Hi Matteo,


On Thu, May 12, 2016 at 7:11 PM, Matteo Busi <basebush@gmail.com> wrote:
Hi Bruce, Matt, and  mailing list readers,

I have one question regarding the XAFS data. I have collected absorption coefficient in fluorescence regime of a CuSO4 penta hydrate sample(however my question can be generalized to any compound). 
Through Athena I then obtained the x\mu(eV) spectra as well as \chi(k) and \chi(R). I am wondering how can I assign a unit to the x\mu values. I would like to have it in the cm^-1 or cm^2 kg^-3 unit so than I can match it with tabulated data for the Copper absorption coefficient and proceed with my studies. 


The analysis tools treat "xmu" as a dimensionless value.   To get values for the mu in cm^-1 or grm/cm^2, you'll have to convert the measured values used for XAFS analysis (say, readings from ion chambers) into actual X-ray intensities, and try to account for anything else in the beam path that will attenuate the X-rays.  For measurements made in transmission, this can be done.   In fact, for most transmission measurements, the *change* in -ln(I0 / I1) is usually a very good measure of the change in absorption of the sample, so that if the observed jump is a value of 2, the sample really did change its absorption by Delta_xmu = 2.    But the absolute values for xmu are definitely not in meaningful units.  Typically, the photo-current from the ion chamber or diode is converted to a voltage and then summed over some time period in a data acquisition system.   You'd have to know the details of that system to convert the "I0" and "I1" numbers into actual beam intensities.

For fluorescence measurements, the situation is worse. You'd need a very good measure of the solid angle of your detector, fluorescence efficiency, and the attenuation of the fluorescence getting from the sample to the detector.  Depending on the detector type, you'd also need a good assessment of the backgrounds.   It's probably not impossible, but I think it would not be easy unless that was the actual point of the experiment and you made many careful measurements of the detection system.    If you're asking this question here, you'd didn't make those measurements.

 

In case a treatment is needed for the normalized \mu , that would also suit my situation.

My idea was to divide by the thickness of the sample but I don't find a reasonable justificiation for this.


If you're measuring in fluorescence, dividing by the thickness of the sample is not what you want.
 

I really hope I made the question clear to any reader this time, and if that is not the case I apologyze in advance.



Cheers,

--Matt