Baseline Fitting
To modify the way
the baseline appears, click on "edit BASELINE params" in "Step
2" on the EXAFS Main Panel.
EXASF123 uses
cubic polynomial spline so the first step is specifying the number of spline
points. In the XAS + Baseline graph, Energy (on x-axis) extends from -200 to
1000 eV. It is advisable to use equal intervals of 300/400 eV so in this case,
three spline points (four intervals of 300eV each) are used. As shown in the
screenshot below, from the "# of spline points" menu, choose "3
equally spaced". The values that appear for Auxes are ignored by the
program. The user can choose to make the 1st/2nd/3rd/4th/5th
derivative continuous at the spline point. As most XAS analysts use the first
derivate, that option is selected in the example.
A table with a
list of baseline parameters appears after hitting "Continue" in the
dialogue box.
When you look at
the XAS + Baseline Graph, it can be seen that the baseline starts from y = 0
but the data curve starts at y = 1.1. To shift the baseline so as to coincide
the points where the two curves start, open the table showing the parameters of
the baseline. As we want to shift the curve by 1.1, insert that number in the
box on the right of the cell that says "Baseline =" as shown in the
figure.
To see the effect
of this change, click on the "Calculate" button located at the lower
right part of the EXAFS123 Main Panel. As shown in the figure below, The
Baseline now shifts up. Note that both of the curves start at 1.1 as shown
below.
Slope of Baseline
The baseline is
perfectly horizontal whereas the steady part of the XAS is not. To match up
the slopes of these two, another Baseline Parameter needs to be altered. Open
the Table of Baseline Parameters again. By trail and error, settle on a number
that gives the baseline a slope close to the steady part of the data curve.
In this case, -0.001 is used.
After hitting "Calculate"
in the Main Panel, it can be seen that the slope of the baseline is approximately
the same as that of the data curve but the starting point of the baseline
itself has shifted up by about 0.15. To get a more accurate result, replace
"1.1" in the box next to "Baseline=" by "0.95".
After following
the steps outlined above, your XAS + Baseline Graph should look like this.
