A simple question. I am have been doing multiple edge fits of different materials for a while in artemis (diana). Now, I would like to analyze a material with inequivalent sites for one of the edges. I understand how to read in the data and have read in the different feff calculations (for each inequivalent site), but what I just want to confirm that artemis/diana/ifeffit will do the "right" thing, namely coherently add the contributions *within* each feff calculation and add the intensities of each feff calculation -- e.g. I don't think there is any interference between the XAFS signal on different crystallographic sites -- or even if there is it is ignorable at least for first neighbor interactions. If there is anyone out there with experience in doing this (or is the right thing done automatically?), drop me (or the list) a line. Thanks a lot! Dr. Paul Fons Senior Researcher National Institute for Advanced Industrial Science & Technology METI Center for Applied Near-Field Optics Research (CANFOR) AIST Central 4, Higashi 1-1-1 Tsukuba, Ibaraki JAPAN 305-8568 tel. +81-298-61-5636 fax. +81-298-61-2939 email: paul-fons@aist.go.jp The lines below are in a Japanese font 〒３０５−８５６８ 茨城県つくば市東１−１−1 つくば中央第４ 近接場光応用工学センター ポール・フォンス主任研究官

Thanks for the message. I guess what I wanted to say was that from what I understand due to lifetime effects the "effective" radius about the absorbing site for which the outgoing scattered wave "exists" is on the order of 2 nn (I have big lattice constant material). It would seem to me then that the inequivalent sites being physically separated by more than this distance (we are talking about crude approximations here) would not interfere. In reality, with bond lengths on the order of 3 Angstroms and 2nn along the lines of 4.5 Angstroms, my case is somewhere in between the coherent interaction and the dilute dopant -- e.g. every site is by itself -- extreme. Certainly, in the latter case it would be wrong to add the signals together coherently! What is the consensus for this sort of thing? The different E0's is a good point too. Paul On 2005/01/14, at 16:16, Scott Calvin wrote:

Hi Paul,

Actually, the inequivalent sites do interfere coherently. EXAFS chi's aren't like quantum mechanical wave functions, for which only relative phase matters. The phase gives information about E0, and that's a physically meaningful quantity. In other words, if one of the sites is producing a peak at 7200 eV and another is producing a trough, they will tend to cancel.

So yes, Artemis and Ifeffit do the right thing, but the right thing is to add everything up coherently regardless of which feff calculation it comes from.

A few other notes on this kind of fit: you must, of course, weight the amplitudes of each feff calculation by the relative number of sites of that type. Also, it is quite possible that you'll need different e0's for the different sites.

--Scott Calvin Sarah Lawrence College

A simple question. I am have been doing multiple edge fits of different materials for a while in artemis (diana). Now, I would like to analyze a material with inequivalent sites for one of the edges. I understand how to read in the data and have read in the different feff calculations (for each inequivalent site), but what I just want to confirm that artemis/diana/ifeffit will do the "right" thing, namely coherently add the contributions *within* each feff calculation and add the intensities of each feff calculation -- e.g. I don't think there is any interference between the XAFS signal on different crystallographic sites -- or even if there is it is ignorable at least for first neighbor interactions. If there is anyone out there with experience in doing this (or is the right thing done automatically?), drop me (or the list) a line. Thanks a lot!

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

Dr. Paul Fons Senior Researcher National Institute for Advanced Industrial Science & Technology METI Center for Applied Near-Field Optics Research (CANFOR) AIST Central 4, Higashi 1-1-1 Tsukuba, Ibaraki JAPAN 305-8568 tel. +81-298-61-5636 fax. +81-298-61-2939 email: paul-fons@aist.go.jp The lines below are in a Japanese font 〒３０５−８５６８ 茨城県つくば市東１−１−1 つくば中央第４ 近接場光応用工学センター ポール・フォンス主任研究官

