Module XAID_Mu2Chi
Module characteristics
- Goal: to obtaing the XAFS signam Chi(k) starting from
the absorption signal Mu(Energy)
- Input: A 2 column file with Mu(Energy) data. It may also receive
data from other XAID modules.
- Output: To a 2-column ASCII file, or passed to the
XAID_FF module, or passed to
XPLOT module for better visualization and
further processing, or a combination of them.
- Tricks:
- Zoom at any time by selecting a box in the window.
- You may use View->XPlot at any time to get more details on your
data.
- It is recommended to display Mu in k scale
before starting the post-edge fitting, although this is not stricktly
necessary.
Follow the steps
- Load a 2-colums ASCII file with input data (Energy in eV in the
first column and Mu(Energy) in the secondone) by using the
File->LoadFile menu. Use for example the file:
Ge_calib.dat.
- Preparation of the data: you may use the
Operation1->Cut item to select the good interval for the
analysis, or may be Operation1->Interpolation to change the
number of points.
- Selecting of Eo: using Operation1->Get Eo
- Pre-edge fitting Usually done by fitting the pre-edge with a
straight line. Use Operation1->Fit Polynomial->Least
Squares
option for that. If the results of the fir is satisfactory, then
substract the Fit from the Mu(Energy) signal using
Operation2->Substraction
- Jump calculation using Operation1->Jump
- Display Mu(k) using the buttons in the lower part of the
window
- Post edge spline This is the more delicate part and the quality
of your Chi(k) signal depens on your ability to perform this fit.
It is more an art than a science. Use Operation1->Fit
Multispline option. The default option may work, but, select the
minimum of the fit interval around 2 or 3 Angstroms. By default, it
presents the full interval. A technical trick: negative values are
presented (although they do not exist physically). This is to obtain
smoother fits in the case of selecting the full interval). You may use
the zoom option (just by selecting the box to be zoomed). Zoom back is
done using View->Refresh or just clicking on the window. If you
do not like this spline, you can perform another one by reselecting
Operation1->Fit Multispline. If you want to carefully study
your data and the spline, it is recommended to use
View->XPlot
option. See an example window.
- Extraction Once you have a satisfactory post-edge spline on
the screen, go to Operation2->Extraction to obtain the
Chi(k) signal. Three options are available, that apply different
extraction algorithms:
- Experimental: {chi(E)=(mu(E)-mu_at(E))/(mu_at(E)-mu_bck(E))
}
- Constant: {chi(E)=(mu(E)-mu_at(E))/jump}
- Lengeler-Eisenberg:
{chi(E)=(mu(E)-mu_at(E))/jump*LE(E)}
with LE(E) = 1- (8/3) [(E-Eo)/Eo]
After the extraction you are asked to either substitute or go to
XPlot for a better look. Click substitute if you are sure
you do not want to go back to post-edge spline.
- End After the extraction you can save the result to a file
using File->Save File or inject the result in the Fourier filter
application XAID_FF using
Operation1->Fourier Filter. In most cases, specially when you
have negative k values, it is recommended to cut your spectrum
to a useful interval before saving.
FAQs
- Can I load a scan from a SPEC file in XAID_MU2CHI?
Not directly. You can however open XPlot (e.g.,
Tools->XPlot, load your SPEC data file in XPlot, and start
XAID_MU2CHI with your selected scan from the XPlot
Calculations->XAFS->Xafs Extraction (xaid_mu2chi)
- Do I need to follow literally the given steps?
No. But you need some logic in your analysis: in a widget
application you risk to get any bad result or even crash the program
because the combination of different possible actions is close to
infinity and the author does not have the time to check all of them.
- What do the strange labels "Operation 1" and "Operation 2" in the
menu bar mean?
"Operation 1" refers to operations with a single data set (i.e.,
when you see only a line in the graphical window). "Operation 2" refers
to operations between two data sets (two lines displayed).
Window example
This is the window (zoomed) at the step 7 using the example data file and
performing the post-edge spline starting at 3 A.
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