On Wednesday 18 February 2004 09:34 pm, Stanislav Stoupin wrote:
Hello all,
I would appreciate if somebody could explain the meaning of imaginary energy correction (Ei parameter). So far I have not found any description of the parameter. If anyone knows of any example on how to use it, please let me know.
Stanislav, There are a lot of things that lead to broadening of the XAS spectrum. Among them are : * The energy resolution of the monochromator * The size of the entrance slits + the divergence of the beam * The mean free path of the photoelectron (i.e. the exp(-2R/lambda(E)) term), which includes such physics as the core hole lifetime, the plasmon excitation, and so on In an ideal situation, the mono resolution would be small compared to the core-hole lifetime and so could be ignored. The entrance slits would be set to optimize the resolution, i.e. to make the effect of the beam divergence small compared to the core-hole lifetime. In that situation, only the lamdba(E) term would contribute to the broadening of the spectrum. lambda(E) is one of the things that we rely upon Feff to compute for us (along with the photoelectron scattering amplitude and phase shift). Again in the ideal world, Feff would compute that correctly within our experimental uncertainties. If all of those things are true, then there is no need for an imaginary energy. Or. more specifically, if you allow an imaginary energy to float in a fit, it will be consistent with zero. Alas, in the world that I live in, I cannot rely on all of that. I occassionally do something (accident, sleepiness, whatever) in my experimental set up that affects the resolution. And Feff does not always compute lambda(E) reliably. Indeed, the folks who write Feff have been working hard of late to improve the loss terms in the calculation. Consequently, it is sometimes prudent to consider an imaginary energy in a fit. So, what does that term mean? Well, in the formalism used by Feff and Ifeffit, the broadening terms are expressed in energy units. And it's imaginary because its a loss term. Hence the name. Because the term in the exafs equation that contains lambda has an R dependence, it effects the data differently than the amplitude terms, S02 and N. Upon examining your data and the statistical results of the fits, you may be able to tell the difference between an amplitude effect and a lambda effect. Finally, I'll direct you to a past discussion on the ifeffit mailing list. In November of 2002 there was an extensive discussion about the correlations between S02, sigma^2, and the imaginary energy. Go here: http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2002-November/ and look for the threads labeled "Debye factor and S02 correlation" Hope that helps, B -- ********* PLEASE NOTE MY NEW PHONE, FAX, & ROOM NUMBERS ****************** Bruce Ravel ----------------------------------- ravel@phys.washington.edu Code 6134, Building 3, Room 405 Naval Research Laboratory phone: (1) 202 767 2268 Washington DC 20375, USA fax: (1) 202 767 4642 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/