[Ifeffit] N K-edge XANES, normalization parameters

[Bruce Ravel]

On Monday 14 November 2005 14:24, Peter Leinweber wrote:
> dear all,
> i am a new XANES and ATHENA user. i didi N K-edge XANES of soil samples.
> while feeling very good with data import, deglitching, truncation,
> merging etc. i am insecure about parameters for normalization.
> in the attached project i did truncation of data before 395900 (it is
> milli-eV), because these seem to be invalid, and i set an E-shift of
> -900 sinc the beamline had -9.9 eV off compared to the N K-edge of N2
> gas. in this way i should be right back on scale ? the i did set the
> pre-edge parameters manually at start of the curve, and around 397000
> since non of the previously measured N standard substances had edge
> steps below this value.
> however, then i am insecure how to set the other parameters for
> normalization properly. depending on how i play with the parameters
> sometimes i get normalized spectra which look pretty reasonable,
> sometimes the curves disappear completely from the graphics window, and
> sometimes i get curves of a shape never senn in the literature. is there
> easily understandable advice for the setting of the normalization
> parameters available; or can anybody more experienced check my project
> and give me recommendations.

Hi Peter,

I didn't write Athena thinking about this energy range, so you are certainly
looking at some of its rough edges.

First off, Athena's unit of currency is eV.  Your data seems to be in meV.
Background removal is certainly not going to work correctly.  Other features
of the program might also make certain assumptions about the data being in eV
units.  One thing you might do to make the program behave less confusingly
might be to convert the data files to eV before reading them into Athena.

If you must treat the data in meV, then I would suggest reading them in as
xanes(E) data rather than mu(E) data.  This will turn off the background
removal, which is likely to be the most confused part of the program by the
meV units.  This can be done in the column selection dialog or by selecting
"Change record type" from the Group menu.  Background weirdness due the meV
units seems the most likely reason that the curves might disappear completely
from view.

I got a fairly reasonable normalization using your eO and [-3695:1910] for
the pre-edge range and [3062:10200] for the normalization range.  I just chose
values using the little blue pluck buttons which seemed to be in a range that
would reasonably represent the pre- and post-edge regions.

In general normalization is kind of tough for these low energy edges.  Using
the CLnorm option which Ifeffit/Athena provides might be useful, but the data
*must* be in eV units for that to work.

B


-- 
 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

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