[Ifeffit] normalization methods

Matthew Marcus mamarcus at lbl.gov
Wed May 15 14:04:26 CDT 2013


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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