[Ifeffit] Peaks in MCA spectra

[Matt Newville]

Hi George,

Calibrated spectra would help, but if we guess the calibration is 0.56
bins/ eV, then we'd have

  line             E (eV)       bin #
---------------------------------------------
Co La1,2       775          434
O Ka1,2         525          294
C Ka1,2         277          155

which looks  pretty good.     This puts the the unknown peak near
380/0.56 which is 678 eV.   Tthat's very close to Co Ll (M1->L3),
which is at 677 eV, and should be a bit less than 10%  of Co La1 and
La2, which is roughly right.

So, I think it's Co Ll.  That says the sample is just Co, C, and O.
Is that reasonable?

What surprises me is that there is no signal from the elastic peak.
Was that somehow filtered out?  The fact that the counts don't go to
zero between C and O could be many factors, including incomplete
charge collection.  This (and Compton scattering) generally make peaks
have a slightly non-Gaussian shape, with a low-energy tail.

Cheers,

--Matt

On Mon, Mar 31, 2014 at 5:01 PM, George Sterbinsky
<GeorgeSterbinsky at u.northwestern.edu> wrote:
> Hello,
>
> I am writing with a general XAS question. It does not necessarily pertain to
> Ifeffit, however, I think the topic is something some, maybe most, list
> members will be knowledgeable about. So it seems like this list is a good
> place to post this question.
>
> On to the question. I have attached a plot of a MCA spectrum collected with
> a vortex silicon drift detector. The spectrum is actually the average of
> several spectra, all collected in the post edge region of the Co L-edge. The
> spectra were averaged to reduce noise. The three peaks result from
> fluorescence from carbon, oxygen, and cobalt. Low-energy shoulders on the Co
> and O peaks are also observed. These can be seen as the regions of the
> spectrum that are not well reproduced by the fit. The main reason I included
> the fit in the plot is to illustrate the presence of these shoulders,
> particularly in the oxygen florescence, where the additional intensity is
> not so obvious.
>
> I am writing to see if anyone has any suggestion as to what the origins of
> these peaks might be. They are not due to additional elements, as they
> appear at the same incident energies as the main florescence peaks, i.e. the
> Co shoulder appears at the same incident energies as the main Co peak, and
> the O shoulder appears at the same incident energies as the main O peak. It
> is possible that the peaks result form other transitions. Considering Co,
> the main peak is due to L3/L2-M4 transitions, and the shoulder is in a
> position that could be consistent with L3/L2-M1 transitions. However, by
> fitting the peaks with Gaussians, one finds an area for the shoulder that is
> about 25% of the area of the main peak. This is significantly larger than
> what one might expect from tabulated transition strengths like those given
> in Hephaestus.
>
> To summarize, does anyone know what these shoulders might result from if not
> lower energy transitions? If they are low energy transitions, why would the
> relative transition strengths differ from tabulated values?
>
> Thank you,
> George
>
>
>
>
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-- 
--Matt Newville <newville at cars.uchicago.edu> 630-252-0431