# xaid_ff

## Module characteristics

- Goal: to calculate the Fourier Transform (FT) of a XAFS spectrum Chi(k) and the Back Fourier Transform (BFT) of a selected region of the FT. Chi(k) can be weighted with powers of k (from 0 to 4) and many weighting windows are available.
- Method: Calculate the Fast Fourier transform using the ftrset and fastftr functions
- Input: A 2 column file with Chi(k) data. It may also receive data from other XAID modules.
- Output: A SPEC-formatted data file with signal, FT and BFT (Use File->Save File).
- Tricks:
  - After changing any parameter, press "Enter" to refresh calculations.
  - The syntax of the intervals is: [min,max]. The square brackets and the separating coma are must always be entered.

## Example of use

1. Load a 2-colums ASCII file with input data (k in A in the first column and Chi(k) in the second one) by using the File->Load File menu. Use for example the file Ge_calib.chi in the Examples directory of the xaid distribution. This file is loaded using File->Load Example File
2. After loading the data, one gets:
3. Change Input parameters:
   - The k limits for the FT (in left window)
   - The R interval in central window (limits for the FT plot).
   - The R limits for the Back Fourier (in central window). The BFT is calculated from the FT within these limits in R. The selected limits appear as vertical lines in the FT window (central window). This helps to select more accurately the wanted limits, normally including one or several peaks in the FT.
   - The k limits for the congugated variable (BFT in the right window). Note that this interval must be smaller than the "k limits for the Fourier Transform" interval in the left window, otherwise some numerical inconsistences appear.
   - The "Shell radius rfor the back phase". This affects only the BFT phase. The BFT is written as A(k)*sin(2kr+Phi(k)). The phase is Phi(k)
4. Press Enter for calculating and updating the windows after entering any new parameter.

## Batch calculations

XAID_FF cannot read directly multiple data sets, but it can be done by calling XAID_FF from DELIA and EXODUS.

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