[Ifeffit] [HOWTO] converting experimental mu(E) in cm^-1

[Matt Newville]

On Thu, 13 May 2004 mauro at rulp.org wrote:

> > But to do the self-absorption corrections on a well-characterized
> > sample, you can use tabulated cross-sections (using barns/atoms is
> > HIGHLY recommended over cm^2/gr or cm^-1 unless you have a pure
> > compound: densities do not add!), and scale the XAFS wiggles to the
> > edge jump of the absorbing atom.  That assumes that the tabulated
> > value for mu are correct except that they ignore the XAFS.
> 
> Ok, this is what I normally do "by hand". The first part of my question - if
> this is right - has been widely answered by you and Tsu-Chien. The second
> part concerned if someone has already written a routine that can match the
> tabulated cross-sections and the XAFS one automatically and then write out a
> file containing the scaled XAFS.

I don't know if this exists right now, but it wouldn't be hard and
would seem to help all self-absorption corrections. Currently,
Ifeffit's bkg_cl()  does the opposite of this: scaling CL
calculations to match experimental data for background subtraction
and normalization.  It would not be hard to have this report the
factors used so that the XAFS could be scaled back to the CL f".  
The CL values for f" are tabularized in units of electrons (as
natural for scattering) but that can be scaled to barns/atom.  It
might be better to use the McMaster or Elam tables.

This would not be too difficult to do with Bruce's perl modules or
in Ifeffit itself.  I'll put this on the Ifeffit to do list, but you
might be able to roll something quick-n-dirty like this yourself
faster than I get it in, especially since you're only working with
one system....  I think all you'd have to do is scale the normalized
XAFS to the edge jump from the tables.

> Yes, you are right. For standing-wave-assisted EXAFS I mean fluorescence
> measurements done at gracing-incidence. In multilayers a standing wave field
> occurs for reflection at the first Bragg reflection order, so a defined
> shift of the standing wave field position leads to different weights of
> absorption.
>
> The experimental setup is phi+theta=90deg with a Ge detector (13 elements),
> incidence angle (theta) in the range 0.400-0.700deg. Samples are 10-periods
> multilayers with the following structure:
> 
> SiO_2(15A)/[Si(42A)/Mo(28A)]_10/Si_substrate
> 
> I would like to simulate (and then correct) the selfabsorption in these
> samples but applying the standard formula doesn't work. In my opinion, I
> think that this is due to the fact that the field intensity in the sample is
> not exponentially decaying but oscillating+exponentially-decaying (because
> it is a standing wave field).

Are you working at the x-ray reflectivity critical angle (probably a 
property solely of the SiO2 layer, though 15A is thin!), or at a 
multilayer Bragg peak?
 
Anyway, this seems wonderfully complicated -- congratulations!!
 
--Matt