you are reasonably happy with the data
    processing.

2.  Use the "Open many files" option in the Files menu to import
    several more of the data files in that ensemble of related data
    files.

3.  When the column selection dialog comes up, make sure that the
    columns are properly selected.  Before hitting the OK button, hit
    the button that says "Set preprocessing parameters".  This will
    expand the column selection dialog, showing a new menu and bunch
    of checkbuttons.

4.  From the menu labeled "Standard" select the first data file that
    you read in.  This will activate all the checkbuttons below.  Now
    select the buttons for "Align to the standard" and "Set parameters
    to the standard".

5.  Now click the OK button.  As each data file is imported, it will
    be aligned and the parameters, including e0, the background
    parameters, and the FT parameters, will all be set to the values
    you settled on for the first data set.  Magic!!

If all went well, then each data set will be calibrated the same way
as the first one and the background and FT parameters will be well
set.  The caveat is that the automated alignment does not always work
well.  A quick plot will let you know if it failed and you need to go
back and align by hand.  But with data that is low noise and similar,
it tends to work ok.

Depending on your data set and what you are aiming to do, you may need
to do some variation of what I described above.  Of course, a change
in Ifeffit's algorithm might still be in order.  But, in any case, I
encourage you to resolve your e0 problems by explicitly examining one
data file and using preprocessing to make the rest consistent with the
first.

HTH,
B


-- 
 Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
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 Naval Research Laboratory                          phone: (1) 202 767 2268
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