I'm not sure what 'flattening' means. Does that mean dividing by a linear or
other polynomial function, fitted to the post-edge?
mam
On 5/15/2013 1:43 PM, George Sterbinsky wrote:
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 <mamar...@lbl.gov
<mailto:mamar...@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 <mamar...@lbl.gov
<mailto:mamar...@lbl.gov> <mailto:mamar...@lbl.gov <mailto:mamar...@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 <earenh...@lbl.gov
<mailto:earenh...@lbl.gov> <mailto:earenh...@lbl.gov <mailto:earenh...@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 the
n 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 <mamar...@lbl.gov <mailto:mamar...@lbl.gov>
<mailto:mamar...@lbl.gov <mailto:mamar...@lbl.gov>> <mailto:mamar...@lbl.gov <mailto:mamar...@lbl.gov>
<mailto:mamar...@lbl.gov <mailto:mamar...@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 <mamar...@lbl.gov
<mailto:mamar...@lbl.gov> <mailto:mamar...@lbl.gov <mailto:mamar...@lbl.gov>> <mailto:mamar...@lbl.gov
<mailto:mamar...@lbl.gov> <mailto:mamar...@lbl.gov <mailto:mamar...@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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