[Ifeffit] Help in background removal with Athena

[Bruce Ravel]

On 09/06/2016 10:55 AM, Irina Pi wrote:
> Thank you for the quick answer. Which range should I have for the EXAFS
> analysis to be meaningful? Like 200eV above the edge?

I am always uncomfortable when newcomers ask for "rules".  Having a rule 
without understanding when and why it might apply seems counter-productive.

Firstly, lets discuss the relation ship between "energy above the edge" 
and photoelectron wave number.  The conversion constant between energy 
and wavenumber is

    k = sqrt( ETOK * (E-E0) )

where ETOK is the appropriate combination of fundamental constants (2m_e 
/ hbar^2, where m_e is the mass of the electron and hbar is Planck's 
constant) about equal to 1 / 3.81.  So, 10 inverse Angstroms is about 
381 volts above the edge.

The extent of the signal in k determines how well you can interpret an 
EXAFS signal in a quantitative sense.  See this page in the Athena manual:

    http://bruceravel.github.io/demeter/documents/Athena/plot/krange.html

Scott Calvin discusses this in his book.  There is more information 
about EXAFS data reduction among the tutorials at http://xafs.org/Tutorials.

As Fred and Robert have said, you need "enough" data range to make a 
useful interpretation of the EXAFS.  200 eV is not a lot -- that will 
leave you with only a small handful of independent measurements in your 
data, possibly not even enough to reliably determine a first shell bond 
length.

B


-- 
  Bruce Ravel  ------------------------------------ bravel at bnl.gov

  National Institute of Standards and Technology
  Synchrotron Science Group at NSLS-II
  Building 743, Room 114
  Upton NY, 11973

  Homepage:    http://bruceravel.github.io/home/
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