Hi All,
I am a little bit surprised how my post had made a nice discussion ;)
I didn't want to discuss about reference spectra and their usefulness.
I only wanted to point that when you merge data with marked reference
groups you get inversed merged groups - in merge group reference and
in Ref merge your real data. Let users decide if it is useful or not.
For me it is! Especially when I work with many repeats, like e.g. in
QEXAFS, when the signal from sample and from reference in single scan is
to noisy.
cheers
darek
-Original Message-
From: ifeffit-boun...@millenia.cars.aps.anl.gov
[mailto:ifeffit-boun...@millenia.cars.aps.anl.gov] On Behalf
Of Scott Calvin
Sent: Friday, November 20, 2009 12:48 AM
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] Bug in Athena?
On Nov 19, 2009, at 2:59 PM, Matt Newville wrote:
For this case, wouldn't it be better to measure the reference
separately to determine the chemical shift, and not rely on the
reference channel for this purpose?
How often is the reference channel both noisy AND improved
by merging?
That would imply a transmission measurement that was poor due to low
flux. But if this is because the sample is thick as you
suggest, the
x-rays hitting the reference could be dominated by
harmonics, and the
reference data may just be bad, not noisy due to counting statistics.
It's a good point. But pick your poison. When I am trying to be
careful about chemical shift, I don't trust that the mono won't just
happen to skip a step between measuring the standard separately and
measuring the sample. So I do both. I measure a standard in
the sample
channel, with a reference in the reference channel. I then leave the
reference in the reference channel, and put my sample in. If the
sample is a reasonable thickness for transmission, but a bit on the
high side (say 2.3 absorption lengths), the photon count is down
pretty far by the time it gets to the reference. The reference
is also
often the worst detector and amplifier that a line has, as the good
stuff is used for I0, It, and If. So the reference channel may well
have a considerable amount of random noise which can be improved by
merging.
If that's the case, and if my sample appears to be suffering no beam
damage (scans when aligned, lie on top of each other), then I align
used the sample data. I then merge the sample data and the reference
data. By comparing the sample to the reference and the previous scans
where I measured the standard to the reference, I can see if there's
been any energy shift between scans. As far as harmonics, this
procedure should detect them. If the merged reference looks different
from sample to sample (including the case where a standard was
also in
the sample channel), that suggests that there are issues with
harmonics. If those issues move the first peak of the first
derivative, I know they're going to affect my determination of
chemical shift. Also, if I get a nonzero chemical shift from this
procedure for the standard, I know there's an issue. If not, they're
not a problem.
The net result is that I have good confidence that I'm getting
accurate chemical shifts, as loss of energy calibration, harmonics,
and noise should all become evident by this procedure.
I'm not recommending this procedure over others; it's just what I do
in some cases. But it doesn't seem like an unreasonable
procedure to me.
--Scott Calvin
Sarah Lawrence College
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