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
_______________________________________________
Ifeffit mailing list
Ifeffit@millenia.cars.aps.anl.gov
http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
