[Ifeffit] polarization on FEFF calculation

Eugenio Paris eugenio.paris at gmail.com
Sun May 24 07:30:00 CDT 2015


Thank you for the answer, now things are more clear.

Now the question is: how to go further on the fitting procedure?

If the Artemis software is weighting the contribution of different paths
using the "Rank" value it sounds like i'm in trouble.

The Fourier transform of my EXAFS signal shows a wide peak between 2 and 3
Angstrom containing contributions from the first 3 scattering paths.
However, it looks like in this geometry i can neglect the 3rd path. That's
the reason why i'm interested in modeling the first two scatterings.

I'll try as follows,
I will import the first 2 scattering paths without taking account of
polarization and try to include such dependency in the S0^2 parameter.

The information that I have now is that the amplitude of the @S1.2@ path is
the key to give the correct weight to the first two paths.

Performing the calculation of the @S1.2@ path with POLARIZATION 1. 0. 0.
and 0. 0. 1. gives Rank = 3.67 and 25.80, respectively.

I wish to assign S0^2 = amp for the @S1.1@ path (in-plane contribution) and
S0^2 = (3.67/25.8)*amp for the @S2.1@ (out-of-plane contribution).

Can this approach be ok?

Eugenio

2015-05-24 13:42 GMT+02:00 Matt Newville <newville at cars.uchicago.edu>:

> Eugenio,
>
>
> On Sun, May 24, 2015 at 5:26 AM, Eugenio Paris <eugenio.paris at gmail.com>
> wrote:
>
>> Dear all,
>>
>> i have a question about polarization vector in performing the FEFF
>> calculation in Artemis.
>> I am studying the system CeOBiS2 using Bi L3-edge EXAFS. Starting from
>> *.cif file i'm creating the *.inp file which atoms list come out as follows:
>>
>>  ATOMS                  * this list contains 35 atoms
>>  *   x          y          z     ipot tag           distance
>>     0.00000    0.00000    0.00000  0  bi            0.00000
>>     0.00000    0.00000    2.50509  4  s2.1          2.50509
>>     2.81711    0.00000   -0.12006  4  s1.1          2.81967
>>    -2.81711    0.00000   -0.12006  4  s1.1          2.81967
>>     0.00000    2.81711   -0.12006  4  s1.1          2.81967
>>     0.00000   -2.81711   -0.12006  4  s1.1          2.81967
>>     0.00000    0.00000   -3.31180  4  s1.2          3.31180
>>     ...
>>
>> Bismuth has a S atom sitting on top of him (s2) and it is square
>> coordinated with other four S atoms (s1).
>> I made measurements on single crystals using fluorescence yield. The
>> experimental geometry consist of the x-ray beam coming at (almost) normal
>> incidence which corresponds to electric field vector lying on the crystal's
>> ab plane.
>>
>> If I perform the FEFF calculation without taking account of polarization
>> I obtain the first scattering paths which are
>>
>> 1) @S2.1@, Degen = 1, Reff = 2.5051, Rank = 100
>> 2) @S1.1@, Degen = 4, Reff = 2.8197, Rank = 100
>> 3) @S1.2@, Degen = 1, Reff = 3.3118, Rank = 14.78
>> 4) ......
>>
>> When I insert the line "POLARIZATION 1 0 0" in the imput file i get
>>
>> 1) @S2.1@, Degen = 1, Reff = 2.5051, Rank = 100
>> 2) @S1.1@, Degen = 4, Reff = 2.8197, Rank = 100
>> 3) @S1.2@, Degen = 1, Reff = 3.3118, Rank = 3.67
>> 4) ......
>>
>> which is exactly the same result for the first 2 scatterings, even if the
>> rank of higher shells is changed...
>>
>> Now, my question is:
>>
>> If the components of the polarization vector has to be written in the
>> same reference frame of the atom list i'm expecting to see a strong
>> suppression of the rank factor for the @S2.1@ scattering whose bond is
>> along the z-direction, is it correct?
>>
>
>
>
> There are two different things going on that can help explain this.    But
> first:  it wasn't clear what you used to specify the polarization in
> feff.inp.  You probably used either
>      POLARIZATION 1. 1. 0.
>
> or
>     POLARIZATION  1. 0. 0.
>
> If that assumption isn't correct, please clarify.
>
> First, Feff's reporting of "Rank" is really "Intensity of this path
> compared to the most intense path seen so far".   The first path is always
> 100.0, and if any later path is 100.0 it indicates that it had more
> intensity than the previous highest-intensity path, but you have no idea of
> how much more intensity.   It's a really lame report on intensity and
> should be fixed.  Because of this, the only meaningful number in your list
> is the Rank of path 3, which does actually go down.
>
> Second, you're asking for a calculation of an L3 edge.  The L3 edge
> includes both p->s and p->d transitions, which means the polarization
> dependence is a mixture of a dipole-like term a constant term.  So,
> polarized measurements at the L3 edge does not completely suppress
> contributions normal to the polarization vector, as it would for a K or L1
> edge.    For your system, it looks like the out-of-plane contribution is
> suppressed by about a factor of 4 relative to the contributions from the
> S1.1 atoms in the X-Y plane.    I think that's not unusual for L3 edges,
> but the literature on this is scant.
>
> Hope that helps,
>
> --Matt
>
>
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