[Ifeffit] Ifeffit Digest, Vol 67, Issue 12

[Adam Webb]

Hi,

Khalid, Syed wrote:
> That is correct: It is a QEXAFS scan of Ni foil as a
> function of time.
>
> Please see the data attached as a .txt file.
> The first column is time, second I0, and third It.
> I can plot this in Athena, time appears at energy axis.
> I can set the first derivative to 8333.0 eV ( changing X-axis from
> time to energy ). If I have another known energy point of Ni scan
> ( which is not a problem ) then I can convert the time axis to
> correct energy axis and proceed with normal analysis.
> Is that possible?
>

The longer answer is yes with a qualifier. It is possible to do many
things in Athena as you can directly use Ifeffit. However, this is not
for the timid. ;-)

**This all assumes that your relationship of time to energy really is
linear or is a known relationship. **

1. open data file
2. select the group and under the edit menu select 'show groups arrays'.
This tells you what the name of the group is for Ifeffit. You will see a
series of similar names. The important part is the four letter part
before the point. For example it might be bpdc.
3. copy the group and get the name as before. It might be ckdl.

You can send commands directly to Ifeffit in the Ifeffit interaction
buffer window that opened in the step that we just did. (at the bottom)
This means that math on the data is possible. This means that you can do
a linear fit to two calibration points or do anything else you want.

4. Pick two points on the data. I used the Ind tab (plotting options) to
select two points and get the values. This would be T1 and T2. You then
decide what the energies (E1 and E2) are.
5. In the buffer, you can now enter the values. Since I assume you want
to do more than one spectrum I would enter the data as variables.
  t1 = (value)
  t2 = (value)
  e1 = (value)
  e2 = (value)
then the equations:
  a = (e2-e1)/(t2-t1)
  b = e1 - a(t1)
  Now you modify the energy scale. For my example names above that would be:
  ckdl.energy = bpdc.energy*a + b
  - plot the data as usual (you will have to change E0)
And you are done!

The nice thing is the a and b are equations. That means that you just
select new values for e1,e2,t1,t2 for each spectrum and enter it. Then
solve for the energy. You can see the values of all scalars using: show
@scalars

Further reading is available online in the usual place:
http://cars9.uchicago.edu/ifeffit/Documentation


I should mention that this sort of sequence (and longer) is exactly what
Athena does with a single click of the mouse on the GUI. It takes care
of all the details like arrays, scalars, equations etc for you. I am
always so glad that I don't have to do these myself each time.
Thanks Bruce!!  :-)

I hope this helps,

Adam