[Ifeffit] Fwd: Re: Athena/Artemis

[Bruce Ravel]

Hi gang,

I received an interesting question this afternoon about EXAFS
parameters and I thought that Olivier would benefit by hearing what
more people than just me would have to say about this topic.

He asks:

   Can you point me to documentation about the third and fourth
   cumulants?  Since I study glasses, which are highly disordered, I
   would like to measure the anharmonicity parameters.  What are
   typical values for C3 and C4?


Grant Bunker has a very useful paper on the cumulants in
Nucl. Instrum, and Meth. (1983) v. 207, p 437-444.  Anatoly's paper on
cumulants Phys. Rev. B 48, 585 - 588 (1993) is also handy and should
give you some sense of what size the C3 and C4 parameters should be.

In my experience, C3 and C4 should be smaller than sigma^2 -- perhaps
by about 1 and 2 orders of magnitude, respectively.  When they get
much bigger than that, one can reasonably ask whether a short cumulant
expansion (that is, an expansion out to only the 4th cumulant) is
really adequate for describing the distribution of atoms in such a
disordered material.  There are people who have been applying reverse
monte carlo or molecular dynamics to come up with better descriptions
of highly disordered materials.

Hopefully others will contribute to answering your questions.

B

Olivier: Sorry for switching languages on you -- hopefully it will
prove useful to hear from others on the list.  Je m'excuse de changer
de langue pour cette conversacion -- on espere que il y aura des
autres response utils ici sur la liste.





----------  Forwarded Message  ----------

Subject: Re: Athena/Artemis
Date: Thursday 24 July 2008
From: Olivier Villain <olivier.villain at impmc.jussieu.fr>
To: Bruce Ravel <bravel at bnl.gov>

Pourrais-tu m'indiquer où est l'aide concernant les paramètres 3rd et  
4th ? Comme j'étudie des verres, donc du désordre, j'aimerais essayer  
des paramètres d'anharmonicité. Quelles sont les valeurs typiques ?  
Quelle est l'unité ?
Je n'ai pas réussi à trouver ça dans l'aide. J'espère que je ne suis  
pas trop lourd...

A bientôt,

Olivier

-------------------------------------------------------

-- 
 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/