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

[Matt Newville]

Hi Mauro,

On Tue, 11 May 2004 mauro at rulp.org wrote:

> I'm dealing with the selfabsorption correction in Mo/Si
> multilayers with standing-wave-assisted EXAFS. In the fluorescence
> formula I have to multiply mu(E) for the thickness of the sample
> to obtain a pure number, so I need (cm^-1)*(cm).

Oh.  You're original question did not mention self-absorption
corrections at all. You asked about converting measured XAFS to mu
in cm^-1.  In general, this conversion (that is, to an absolute
measure of absorption) is challenging for transmission data and
essentially impossible for fluorescence data.
 
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.

I'm not sure I understand 'standing-wave-assisted EXAFS'.  I'd
interpret this to mean the measurements are done near the critical
angle to control the penetration depth. Is that right?  If so, and
if you're asking for advice on self-absorption corrections, you'd
need to give more details layer thickness, and measurement geometry.

Hope that helps,

--Matt