[Ifeffit] RE: Ifeffit Digest, Vol 17, Issue 23

yaelp yaelp at wicc.weizmann.ac.il
Thu Jul 29 08:49:08 CDT 2004 

didnt get any instruction
thanks yael
>===== Original Message From ifeffit at millenia.cars.aps.anl.gov =====
>Send Ifeffit mailing list submissions to
>	ifeffit at millenia.cars.aps.anl.gov
>
>To subscribe or unsubscribe via the World Wide Web, visit
>	http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
>or, via email, send a message with subject or body 'help' to
>	ifeffit-request at millenia.cars.aps.anl.gov
>
>You can reach the person managing the list at
>	ifeffit-owner at millenia.cars.aps.anl.gov
>
>When replying, please edit your Subject line so it is more specific
>than "Re: Contents of Ifeffit digest..."
>
>
>Today's Topics:
>
>   1. Re: weights data processing (Bruce Ravel)
>   2. Re: EXAFS question (Bruce Ravel)
>
>
>----------------------------------------------------------------------
>
>Message: 1
>Date: Mon, 26 Jul 2004 09:06:13 -0400
>From: Bruce Ravel <ravel at phys.washington.edu>
>Subject: Re: [Ifeffit] weights data processing
>To: XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>
>Message-ID: <200407260906.13553.ravel at phys.washington.edu>
>Content-Type: text/plain;  charset="iso-8859-1"
>
>
>I wanted to add a coupe more comments on Stafano's questions regarding
>k-weighting in Athena and Artemis.
>
>Regarding k-weights in Athena: Athena's default for the k-weight using
>background removal is 1.  I chose that pretty much for the reason that
>Scott gave.  Ifeffit's default is 0, so this is a situation where
>Athena and Ifeffit have different default behavior.  As for the
>insensitivity of your data to this value, I would say that it is
>situational.  Sometimes, the k-weight can have a dramatic influence on
>the background removal.  Stability of the background removal and its
>impact on subsequent analysis has been a frequent topic of discussion
>in this forum.  k-weight, then, is yet another parameter that can
>impact the stability of your spline.
>
>Rgearding k-weights in Artemis:  Scott and Matt covered the "why" of
>k-weights.  How k-weights are actually used by Artemis when fitting
>and plotting is a bit complicated.  See section 5.8.4 of the
>(incomplete) artemis document for a detailed explanation of how
>k-weighting is used in Artemis.
>   http://leonardo.phys.washington.edu/~ravel/software/exafs/doc/artemis.pdf
>
>B
>
>--
> Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
> Code 6134, Building 3, Room 405
> Naval Research Laboratory                          phone: (1) 202 767 2268
> Washington DC 20375, USA                             fax: (1) 202 767 4642
>
> NRL Synchrotron Radiation Consortium (NRL-SRC)
> Beamlines X11a, X11b, X23b
> National Synchrotron Light Source
> Brookhaven National Laboratory, Upton, NY 11973
>
> My homepage:    http://feff.phys.washington.edu/~ravel
> EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/
>
>
>
>------------------------------
>
>Message: 2
>Date: Mon, 26 Jul 2004 09:37:31 -0400
>From: Bruce Ravel <ravel at phys.washington.edu>
>Subject: [Ifeffit] Re: EXAFS question
>To: "Norman White" <gn-white at tamu.edu>
>Cc: XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>
>Message-ID: <200407260937.31266.ravel at phys.washington.edu>
>Content-Type: text/plain;  charset="iso-8859-1"
>
>Norman,
>
>Your question is the sort of thing that is appropriate for the Ifeffit
>mailing list, so I am CCing my answer there.  You should consider
>joining the mailing list.  It is a great resource and gives you access
>to a large number of experts who can answer your questions.
>
>
>On Friday 23 July 2004 03:38 pm, Norman White wrote:
>NW> In going through Matt Neville's xas_fun_slides.pdf file that was
>NW> included with the Athena software, I noticed on slide 36:
>
> MN > The Fe-O distance in FeO is 2.14 ° A, but the first peak is at 1.6 ° A.
> MN > This shift in the first peak is due to the phase-shift,
> MN > delta(k): sin[2kR + delta(k)] .
> MN > A shift of -0.5 ° A is typical.
>
>
>NW> I asked him in an e-mail about the shift and he said (edited):
>
> MN > For traditional XAFS analysis, we do a relatively simple Fourier
> MN > transform.  This simple FT would give a peak in chi(r) at 'R' if
> MN > the oscillatory part was sin[2kR].  Since it's sin[2kR + delta(k)]
> MN > instead, there's a shift.
> MN >
> MN > It turns out that, to first order, delta(k) is linear and has a
