[Ifeffit] Absorption-coefficient data repository?

[Matt Newville]

Hi Damon,


On Mon, Jan 27, 2014 at 12:30 PM, Damon Turney <dturney at ccny.cuny.edu>wrote:

> Dear XAFS Community,
>
> I'm looking for values of absorption coefficient (or mass absorption
> coefficient) of ZnO in the XANES region of the Zn edge, but all the
> publications give XANES plots in "arbitrary units", so I have no way
> of knowing the absorption coefficient for ZnO.
>
> Does anybody know a data repository where absorption coefficients are
> collected?
>
>
Scott and Bruce gave pointers to where tables of absorption coefficients
can be found.    These are per-atom, and ignore any fine structure or
chemistry effects.  XANES measurements are typically labeled as "arbitrary
units" because it is both surprisingly tricky and unnecessary for (most)
XANES or EXAFS analysis to actually have an accurate absolute measure of
the absorption coefficient (either in mass/area or inverse length).

We can easily measure XANES features at ~1% of the edge jump.   But to get
this signal we've (generally, without loss of generality) sampled the
intensity of the X-ray beam with an ion chamber that typically absorbs a
few percent of the X-rays upstream of our sample, and an ion chamber
downstream of our sample (that might absorb more of the X-rays hitting
it).  The beam travels through, and is partially absorbed by, any air paths
and other material (plastics, sample containers) as well as our sample.
Typically, the sample attenuates the beam by between a factor of 2 and a
factor of 100.  The additional stuff in the beam may attenuate by this much
as well.

We typically take "Current from upstream ion chamber" as I0 (it is NOT the
absolute flux hitting the sample) and "Current from downstream ion
chamber"  I1 (again, NOT the flux leaving the sample), and call -log(I1/I0)
"mu" -- it is not in any real units.

To get the real attenuation (thickness  * absorption coefficient) for the
sample, we'd have to take many things (some rather subtle) into account.
And because attenuation is exponential, you actually have to be pretty
careful to get this right.  On the other hand, for analyzing XANES and
EXAFS we really only care about the change in the absorption coefficient
from the element of interest, and are willing to normalize to 1 atom of the
absorbing species.   This can be tricky enough, but it means all that other
stuff cancels out.    To get the absorption coefficient, you have to divide
by the sample thickness (and/or know the sample's density).   But sample
thicknesses are typically ~10 microns, and if you think you know the
average thickness of a sample to 1% of 10 microns, you must have made a
very uniform and precise sample, at least compared to what most people
study with EXAFS.

That's not to say it can't be done.  A few people, notably Chris Chantler's
group at U Melbourne have made some such measurements.   They're
challenging, and most beamlines are not set up to do this easily.

Out of curiosity, why do you need to know the (absolute) values for
absorption coefficient for ZnO?

-- 
--Matt
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20140127/1c09f99e/attachment.html>