[Ifeffit] Reference for Setting k range in EXAFS analysis

[Bruce Ravel]

To add to what Robert said...

Section 12.2.3 of Scott Calvin's textbook, XAFS for Everyone, may be
exactly what you are looking for.  You can find it on Amazon and
elsewhere.

I am not sure if you are actually asking about the lower end of the
k-range or the upper end.  If the question is about the upper end,
then, as Robert, said, the answer is simple.  More data is better, so
long as you are mindful of the size of the signal relative to
statistical and systematic noise.

As for the lower end, you need to remember that the background removal
algorithm uses a rather simple-minded, Fourier analysis to distinguish
background from XAFS signal.  In the region close to the edge -- where
the data are changing most rapidly -- the spline probably does not
have the freedom to approximate the background function well.  Thus,
the extracted chi(k) are quite a bit more unreliable below 2 or 3
inv. Ang. than they are at higher k.

This may sound like a horrible problem for EXAFS analysis, except that
the theory is also unrelible at low k.  When the photoelectron has low
kinetic energy (i.e. just above the edge), it is very sensitive to the
details of the potential surface.  Given that Feff uses spherical
muffin tins and that spherical muffin tins are a pretty crude
approximation, the theory is unreliable below 2 or 3 inv. Ang.

Happily, about 2 or 3 inverse Angstroms, both the theory and the
extracted data are pretty reliable, which is why EXAFS analysis works.

The default kmin in Artemis is 3.  You should choose kmax to be as big
as possible without extending into a regime where the signal is
dominated by statistical or systematic noise.  (Section 5.9.1 in
Scott's book is helpful in that regard.)

HTH,
B



On 06/18/2015 12:33 PM, Robert Gordon wrote:
> Hi Navneet,
>
> You have asked a rather open-ended question.
> There are numerable resources available for learning about XAFS analysis
> that are straightforward
> to find. Setting the k-range is more specific and governed by several
> factors.
>
> The range chosen affects resolution and precision. In general, you would
> like as large a k-range
> as you can reasonably obtain. Practically, this is limited by the sample
> (order/disorder, mean-square-
> relative-displacement, Debye-Waller term) and by finite limits on beamtime.
>
> My suggestion, as a starting point, is to consider what type of
> materials you plan on studying,
> then check the literature for similar systems to see what results others
> have been able to obtain.
>
> -R.
>
> On 6/18/2015 3:42 AM, Navneet Singh wrote:
>>
>> Dear All
>>
>> Can I get some study material mentioning how to set the K-range for
>> EXAFS analysis.
>>
>> With best regards
>>
>> Navneet Singh
>>
>>
>>
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>
>
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-- 
  Bruce Ravel  ------------------------------------ bravel at bnl.gov

  National Institute of Standards and Technology
  Synchrotron Science Group at NSLS-II
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  Upton NY, 11973

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