[Ifeffit] standard values

Bruce Ravel bravel at bnl.gov
Sun Feb 21 09:18:16 CST 2010

On Sunday 21 February 2010, 07:04:05 am, Neetu parsai wrote:

> Hi all,
> I am searching for the values of mean free path and Debye waller factor of
>  transition metals. Is there any citation available on these values? I want
>  to understand how these are calculated.

Feff uses the values for mean free path from K. Rahkonen and K. Krause, Atomic 
Data and Nuclear Data Tables, Vol 14, Number 2, 1974.  (It is easier to just 
dig through the Feff source code for the actual numbers that it is to try to 
find a copy of that paper.)  Feff also uses a relatively simple plasmon-pole 
model to compute other sources of loss.  Note, though, that mean free path and 
other forms of loss are quite hard to distinguish in an EXAFS analysis.  
Sorini et al, Phys. Rev. B 74, 165111 is a much more modern take on the issue, 
although their many-pole approach has not yet found its way into common use in 
Ifeffit and friends.

As for sigma^2 values -- the notion of "standard values" is not really very 
meaningful.  sigma^2 depends on coordination environment, bonding strength, 
temperature, pressure, and probably other things as well.  In practice, it is 
rather difficult to distinguish between mean square variation in bond length 
arising from thermal fluctuations and from structural disorder.  Although 
there are a few examples in the literature of ab initio calculations of 
sigma^2 values, in practice sigma^2 is something we measure in an EXAFS 
experiment, rather than something we take as prior knowledge.  That said, 
sigma^2 values tend to be of the order of 10^(-3) to 10^(-2) in units of 
Angstrom^2 as used in the formulation of the EXAFS equation implemented in 


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

 National Institute of Standards and Technology
 Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
 Building 535A
 Upton NY, 11973

 My homepage:    http://xafs.org/BruceRavel
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/

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