[Ifeffit] EXAFS simulations

[Cammelli Sebastiano]

Dear IFFEFIT user

I do some EXAFS simulations (FEFF8.4) of small pure Cu clusters (fcc
bulk structure) and then I try to compare them with experimental data. 

The simulations are performed on all the shells, then all the
contribution are added according to the number of atoms per shell (so
the total xmu of a cluster of 19 atoms is = [xmu0+12*xmu1+6*xmu2]/19
where xmu0 is the xmu of the atom in the center of the cluster and xmu1
of the atoms in the first shell and xmu2 for the second shell).

The first and obvious result concerns the peaks amplitude of the FT
which is proportional to the number of atoms involved in the
simulations. I have calculated the EXAFS spectra from the xmu.dat files.
As you know in EXAFS simulations there is not pre-edge so finally is not
easy to find the right value of E0. 

In the Athena.prj file I attached you can easily see this effect
changing the absorption energy of Cu Bulk 4  from  8988.430eV to
8988.431eV. The amplitude of the first peak of the FT changes from 100
to 60! In order to overcome this problem I recalculated the FT using the
chi.dat and not the xmu.dat. 

In the Athena file you can find the 8 simulations based on the xmu.dat
(and they are called Cu bulk 1.... Cu bulk 8) and on the chi.dat (called
CHI_Cu bulk 1....8). Of course they refer to the same cluster and they
were calculated in the same way. 

Since these simulations, calculated on the Chi.dat, are very different
from those calculated on xmu.dat, some questions rise. Moreover if you
compare the FT of the simulations Chi_Cu bulk 5 and Chi_Cu Bulk 6 you
can see that the amplitude of the first peak is larger for the smaller
cluster. 

1.                   Should I use the spectra I got from the xmu.dat or
from the chi.dat?

2.                   Are the difference between Chi_Cu 5 and Chi Cu 6
(even if small) due to the E0 calculation? 

3.                   How shall I behave if the first derivative of some
simulations does not show a peak where it is "reasonably" expected
(compare Cu Bulk 4 and Cu Bulk 7) 

4.                   Do you have any other suggestions? 

 

Thank you in advance for any suggestion, 

Best regards

Sebastiano Cammelli

 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20081024/13e238ff/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: athena_Cu.prj
Type: application/octet-stream
Size: 34413 bytes
Desc: athena_Cu.prj
URL: <http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20081024/13e238ff/attachment.obj>