[Ifeffit] normalization methods
George Sterbinsky
GeorgeSterbinsky at u.northwestern.edu
Wed May 15 11:58:53 CDT 2013
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> 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> 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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