[Ifeffit] normalization methods

Wed May 15 13:13:10 CDT 2013

Hi Matthew,

On Wed, May 15, 2013 at 1:20 PM, Matthew Marcus <mamarcus at lbl.gov> wrote:

> You say that the flipping difference (p - n) is 0 in pre-edge and far
> post-edge regions, which is as it should be, but then say that the
> slopes of p- and n- post-edges, considered separately, are different.  I
> must be misunderstanding because those two statements would seem to be
> inconsistent.

Sorry, I think my wording wasn't particularly clear here. What I should
have said is:

"The goal then is to subtract the *normalized* XAS measured in a positive
field (p-XAS) from *normalized* XAS measured in a negative field (n-XAS)
and get something (the XMCD) that is zero in the pre-edge and post-edge
regions. *However, standard normalization does not give this result*"

Italics indicate new text.

> I wonder if the sensitivity of the TEY changes with magnetic field because
> of the effect of the field on the trajectories of
> the outgoing electrons, which would explain the differing curves.

I would agree, I think the effect of the magnetic field on the electrons is
the likely source of the differences in background.

> A possibility - if you divide the p-XAS by n-XAS, do you get something
> which is a smooth curve everywhere but where MCD is expected?  Does that
> curve match in pre- and far post-edge regions?

No, after division of the p-XAS by the n-XAS, both the pre and post-edge
regions are smooth, but one would need a step-like function to connect
them. I've attached a plot showing the result of division.

If that miracle occurs,
> then perhaps you could fit that to a polynomial, except in the MCD region,
> then divide the p-XAS by that polynomial, to remove the effect of
> the differing sensitivities.
>
> There are people here at ALS, such as Elke Arenholz <earenholz at lbl.gov>,
> who do this sort of spectroscopy.  I suggest asking her.
>         mam

Thanks for the suggestion and your reply.

George

>
> On 5/15/2013 9:58 AM, George Sterbinsky wrote:
>
>> The question of whether it is appropriate to use flattened data for
>> quantitative analysis is something I've been thinking about a lot recently.
>> In my specific case, I am analyzing XMCD data at the Co L-edge. To obtain
>> the XMCD, I measure XAS with total electron yield detection using a ~70%
>> left or right circularly polarized beam and flip the magnetic field on the
>> sample at every data point. The goal then, is to subtract the XAS measured
>> in a positive field (p-XAS) from XAS measured in a negative field (n-XAS)
>> and get something (the XMCD) that is zero in the pre-edge and post-edge
>> regions. I often find that after removal of a linear pre-edge, the spectra
>> still have a linearly increasing post edge (with EXAFS oscillations
>> superimposed on it), and the slope of the n-XAS and p-XAS post-edge lines
>> are different. In this case simply multiplying the n-XAS and p-XAS by
>> constants will never give an XMCD spectrum that is zero in the post edge
>> region. There is then some component of the
>>
> XAS background that is not accounted for by linear subtraction and
>> multiplication by a constant. It seems to me that flattening could be a
>> good way to account for such a background. So is flattening a reasonable
>> thing to do in a case such as this, or is there a better way to account for
>> such a background?
>>
>> Thanks,
>> George
>>
>>
>> On Wed, May 15, 2013 at 11:41 AM, Matthew Marcus <mamarcus at lbl.gov<mailto:
>> mamarcus at lbl.gov>> wrote:
>>
>>     The way I commonly do pre-edge is to fit with some form plus a
>> power-law singularity representing the initial rise of the edge, then
>>     subtract out that "some form".  Now, that form can be either linear,
>> linear+E^(-2.7) (for transmission), or linear+ another power-law
>>     singularity centered at the center passband energy of the
>> fluorescence detector.  That latter is for fluorescence data which is
>> affected by
>>     the tail of the elastic/Compton peak from the incident energy.
>>  Whichever form is taken gets subtraccted from the whole data range,
>> resulting
>>     in data which is pre-edge-subtracted but not yet post-edge
>> normalized.  The path then splits; for EXAFS, the usual conversion to
>> k-space, spline
>>     fitting in the post-edge, subtraction and division is done, all
>> interactively.  Tensioned spline is also available due to request of a
>> prominent user.
>>     For XANES, the post-edge is fit as previously described.  Thus,
>> there's no distinction made between data above and below E0 in XANES,
>> whereas
>>     there is such a distinction in EXAFS.
>>              mam
>>
>>
>>     On 5/15/2013 8:25 AM, Matt Newville wrote:
>>
>>         Hi Matthew,
>>
>>         On Wed, May 15, 2013 at 9:57 AM, Matthew Marcus <mamarcus at lbl.gov<mailto:
>> mamarcus at lbl.gov>> wrote:
>>
>>             What I typically do for XANES is divide mu-mu_pre_edge_line
>> by a linear
>>             function which goes through the post-edge oscillations.
>>             This division goes over the whole data range, including
>> pre-edge.  If the
>>             data has obvious curvature in the post-edge, I'll use a
>> higher-order
>>             polynomial.  For transmission data, what sometimes linearizes
>> the background
>>             is to change the abscissa to 1/E^2.7 (the rule-of-thumb
>> absorption
>>             shape) and change it back afterward.  All this is, of course,
>> highly
>>             subjective and one of the reasons for taking extended XANES
>> data (300eV,
>>             for instance).  For short-range XANES, there isn't enough
>> info to do more
>>             than divide by a constant.  Once this is done, my LCF
>> programs allow
>>             a slope adjustment as a free parameter, thus muNorm(E) =
>>             (1+a*(E-E0))*Sum_on_ref{x[ref]**__*muNorm[ref](E)}.  A sign
>> that this degree of
>>
>>             freedom
>>             may be being abused is if the sum of the x[ref] is far from 1
>> or if
>>             a*(Emax-E0) is large.  Don't get me started on overabsorption
>> :-)
>>                       mam
>>
>>
>>         Thanks -- I should have said that pre_edge() can now do a
>>         victoreen-ish fit, regressing a line to mu*E^nvict (nvict can be
>> any
>>         real value).
>>
>>         Still, it seems that the current flattening is somewhere between
>>         "better" and "worse", which is unsettling...  Applying the
>>         "flattening" polynomial to the pre-edge range definitely seems to
>> give
>>         poor results, but maybe some energy-dependent compromise is
>> possible.
>>
>>         And, of course, over-absorption is next on the list!
>>
>>         --Matt
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