[Ifeffit] Question about Io during data collection

[Matt Newville]

Hi Todd,



On Thu, Mar 31, 2016 at 9:27 AM, Luxton, Todd <Luxton.Todd at epa.gov> wrote:

> All:
>
>
>
> Recently our group was at the APS collecting Pb L(III) spectra on an ID
> line using quick XAFS.  Data was collected from -200 to +800 eV for Pb
> L(III) at 0.2 eV steps with a count rate of 0.025 seconds.  Each scan took
> about 2.5 minutes to complete.   During our measurements we began noting
> issues with the linearity of scans collected in the extended region of the
> same sample.  After poking around in the data we noticed that Io was not
> linear throughout the measurement for a portion of the scans.  This was not
> always the true (see Athena project attached to the email, and images
> attache to the email).  I was always under the impression that Io should
> ideally remain linear throughout the energy region scanned, or at least
> remain unchanged between replicate scans.  We worked with the beamline
> scientists to try alleviate/fix the issue, but it kept persisting.   I
> realize this is not an IFEFFIT issue, but I was hoping someone might be
> able to help me understand what was going on and why this was happening?
> If I have not included enough information for the question please let me
> know.
>

It's a little hard to see much detail from the blurry images, but I0 is
definitely falling in dramatically different ways for the different scans.

My guess (from similar experience trying to do continuous XAFS scans at
another APS ID line) is that the undulator is not tracking with the
monochromator correctly in some of the scans. This tracking is definitely
challenging for us. I suspect MR-CAT does small, constant energy steps per
time point, but I'm not sure of this.  There is no way I can scan my
undulator at 25 ms per energy point.

What I see (and from talking to the undulator control folks at the APS),
there is about a 0.5 second "settling time" for the devices during which
time is pointless to ask for another move.
So, when I do QXAFS at 13-ID-E, I set up "a normal XAFS scan", with 0.05
Ang^-1 steps and move the mono energy between these points in a fixed time
-- so doing a ~4 eV move per time point at 10 Ang^-1.   But I find that if
I try to go faster than about 100 ms per point, I see oscillations in I0,
as the undulator  lags behind and then tries to catch up.   That can put
oscillations in I0, but what you're seeing is it just falling off, like the
tracking just isn't working.

For those not at the APS, the undulators cannot be hardware-synchronized
(perhaps that is "yet", but we've been waiting a long time).  One can move
the undulator at a constant rate in gap (mm) velocity, and then try to
synchronize the mono energy to that... I don't do that and I'm pretty sure
that MR-CAT does not either.

Anyway, I would suspect the tracking of the undulator.

Hope that helps,

--Matt
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