Mingliang, For more information on self-absorption, read the links found here: http://www.xafs.org/Experiment/OverAbsorption
This includes a link to a discussion on the mail list about self-absorption and how to understand it. -Richard 2010/1/29 Welter, Edmund <edmund.wel...@desy.de> > Dear Mingliang, > > Thanks for your explain. But I am still puzzled. Following your >> instruction, I performed *Self-Absorption *for the samples, choosing >> *XANES(Fluo) *from *algorithm*, but I did not know the real meaning of >> *Formula*. Does it mean the species formula which I wish to detect? Or the >> whole composition of the sample? >> > The composition of the whole sample. You do not need to know the species, > you "only" need to know thew elemental composition of your sample. A hairy > task if you don't digest your sample completely and put it into an ICP-MS or > anything like that. I don't know which precision is necessary to yield > usable results. Maybe someone else tested or calculated it. > In principle the matrix produces just a smooth background absorption and > knowing the composition it is easy to calculate the amount of this > absorption using tabulated scattering factors. But it should also be > possible to determine this background experimentally, for instance > extrapolating the background before the edge. But for this you would need a > blank spectrum to "calibrate" the mued value. > If you follow the link to the "document section" in Athena a very > instructive webpage will open and among other things you will find citations > of the original papers underlying the four different algorithms used for the > self absorption correction. > > > I used natural mineral to interact with selenite, then I wish to check the >> speciation of the selenium reaction product. For my case, should I fill the >> blank with Se or FeSe or both of them (because currently I do not know the >> exactly containing speciation)? >> > As said before, the "chemical Formula" of the entire sample. "Chemical > formula" here does not mean you have any kind of chemical bond, it just > means molar ratio. > > For the angles, normally are they 45? Becuase I measured the samples in >> fluorescence mode, and didnot measure the angles between sample, incoming >> beam and detector. I tried to input *Se *in the *Formula* entry, and select >> 45 for the angles, then I got a corrected spectra which was higher than the >> original one. >> > > 45 degrees is the usually used geometry. However, the program asks for the > geometry which you used not for the usually used geometry! If it was 45 > degrees than its fine. Obviously the algorithm does what it is supposed to > do, it amplifies the oscillations. But this does not mean that the result is > correct! It will only be "correct" if the input parameters were correct. > > But when I using this spectra for LCF analysis, I still could not got >> overlapping line for the fitted spectra with the sample spectra... >> > Using pure Se in the self absorption correction is definitely wrong, it > overestimates the effect dramatically. Actually the effect more or less > disappears for diluted samples. But even if the spectra were properly > corrected I can not see any reason why the used set of reference spectra > MUST fit the spectrum of your sample. There are many possible reasons for > this, the most likely is that you are missing one or more important > reference spectrum (Or you are not using enough references/free parameters > in your fit...;-) > > Best regards, > Edmund Welter > > _______________________________________________ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > >
_______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit