[Ifeffit] Transmission EXAFS sample

[Welter, Edmund]

Dear Jatin,

the optimum mued of 2.x is not just derived by simple photon counting 
statistics. As Matt pointed out, for transmission measurements at a 
synchrotron beamline in conventional scanning mode this is seldom a 
matter. Nevertheless, one should avoid to measure subtle changes of 
absorption at the extreme ends, that is, transmission near 0 % or 100 %. 
In optical photometry this is described by the more or less famous 
"Ringbom plots" which describe the dependency of the accuracy of 
quantitative analysis by absorption measurements (usually but not 
necessarily in the UV/Vis) from the total absorption of the sample.

This time the number is only near to 42, the optimum transmission is 
36.8 % (mue = 1). So, to achieve the highest accuracy in the 
determination of small Delta c (c = concentration) you should try to 
measure samples with transmissions near to this value (actually the 
minimum is broad and transmissions between 0.2 and 0.7 are ok). In our 
case, we are not interested in the concentration of the absorber, but we 
are also interested in (very) small changes of the transmission resp. 
absorption in our samples. Or, using Bouger, Lambert Beer's law, in our 
case mue (-ln(I1/I0) is a function of the absorption coefficient (mue0). 
The concentration of the absorber and the thickness (d) of the sample 
are constant.

-ln(I1/I0) = mue0 * c * d

But then: If the optimum is a mue between 0.35 and 1.6 why are we all 
measuring successfully (ok, more or less ;-) using samples having a mue 
between 2 and 3? ...and 0.35 seems desperately small to me! Maybe sample 
homogeneity is an issue?

Cheers,
Edmund Welter