[Ifeffit] Peaks in MCA spectra

George Sterbinsky GeorgeSterbinsky at u.northwestern.edu
Wed Apr 2 14:56:19 CDT 2014


Matt and Matthew,

Thanks for your suggestions. I will try to collect some spectra well above
the Co L-edge and see if they provide additional insight.

Best,
George


On Tue, Apr 1, 2014 at 6:07 PM, Matthew Marcus <mamarcus at lbl.gov> wrote:

> I don't think so.  This may be a better question for M. Newville, or
> whoever wrote the program.
>         mam
>
>
> On 4/1/2014 9:56 AM, George Sterbinsky wrote:
>
>> Hi Matthew,
>>
>> Hephaestus shows the strength of the Ll emission to be about 10% of the
>> La2 emission. When fitting the spectrum as a sum of Gaussians, I find that
>> the area of of the Gaussian used to fit the Ll is 24% of that used to fit
>> the La2. What would cause the ratio determined from tabulated data to
>> differ from that found in the data I collected? Am I misunderstanding the
>> meaning of the "strength" value in Hephaestus?
>>
>> Thanks,
>> George
>>
>>
>> On Tue, Apr 1, 2014 at 11:45 AM, Matthew Marcus <mamarcus at lbl.gov<mailto:
>> mamarcus at lbl.gov>> wrote:
>>
>>     OK, I think the "other Matt" has solved it - it's the L1 peak.  I was
>> confused by an inaccurate attempt at reading the energy scale.
>>     Also, I somehow didn't read correctly the branching ratio from
>> Hephaestus.
>>              mam
>>
>>
>>     On 3/31/2014 8:57 PM, George Sterbinsky wrote:
>>
>>         Hi Matt,
>>
>>         Thanks for your reply. Please see below.
>>
>>         On Mon, Mar 31, 2014 at 8:10 PM, Matt Newville <
>> newville at cars.uchicago.edu <mailto:newville at cars.uchicago.edu> <mailto:
>> newville at cars.__uchicago.edu <mailto:newville at cars.uchicago.edu>>> 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
>>
>>              On Mon, Mar 31, 2014 at 5:01 PM, George Sterbinsky
>>              <GeorgeSterbinsky at u.__northwestern.edu <mailto:
>> GeorgeSterbinsky at u.northwestern.edu> <mailto:GeorgeSterbinsky at u.__n
>> orthwestern.edu <mailto: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 <
>> http://cars.uchicago.edu> <http://cars.uchicago.edu>> 630-252-0431 <tel:
>> 630-252-0431> <tel:630-252-0431 <tel:630-252-0431>>
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