Hi Julian,

Carlo is completely right about the S0^2, and the common way to handle it (measure it for a standard and then transfer it, keeping it fixed, to the other measured materials).

I would just like to add that it is preferable that the standard and the materials have similar thickness (absorption coefficient) and especially good homogeneity. In fact, while the first one has an important influence on the amplitude of the EXAFS oscillations (especially when you deal with thick samples at low energies), the second one might also induce you in a wrong evaluation of the real value of the edge jump and thus of the amplitude of the oscillations (with inhomogenoeus samples, you may obtain edge jumps of acceptable values with attenuated oscillations).

This just to stress that sample preparation is really extremely important also to obtain a correct evaluation of the S0^2 from a reference compound.

Yours, Lorenzo

Hi Julian:

I would be happy to help with this.  Let me answer briefly on the list and afterward, if you like, just contact me privately and we can discuss in more depth.

S02 is a property of the absorbing atom and is due to the change in electronic energy levels because of the present  of a core hole after the photoabsorption event.  It is usually considered to be transferable from material to material with the same element because it is really a local property.  However, in practice it does depend on the type of compound you are measuring and the beamline as well as the type of measurement you are making (transmission, fluorescence).  All of these factors can masquerade a amplitude reduction factor.

What I do in practice is to measure a standard which has the same local environment and fit the EXAFS with the known structure to extract an S02 which I then use transferably.

As for your question about delta R.  If your EXAFS cannot be fit using an isotropic expansion model (not always the best way anyway) then you most likely need to make the delta R parameters independent.  There is absolutely no reason to correlate a shorter path with a longer path even if it is the same element, unless you are using a known structure model out of which you are trying to extract specific parameters.

Hope this helps.

Carlo

On Thu, 10 May 2018, Julian Ehwald wrote:

Hi Carlo

Thank you for your answer. I have completely different question as well: I
just started using Artemis, and have a question regarding the amplitude
reduction factor: I use different So^2 for different elements, correct? If I
start fitting and have paths involving the same kind of atom but once at
distance 2A, and once maybe 3A, is this then the same So^2 or am I supposed
to model the behaviour of So^2 somehow?
Similar for \Delta R: If I don?t have a nice simple cubic structure and
can?t use isotropic expansion similar like alpha*R_eff, what would be a
smart(or physically correct) way to connect \Delta R (as well as sigma^2)
for the same element for different distances? The material I am looking at
is Li2O2 and Li2IrO3, which don?t have a very simple structure.

I am aware that maybe I am asking a bit much, but if you could help me in
any way I would be very delighted.

Best, Julian

-----Ursprüngliche Nachricht-----
Von: Ifeffit [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] Im Auftrag
von Carlo Segre
Gesendet: Sonntag, 6. Mai 2018 01:07
An: XAFS Analysis using Ifeffit <ifeffit@millenia.cars.aps.anl.gov>
Betreff: Re: [Ifeffit] Energy Resolution


Hi Julian:

With an edge energy of 543 eV, your monochromator resolution should be at
least 0.1 eV.  If you are interested in XANES, you need to take data at this
resolution or maybe even better.  ONce past the edge, there is no need to
keep that step size.  If your beamline permits an EXAFS-type step scan with
steps in k-space, you want to use a dk of 0.05 which is what IFEFFIT
interpolates to.  Smaller step sizes are not really useful.

Carlo

On Sat, 5 May 2018, Julian Ehwald wrote:

Dear all,



This might be a very general question, but: I was wondering what
energy resolution you typically use at beamlines to obtain your raw data

for exafs.

Since we have limited Beamtime only and other experiments that we want
to carry out, I was wondering what kind of resolution is common. Or
differently: Is it predictable what happens if I for example change
from steps of 0.2eV to 0.4eV? We want to obtain spectra up to 500eV
above Oxygen K, if that is of relevance.

Best, Julian

--
Carlo U. Segre -- Duchossois Leadership Professor of Physics Interim Chair,
Department of Chemistry Director, Center for Synchrotron Radiation Research
and Instrumentation Illinois Institute of Technology
Voice: 312.567.3498            Fax: 312.567.3494
segre@iit.edu   http://phys.iit.edu/~segre   segre@debian.org
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