Hi Gabriel, My two cents worth:

When using ATHENA, a circle appears approximately where the edge should be,

That circle you refer to is placed at the value of E0, i.e. the energy value which is in the E0 field in the upper left of Athena's main window. If you change that number by hand, you will change the location of that circle in the next plot. By default, Athena uses Ifeffit's determination of E0. The algorithm in Ifeffit looks for the maximum in the first derivative of mu(E). As I recall, there are some sanity checks to make sure that there are a few monotonically increasing points before the maximum and a few monotonoically decreasing points after (i.e. that it is a *peak* in the derivative and not an outlying point). This algorithm is usually pretty good, but can certainly fail. A known example of a common situation where it fails is in determining E0 for a zirconium foil. In that spectrum, the maximum of the first derivative is a peak several volts after the point that physics tells us is the Fermi energy. In that case, the "first large peak" in the derivative spectrum rather than the "largest peak" should be chosen. Ifeffit's algorithm may not, then, be accurate but it is predicatable, repeatable, and easy to explain. Athena has a configuration option that overrides this behavior and instead always sets E0 to the energy value at which the spectrum is 1/2 the edge step. This also is inaccuate but also is predicatble and easy to explain.

whereas a different result for the edge position appears when analyzing with UWEXAFS package – by viewing the maximum point of the first derivative of mu(E). I would also like to know, what is the correct definition of the edge position.

I would say that there are two possible answers to this question. One is to pick a standard and treat it rigorously consistently. In that case, you may make a small mistake in the *absolute* energy, but you will measure energy shifts between spectra correctly. One way you might do this is to always measuring reference spectra. Align the reference spectra then the data spectra will be internally consistent. I don't see how you can go wrong doing that. The second answer is somewhat more of a physics lecture. The edge is the Fermi energy. The Fermi energy is the energy at whih the filled states end and the empty states begin at 0 Kelvin. If you can imagine deconvolving your measured spectrum by a Lorentzian whose width considers all the physical broadeing terms (self-energy, mean free path, monochromator resolution, slit height, and so on), then there will be a sharp rise somewhere near the peak of the first derivative. That sharp rise would be the beginning of the unoccupied states adn the beginning of the allowed transition from the deep core states. That energy can be computed by FEFF8 and other theory programs and almost always shows up near the peak of the first derivative. Thus the use of the first drivative is a handy, emnpirical way of approximating the Fermi energy. But it's not the Fermi energy -- merely close to the Fermi energy. HTH, B -- Bruce Ravel ----------------------------------- ravel@phys.washington.edu Code 6134, Building 3, Room 222 Naval Research Laboratory phone: (1) 202 767 5947 Washington DC 20375, USA fax: (1) 202 767 1697 NRL Synchrotron Radiation Consortium (NRL-SRC) Beamlines X11a, X11b, X23b National Synchrotron Light Source Brookhaven National Laboratory, Upton, NY 11973 My homepage: http://feff.phys.washington.edu/~ravel EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/