[Ifeffit] Help in background removal with Athena

[Irina Pi]

Alright thanks to all of you. I already knew that there is a big  relationship between the Fourier transform k-range and spectral resolution, but we couldn't get more energy range in the lab and wanted to see if EXAFS analysis was trustworthy this way or not.

A last question then: in the project I'm attaching 1_data.prj (other data, where I know I don't have enough energy range, but I wanted to try it), I get a FT plot with two well resolved peaks at around the bond lengths of the graphite sample (1.42Å in the same layer and 3.35Å between layers). Then as there is not enough k-range, we cannot extract any conclusions on that? (I guess we can but without much confidence, because then with another background removal, 2_data.prj, we obtain something different). Sorry for all the emails but it seemed so confusing to me..

Thank you again a lot,
Irina.
________________________________
From: Ifeffit <ifeffit-bounces at millenia.cars.aps.anl.gov> on behalf of Bruce Ravel <bravel at bnl.gov>
Sent: 06 September 2016 04:39:11
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] Help in background removal with Athena

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/
  Software:    https://github.com/bruceravel
  Demeter:     http://bruceravel.github.io/demeter/
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