[Ifeffit] consultation

[Richard Mayes]

Mingliang,

For more information on self-absorption, read the links found here:
http://www.xafs.org/Experiment/OverAbsorption

This includes a link to a discussion on the mail list about self-absorption
and how to understand it.

-Richard

2010/1/29 Welter, Edmund <edmund.welter at desy.de>

> Dear Mingliang,
>
> Thanks for your explain. But I am still puzzled. Following your
>> instruction, I performed *Self-Absorption *for the samples, choosing
>> *XANES(Fluo) *from *algorithm*, but I did not know the real meaning of
>>  *Formula*. Does it mean the species formula which I wish to detect? Or the
>> whole composition of the sample?
>>
> The composition of the whole sample. You do not need to know the species,
> you "only" need to know thew elemental composition of your sample. A hairy
> task if you don't digest your sample completely and put it into an ICP-MS or
> anything like that. I don't know which precision is necessary to yield
> usable results. Maybe someone else tested or calculated it.
> In principle the matrix produces just a smooth background absorption and
> knowing the composition it is easy to calculate the amount of this
> absorption using tabulated scattering factors. But it should also be
> possible to determine this background experimentally, for instance
> extrapolating the background before the edge. But for this you would need a
> blank spectrum to "calibrate" the mued value.
> If you follow the link to the "document section" in Athena a very
> instructive webpage will open and among other things you will find citations
> of the original papers underlying the four different algorithms used for the
> self absorption correction.
>
>
> I used natural mineral to interact with selenite, then I wish to check the
>> speciation of the selenium reaction product. For my case, should I fill the
>> blank with Se or FeSe or both of them (because currently I do not know the
>> exactly containing speciation)?
>>
> As said before, the "chemical Formula" of the entire sample. "Chemical
> formula" here does not mean you have any kind of chemical bond, it just
> means molar ratio.
>
> For the angles, normally are they 45? Becuase I measured the samples in
>> fluorescence mode, and didnot measure the angles between sample, incoming
>> beam and detector. I tried to input *Se *in the *Formula* entry, and select
>> 45 for the angles, then I got a corrected spectra which was higher than the
>> original one.
>>
>
> 45 degrees is the usually used geometry. However, the program asks for the
> geometry which you used not for the usually used geometry! If it was 45
> degrees than its fine. Obviously the algorithm does what it is supposed to
> do, it amplifies the oscillations. But this does not mean that the result is
> correct! It will only be "correct" if the input parameters were correct.
>
> But when I using this spectra for LCF analysis, I still could not got
>> overlapping line for the fitted spectra with the sample spectra...
>>
> Using pure Se in the self absorption correction is definitely wrong, it
> overestimates the effect dramatically. Actually the effect more or less
> disappears for diluted samples. But even if the spectra were properly
> corrected I can not see any reason why the used set of reference spectra
>  MUST fit the spectrum of your sample. There are many possible reasons for
> this, the most likely is that you are missing one or more important
> reference spectrum (Or you are not using enough references/free parameters
> in your fit...;-)
>
> Best regards,
> Edmund Welter
>
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