[Ifeffit] Negative ss problem

Kropf, Arthur Jeremy kropf at anl.gov
Wed Jul 14 08:46:15 CDT 2010

Consider the energy resolution of the dispersive EXAFS beamline.  A poor
energy resolution could suppress the low-k amplitude causing the
Debye-Waller factor to compensate by being articifically low.  However,
you also have an amplitude larger than 1.  Adding an Ei component will
address the problem of poor energy resolution, but I would suggest that
it should be based on the measured energy resolution rather than being a
"guessed" variable.  You can also measure the appropriate Ei using a
known sample, such as a copper foil for which you have comparable
spectra on a beamline with better energy resolution.
What scattering paths and at what bond lengths are responsible for the
peak below 1 angstrom?  It seems unlikely to me that a Cu-O or Cu-N path
would produce such a peak.  These peaks would more typically be closer
to 1.5 angstroms.
Jeremy Kropf
Chemical Sciences and Engineering Division
Argonne National Laboratory
Argonne, IL 


	From: ifeffit-bounces at millenia.cars.aps.anl.gov
[mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf Of Abhijeet
	Sent: Wednesday, July 14, 2010 2:00 AM
	To: ifeffit
	Subject: [Ifeffit] Negative ss problem
	Hi all,
	          I read the discussion on negative Debye waller factor.
As per the discussion the negative value of this factor shows that the
model has shortcomings, so it should be corrected.
	         I am also getting the same problem while fitting one of
my samples. It is a Cu complex whose EXAFS data is taken at Dispersive
EXAFS beamline at RRCAT, Indore, India. 
	        In the complex the nearest neighbours of Copper atom are
Nitrogen and Oxygen.
	        I am getting a very good fit upto 3rd shell but the
problem is that all ss parameters are coming negative. I checked the
model it seem correct.
	        Also I am getting a peak below 1 Angs which is also
getting fitted but I am not able to get that whether it is a real or due
to some noise. 
	        I am attaching herewith the results of data analysis and
	        Thanks in advance
	        With regards
	     Abhijeet Gaur
	Vikram University
	Ujjain, INDIA

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