xaid_mu2chi
Module characteristics
- Goal: to extract the XAFS
signal Chi(k) from the absorption spectrum Mu(Energy)
- Method: Preedge linear
fit substraction, postedge multi-spline fit. Total control on fit limits
and knots, spline polynomial degree, jump and edge values. Three types
of normalization: experimental, constant and Eisenberg-Lengeler.
- Input: A single two column
file with Mu(Energy) data. It may also receive data from other
XAID modules (like DELIA and EXODUS). It can process
multiple files.
- Output: Export signal to
a spec file, or passed data to other the XAID
applications (XAID_FF,
EXODUS, XPLOT) module
for better visualization and further processing.
-
Tricks:
- After changing any parameter, press "Enter"
to refresh calculations.
Example of use
Quick start
Load the file Ge_calib.dat
in the Examples directory of the xaid installation
directory. Fot that, use File->Load
Data->Load Example File. A plot of the spectrum is obtained:

Click the
button. Default values of preedge limits
(Emin and Emax),
Eo, jump
and knot positions are filled.
The extraction is performed and the plot is changed to visualize Chi(k):

Play with the plot X value and Y value to visualize
intermediate steps of the calculation. Change the parameters.
Compute the Fourier Transform by exporting Chi(k)
to XAID_FF (using Tools->xaid_ff
(FT)), or save data using File-Export
to spec file
Step by step
- Load a 2-colums ASCII file
with input data (Energy
in the first column and Mu(Energy)
in the second one) by using the File->Load data->from file(s) menu.
- Preparation of the data (optional):
XAID_MU2CHI must use eV
as energy units. If the input file has another unit, fill the conversion
factor in the box: Abscissas factor to eV.
For example, set 1000 if your file is in keV.
You may use the Raw_Data->Cut
item to select the good interval for the analysis, or may be Raw_Data->Interpolation to change the
number of points. The option Raw_Data->Generic Operation
allows to redefine the input set by performing arithmetic operations.
- Pre-edge fitting Select
the Emin and Emax limits, either by entering
the values, or by clicking two points (using
)
- Selecting of Eo and Jump, perhaps with the help of
the
buttons
- Post edge spline This is
the more delicate part and the quality of your Chi(k) signal depends
on the ability to perform a good fit. Use The default option using
puts the knots equi-spacied
in the available k
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 options to better
visualize the fits.
-
Extraction Once you have a
satisfactory post-edge spline on the screen, a normalization is applied
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]
- End After the extraction you can save the result to
a file using File->Export
to spec file or inject the result in the Fourier filter application
XAID_FF. By default the negative k values
are kept, but it will be possible to cut your spectrum in another application
(XPLOT or EXODUS).
Preference parameters
The File->Preferences...
option starts a window with the application preferences:

The start and end
preedge points define the default limits when using
. The first one is the number of eV after
the first point of the spectrum. The second one is the number of eV below
the edge (maximum of the derivative).The postedge defaults are equi-spaced
knots in k, from Eo+(end
preedge) to the maximum k available.
Then the K limits for the plot and the Fourier Transform parameters
are defined. The Fourier Transform is plotted when the plot X
value is set to R.
The last option is important for batch calculations, as it will allow
automatic computation of the default
Emin, Emax, Eo, Jump and knot
positions after loading each individual spectral data.
Batch calculations
XAID_MU2CHI can perform extractions for multiple data:
Note that after loading individual data, the extraction parameters are
set to the default if selected in the Preferences.
Otherwise are kept unchanged prom the previous spectrum.