Re: [Ifeffit] Inequivalent sites and multiple shell fitsPaul, may be it will be more clear what you mean by interference of inequivalent sites if you explain it on the example of equivalent sites. In a bulk fcc metal, contributions to EXAFS from neighboring (equivalent) sites do not interfere - and it is why we can use scattering amplitudes and phases extracted from model compounds (or FEFF theory) with one structure to model unknown compound with a (not too) different structure. Had it not been true, the chemical transferability of scattering phase and amplitude wouldn't be working but it does in most cases. So, if it is working for equivalent sites, why should it not for inequivalent sites? Moreover, this possibility (of interference) is almost always ruled out in FEFF at the state when we assign unique potentials to different atoms, the absorber (0) and its neighbors (1). Sometimes we assign a unique potential 0 to the central atom, and, another unique potential 1 to its first nearest neighbors, to correct for the muffin tin approximation, while the rest of the atoms have the same potential 2 (if there is just one element in the sample). For fcc copper, both models give very similar results for FEFF paths and for the fits. The reason the central atom has different potential than the 1NN is because it is ionized. Had its 1NN been ionized also, its potential would be different too, but we know that we can get away just fine by assigning a neutral atom potential to all the neighbors. Therefore, if FEFF does not worry about interference, how can it be taken into account in principle, and why? Regards, Anatoly Anatoly Frenkel Yeshiva University [Anatoly Frenkel] -----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov]On Behalf Of Scott Calvin Sent: Friday, January 14, 2005 12:49 PM To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] Inequivalent sites and multiple shell fits Hi Paul, I think we're getting confused over what we each mean by "interfere." Whether sites are equivalent or inequivalent, neighboring or far apart, the wave functions do not interfere at all, in part because the odds of two x-ray photons being absorbed by sites close to each other at nearly the same time is very very small--if it wasn't, your material would disintegrate in an instant! The idea behind the path expansion used in EXAFS analysis by ifeffit, however, is that the chi(k) for each possible scattering event by a single electron can be computed separately, and then the results added (I'll defer to John or Matt or whoever for a clear explanation of why this is justified). In other words, the EXAFS spectra can be thought of as if there were a contribution from an electron scattering off a near-neighbor, another scattering off a 2nd-nearest-neighbor, another doing a multiple-scattering thing, etc. (all weighted, of course, by the relative contributions of these scattering events). These are literally just added. Given that system, if there really are multiple sites that are each having their own scattering events, then it is perfectly appropriate to treat them in exactly the same way--just add. So, depending on your terminology, either we are combining all scattering paths coherently or we are adding none of them coherently, regardless of whether they correspond to the same absorbing site or not. It would not be correct to add, say, the magnitudes of the FT's for the separate contributions...the phase of the FT indicates where the peaks and troughs are in k-space (and thus energy-space), and it certainly matters whether a given scattering path has enhanced or suppressed absorption at a given energy! Hope that helps... --Scott Calvin Sarah Lawrence College P.S. I've published several analyses of systems with inequivalent sites. If you'd like a pdf of one of those papers, let me know. Thanks for the message. I guess what I wanted to say was that from what I understand due to lifetime effects the "effective" radius about the absorbing site for which the outgoing scattered wave "exists" is on the order of 2 nn (I have big lattice constant material). It would seem to me then that the inequivalent sites being physically separated by more than this distance (we are talking about crude approximations here) would not interfere. In reality, with bond lengths on the order of 3 Angstroms and 2nn along the lines of 4.5 Angstroms, my case is somewhere in between the coherent interaction and the dilute dopant -- e.g. every site is by itself -- extreme. Certainly, in the latter case it would be wrong to add the signals together coherently! What is the consensus for this sort of thing? The different E0's is a good point too. Paul

Wow. An interesting discussion has started. I am afraid my original question was not very clear (my fault). I just wanted to be sure that athena (diana) was doing the "right thing", e.g adding the contributions from separate feff calculations, e.g. treating them as incoherent sums where the phase information is not preserved. In retrospect, it seems obvious that it must be doing so, but in a moment of doubt I dashed off a quick email. Thanks for the help.