> MN > negative slope of about 1:
> MN >     delta(k) ~= -k ~= -2k (0.5),
> MN >
> MN > so that the shift in phase is about 0.5A.
>
>NW> I asked if this was true for all elements and he said:
>
> MN > It's a typical value for most bonds, but shouldn't be taken as an
> MN > accurate number (and it's easy enough to calculate).
>
>NW>
>NW> How is an accurate shift calculated?  Is this correction performed
>
>This calculation is computed quite accurately by Feff.  Indeed,
>getting that calculation (and the related calculation of the
>scattering amplitude) correct is the raison d'etre of Feff.
>
>NW> automatically in Athena?  If it is a constant, why is it not simply
>NW> included in the step producing the radial distribution function from
>NW> the k values? Can it be accurately calculated?  The manual had
>NW> statements that seemed to imply that it could be calculated with the
>NW> Atoms program but did not explain how at a level I understood.
>NW>
>NW> Matt neville stated in an e-mail that:
> MN > The phase shift correction can be made fairly easy with
> MN > SixPack and with Artemis as well as Ifeffit.
>
>It's not a constant and treating it as such would be specious at best,
>and it is not something that Athena intends to compute.  However,
>since it is something that Feff computes, one can imagine using the
>results of Feff.  How this is done is slightly different in Athena as
>in Artemis.
>
>At the level of Athena, we know the absorbing atom but it is not
>actually necessary to know what the material is to use Athena
>effectively.  In Artemis, on the other hand, we do have to know
>something about what the materials is.  This difference between the
>two explains the rationale for how phase correction works i the two
>programs.
>
>In Athena, only a central atom phase correction is supported.  This is
>enabled by clicking the button that says "phase correction" in the
>forward trasform section of the mail window.  With that button
>clicked, the central atom phase shift is subtracted from the data
>before the transform is made.  When you plot chi(R), then, the
>transform will be peaked a little bit to the right of where it was
>peaked without that button clicked.
>
>Artemis does a somewhat better job of this.  In Artemis, you can
>select a path to use for pase correction.  With a path selected, the
>central *and* scattering atom phase shifts will be subtracted before
>transforming.  This is possible in Artemis because we have the fuill
>information about the material and the full feff calculation at our
>disposal.  In Athena, in contrast, we only know about the absorber and
>I have never implemented (and don't really intend to do so) a way of
>reading in a feff path for the purpose of phase shifted transforms.
>
>NW>
>NW> I could not find where it explained how to do this.
>
>Did you read any of the documentation?
>
>For Athena:
>http://leonardo.phys.washington.edu/~ravel/software/exafs/doc/Athena/athena_m
isc.html
>  The document is also available within Athena via the Help menu.
>
>For Artemis:  Section 5.8.5 of
>http://leonardo.phys.washington.edu/~ravel/software/exafs/doc/artemis.pdf
>
>
>NW> I am looking at As adsorption on Fe-Al oxides and other minerals
>NW> I know what the central atom is As.
>NW> I can also estimate the oxygen bondlengths and the As-cation distance 
from
>NW> the "corrected" EXAFS pattern and the literature. Is it possible to
>NW> simulate an EXAFs pattern?
>NW> Is so, how?
>
>Yes.
>
>Have you read the Artemis manual?
>
>B
>
>
>
>--
> Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
> Code 6134, Building 3, Room 405
> Naval Research Laboratory                          phone: (1) 202 767 2268
> Washington DC 20375, USA                             fax: (1) 202 767 4642
>
> NRL Synchrotron Radiation Consortium (NRL-SRC)
> Beamlines X11a, X11b, X23b
> National Synchrotron Light Source
> Brookhaven National Laboratory, Upton, NY 11973
>
> My homepage:    http://feff.phys.washington.edu/~ravel
> EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/
>
>
>
>
>------------------------------
>
>_______________________________________________
>Ifeffit mailing list
>Ifeffit at millenia.cars.aps.anl.gov
>http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
>
>
>End of Ifeffit Digest, Vol 17, Issue 23
>***************************************

More information about the Ifeffit mailing list