Re: [Ifeffit] Peaks in MCA spectra

Hi Matt, Thanks for your reply. Please see below. On Mon, Mar 31, 2014 at 8:10 PM, Matt Newville wrote:

Hi George,

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

I've attached a plot of the data showing the x-axis in keV. Also, as requested by Zack, I've attached a two column data file.

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.

Based on fitting the spectrum with Gaussians, the Co LI is 24% of the La2. As you mention, a value of roughly 10% is expected. What could cause such a discrepancy?

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

Yes, it is. I think maybe a little fluorine too, but it is very weak as I mention in my response to Matthew.

What surprises me is that there is no signal from the elastic peak. Was that somehow filtered out?

No, if there is an elastic peak it is probably lost under the Co La2. Thanks, George

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

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Hello,

I am writing with a general XAS question. It does not necessarily

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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

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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.

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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

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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

On Mon, Mar 31, 2014 at 5:01 PM, George Sterbinsky wrote: pertain to the Co the that is the

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relative transition strengths differ from tabulated values?

Thank you, George

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-- --Matt Newville <newville at cars.uchicago.edu> 630-252-0431 _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit