[Ifeffit] Spectra for ions?

[John J. Rehr]

Dear Kristine,

 FEFF is not set up to calculate single Mn ions. However, you could
try to simulate this approximately by taking a Mn cluster, varying
the Fermi energy (which adjusts the ionicity) and plotting only
the atomic-like backgound mu_0 (col 5 in xmu.dat).

 Unfortunately, for Mn and other transition elements there are large "local field
effects," i.e., screening of the x-ray field at the L_23 edge, which cause the
L3/L2 ratio to vary. For this reason independent electron calculations
are not likely to be reliable for such transition elements.

 For details, see e.g., ``Dynamic screening effects in x-ray absorption spectra,"
A.\ L. Ankudinov A.\ I. Nesvizhskii, and J. J. Rehr, Phys. Rev. B. {\bf67},
115120 (2003).

 Also the absorption of the Mn will depend strongly on the environment.
Thus it's important to know what material the Mn is embedded in.

  Cheers,
  John Rehr



> Hi ,
> 
> I'm trying to help a collegue who would like to use FEFF to simulate
> X-ray absorption spectra for a single ion (i.e. Mn+2, Mn+3, Mn+4...)
> through the Mn L3/L2 edge.  Since this is quite different from using FEFF 
> for EXAFS like I'm used to... I could really use some help in figuring
> out how to accomplish this.
> 
> We were also trying to figure out a way to remove the background from his 
> experimental absorption spectra -- can Athena do this (or does Athena
> only work on EXAFS spectra)?  If not, does anyone do this type of thing
> and have suggestions?
> 
> Thanks!!!
> Sincerely,
> 
> Kristine Witkowski
> Northwestern University
> 
> 
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