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. 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. 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? 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>, who do
this sort of spectroscopy. I suggest asking her.
mam
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 t
he
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>> 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>> 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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