# 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

1. 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.
2. 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.
3. Selecting of Eo: using Operation1->Get Eo
4. 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
5. Jump calculation using Operation1->Jump
6. Display Mu(k) using the buttons in the lower part of the window
7. 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.
8. 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.
9. 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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