[Ifeffit] normalization methods
George Sterbinsky
GeorgeSterbinsky at u.northwestern.edu
Wed May 15 15:43:42 CDT 2013
By standard normalization, I meant subtraction of a linear pre-edge and
multiplication by a constant. If this treatment is applied to the XAS
spectra before subtraction, one does not obtain an XMCD spectrum that goes
to zero in the post edge region for the data I described. As you noted,
that is what would be expected given the p-XAS and n-XAS have different
slopes in the post-edge region.
On the other hand, standard normalization + flattening does result in pre
and post-edge regions that go to zero, again as one might expect. So
perhaps, the background modeled by standard normalization + flattening is
an accurate representation of the real background in some cases and can be
used in quantitative analysis. Is there reason to believe that cannot be
the case?
Thanks,
George
On Wed, May 15, 2013 at 3:04 PM, Matthew Marcus <mamarcus at lbl.gov> wrote:
> OK, I guess I don't know what 'standard normalization' is. It looks from
> the quotient that you'll need some sort of curved post-edge.
> I guess the division didn't work because the electron energy distribution
> is different pre- and post-edge, so the magnetic effects are
> different and vary across the edge. Thus, the shapes of the MCD peaks
> will be at least a little corrupted even if the pre- and post-edge
> spectra are taken into account. I don't know what to do about this. Did
> you try asking Elke?
> mam
>
>
> On 5/15/2013 11:52 AM, George Sterbinsky wrote:
>
>> Hi Matthew,
>>
>>
>>
>> On Wed, May 15, 2013 at 1:20 PM, Matthew Marcus <mamarcus at lbl.gov<mailto:
>> 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 (before any normalization),
>> 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 <mailto: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> <mailto: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> <mailto: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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