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

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@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 - Physics student University of Rome "Tor Vergata", ESRF Beamline BM8 - GILDA _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit