[Ifeffit] Energy Resolution

[Carlo Segre]

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 at millenia.cars.aps.anl.gov] Im Auftrag
> von Carlo Segre
> Gesendet: Sonntag, 6. Mai 2018 01:07
> An: XAFS Analysis using Ifeffit <ifeffit at 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 at iit.edu   http://phys.iit.edu/~segre   segre at debian.org
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>

-- 
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 at iit.edu   http://phys.iit.edu/~segre   segre at debian.org