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

[Matt Newville]

Dead Mauro,

> I would like your opinion on the procedure to convert the experimental
> mu_exp(E) in arbitrary units (counts) to mu(E) in cm^-1.
 
Why you want mu in cm^-1?  This is not necessarily easy to get from
an XAFS measurement.  

For transmission data, what is recorded is counts in an ion chamber
or PIN diode that sample the x-ray intensity before and after the
sample.  Calling these intensities V0 and V1, we typically do the
analysis with log(V0/V1).  This would be _proportional_ to x*mu, but
not an absolute measure of x*mu.  To get true measure of absorbance,
you'd need to include the ion chamber efficiencies, amplifier gains,
and any absorption losses (windows, air) between the two ion
chambers as well as the sample thickness to convert the ion chamber
currents into a direct measure of mu.  A good start would be to
measure log(V0/V1) on a blank sample -- something that is rarely
done for XAFS.  Even then, the average sample thickness would need
to be known to pretty high precision to get an absolute measure of
mu, as log(I0/I1) is xmu (which is unitless), not mu(E).  Finally,
mu(E) in cm^-1 has to assume a density for the material.  The result
is that log(V0/V1) is pretty far removed from an absolute measure of
mu in cm^-1.

Ifeffit does include the Cromer-Libermann data to help with the
corrections as you outlined, but it's not enough to get you to cm^-1
by itself.  Perhaps you could explain what is you are trying to do
and why?  Thanks,

--Matt

On Fri, 7 May 2004 mauro at rulp.org wrote:

> Hi,
> 
> I would like your opinion on the procedure to convert the experimental
> mu_exp(E) in arbitrary units (counts) to mu(E) in cm^-1.
> 
> This is my "student" procedure:
> 
> 1. take mu_exp(E) - with EXAFS - spectrum in counts
> 
> 2. take mu_th(E) - without EXAFS - spectrum in cm^-1 (from MacMaster or XOP
> software)
> 
> 3. plot 1+2
> 
> 4. while mu_exp(E) != mu_th(E) do (by hand!)
> 
>    4a. mu(E) = A * mu_exp(E) + B
> 
> When the two spectra coincide (more or less) I assume that the mu(E) is
> converted in cm^-1 units. Is this right?
> 
> Does it already exist a software that makes this work automatically? Could
> be interesting to implement it in Athena?
> 
> Thanks,
> M.
> 
> --
>   Mauro Rovezzi <mauro at rulp.org> - Physics student
>   University of Rome "Tor Vergata", ESRF Beamline BM8 - GILDA 
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