[Ifeffit] Units for various parameters of ‘feffNNNN.dat’ file

Matt Newville newville at cars.uchicago.edu
Tue Feb 28 10:08:02 CST 2017


On Tue, Feb 28, 2017 at 9:48 AM, Bruce Ravel <bravel at bnl.gov> wrote:

>
> On 02/28/2017 10:10 AM, Samy Ould-Chikh wrote:
>
>> Dear Ifeffit mailing list,
>>
>>
>>
>> I have a question which concern units and I can’t find the answer in the
>> Feff manual.
>>
>> In a FeffNNN.dat output file, what are the unit of the numbers (if any)
>> corresponding to
>>
>> φc,
>>
>> |Feff|,
>>
>> φeff,
>>
>> the reduction factor,
>>
>> λ,
>>
>> Re(p)
>>
>
> Samy,
>
> This is a peculiar reference, but I think the clearest written explanation
> of this is in the manual for the ancient "feffit" program. Here is a copy
> of its manual:
>
> http://cars.uchicago.edu/~newville/feffit/feffit.pdf
>
> You will want to look at page 22.  While the rest of the manual is not so
> useful anymore (in the sense that there are very few users of the old
> feffit remaining), it's explanation of how the feffNNNN.dat files are used
> is excellent.
>
> Eq. 6.1 should clarify how each term is used and what the units are.
> Column 1 is, of course, inverse Angstroms.  Columns 2, 3, and 4 are
> unitless.  The two phase terms go into the argument of the sine, thus must
> be unitless.  The amplitude term is multiplied by N and S02, thus is also
> unitless.  The reduction factor, as well, is unitless -- it is used in the
> computation of the amplitude term in equation 6.1.
>
> Lambda has units of angstroms, the real part of p (which is the complex
> wavenumber used in the Feff calculation) has units of inverse Angstrom.
>
> I am pretty certain I got all that right.
>
> I think it would be a good idea to capture all of this information as well
> as eq. 6.1 in the Artemis manual.
>
> HTH,
> B
>
>
Yes, the units for the data in the columns are:
  mag[feff] and red factor: unitless terms for amplitude
  real[2*phc] and phase[feff]: radians
  k, real[p]: inverse Ang
  lambda: Ang

See  http://cars.uchicago.edu/xraylarch/xafs/feffpaths.html and
http://cars.uchicago.edu/xraylarch/xafs/feffpaths.html#the-exafs-equation-using-feff-and-feffpath-groups

--Matt
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