Re: [Ifeffit] Peaks in MCA spectra

[George Sterbinsky]

Hi Matthew,

Thanks for your reply. see below.

On Mon, Mar 31, 2014 at 6:30 PM, Matthew Marcus  wrote:

> Hmmm.  An interesting brain-teaser.  Scaling very roughly from the graph,
> the mystery peak shows up at about 715eV.  This is consistent with Fe.
> Could there be a background from Fe somewhere in the detection systen?


I don't think so. The shoulder appears when the incident energy corresponds
to the Co L-edge, at the same time as the main Co peak. Also, MCA spectra
collected above and below the Fe L-edge are pretty much identical.


> The intensity between the O and Co peaks doesn't really look like
> a peak, but just like a background tail, perhaps from the strong Co peak.
>  It seems to be a little to the right of where N (from, for instance,
> a kapton window) would be.  I could be wrong, in which case my Fe theory
> goes away.


I'm assuming you mean the intensity between C and O. I've attached a plot
of a MCA spectrum from the Co L pre-edge. This is again the sum of several
spectra collected in the pre-edge region to reduce noise.The intensity
between C and O is still present, so I don't think it is due to a tail from
the Co peak. I also agree that is to the right of where one expects N.


> Now, I thought about an escape peak from Si, but I suspect that
> this is really small down here and anyway, the Si L-fluorescence energy is
> 92eV or so, and the extra peak is about 55eV below the main peak.
>
> Did you duck below the Fe edge to see if the peak to the L of Co goes away?
>

Yes, as noted above, the attached MCA spectrum from the Co pre-edge is
pretty much identical to spectra collected below the Fe L-edge. A peak is
seen at energies consistent with Fe and/or Co florescence. I think it is
actually a combination of 2nd order cobalt, a small amount of fluorine, and
maybe some elastic scatter. However regardless of its origin, it is much
weaker than the Co L3/L2-M4 and the shoulder on that peak, so I don't think
they are related.


> The peaks in the fit seem to be consistently to the left of the ones in
> the data.  Any idea why?  Was the energy scale not a fit parameter?
>

My guess is that they are shifted because they are trying to fit the
intensity of the shoulders.

In order to analyze the 'extra' peaks, it may be useful to optimize the fit
> to the main peaks, then subtract to get the residual, including
> the 'extras'.  What would happen if you added Fe and maybe N to the fit?
>

Yes, I do get a good fit when I do that. In fact, that is what I originally
thought before I started to look at the data a little more closely and
ruled out Fe in the ways explained above. I'll take a look at the
residuals, but I need to play around with the fit to get rid of the
shifting to left of the peaks.


Thanks,
George


>
> I admit that I've never done soft X-ray fluorescence, so my ideas come
> from hard X-ray experience, which may not totally apply.
> mam
>
>
> On 3/31/2014 3:01 PM, George Sterbinsky 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
>>

Re: [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
 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  630-252-0431
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Re: [Ifeffit] Peaks in MCA spectra

[Matthew Marcus]

Hmmm.  An interesting brain-teaser.  Scaling very roughly from the graph, the 
mystery peak shows up at about 715eV.  This is consistent with Fe.
Could there be a background from Fe somewhere in the detection systen?  The 
intensity between the O and Co peaks doesn't really look like
a peak, but just like a background tail, perhaps from the strong Co peak.  It 
seems to be a little to the right of where N (from, for instance,
a kapton window) would be.  I could be wrong, in which case my Fe theory goes 
away.  Now, I thought about an escape peak from Si, but I suspect that
this is really small down here and anyway, the Si L-fluorescence energy is 92eV 
or so, and the extra peak is about 55eV below the main peak.

Did you duck below the Fe edge to see if the peak to the L of Co goes away?

The peaks in the fit seem to be consistently to the left of the ones in the 
data.  Any idea why?  Was the energy scale not a fit parameter?
In order to analyze the 'extra' peaks, it may be useful to optimize the fit to 
the main peaks, then subtract to get the residual, including
the 'extras'.  What would happen if you added Fe and maybe N to the fit?

I admit that I've never done soft X-ray fluorescence, so my ideas come from 
hard X-ray experience, which may not totally apply.
mam

On 3/31/2014 3:01 PM, George Sterbinsky 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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[Ifeffit] Peaks in MCA spectra

[George Sterbinsky]

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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[Ifeffit] Vacancy

[Khalid, Syed]

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LABEX PLAS@PAR, University Pierre et Marie CURIE
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Vacancy at LABEX PLAS@PAR
Three years fixed term vacancy, from May 2014
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