EXAFS123 Overview
EXAFS123 is an IGOR Pro
Program that can be used to data-average and analyze the XANES and EXAFS
regions of K-shell X-ray absorption spectra.
There are two parts of the
program EXAFS123:
a. The LX
Panel: Generates XAS (X-ray Absorption Spectrum) from raw, experimental data.
b. The EXAFS Panel: analyzes the XAS spectrum generated by the LX Panel.
Features
As
you follow the steps outlined in the Tutorial, you'll see how EXAFS123 lets you
analyze your data differently and hence more accurately than many other EXAFS
Data Analysis Programs.
XAS Data Workup
c. Designed to read X-ray Absorption scan files
collected at NSLS.
d. You can examine data from a single scan or a
single detector at a time in order to find glitches. This helps you pinpoint
the faulty scans and detectors and remove them from averaging. You
can also remove data points from specific energies (for instance when there is
a glitch due to secondary diffraction from the monochromator).
e.
If
you are using a solid state fluorescence detector and collected the ungated
(ICR) as well as energy-gated fluorescence signal, you can use the ICR to
correct the fluorescence data for saturation effects using a feature called deadtime correction.
f.
You
can use what you know about the composition of the sample (and windows and air
path between the I0 detector and the sample), along with the experimental transmission spectrum, to correct the
fluorescence data for thickness effects.
g.
You
can use in-line calibration data collected simultaneously with the sample XAS
data to calibrate the monochromator energy and correct the energy in the
XAS. If the monochromator is unstable, you can even shift the energy from
successive scans by different amounts. This feature is called de-drift correction.
h.
The
XAS data can be corrected for the theoretical fall-off of the K-shell
absorption coefficient above the edge. By doing this, the baselines below
and above the edge become parallel curves.
Baseline and XANES
analysis
1.
The
baseline is fit as a polynomial (cubic) spline function, but the same function
is used above and below the edge (with, of course, a step-up at the edge).
2.
The
edge itself can be modeled as the integral of a Lorenzian
peak, integral of a Gaussian peak, or some combination of these (75% Gaussian
works well).
3.
Pre-edge
peaks can be modeled as Gaussian or Lorenzian (or combination)
functions, with position, width, and either area or height refined.
4.
When
there are multiple pre-edge peaks, they can be assigned to have the same width
(using "Restraints").
EXAFS Analysis
1.
Uses
amplitude and phase functions based on FEFF calculations. These are
generated starting with a set of crystal-structure coordinates (for a model
compound) using the f7and8runner routine.
2.
Allows
flexible application of restraints, such as that the number of atoms in the 3rd
shell = n(3rd) = 6 - n(1st) - n(2nd).
3.
Results
can be archived so that different types of fits can be performed and compared
for each dataset.
4.
The
fits to EXAFS data (optionally multiplied by k^n) can
be performed simultaneously with or after fitting the baseline and XANES.
After fitting the baseline and XANES, the Fourier-transformed or
Fourier-filtered data may be fit. Weighting schemes may be applied to any
of these fits (in the form of specifying an esd of
the data).
5.
The
noise of the EXAFS spectrum may be reduced by interpolating the data to a 0.1
Angstrom (or similar) grid.
To learn to start using the program, click on the right arrow.