February 2007 - Ifeffit - millenia.cars.aps.anl.gov

Debye-Waller factors for metal oxides ?
by Ian James Drake 16 Apr '09

16 Apr '09

Hi, There is little discussion over the significance of fitted Debye-Waller factors (ss2) found for metal centers in metal oxides of heterogeneous catalysts. I am searching for a better way of determining how reasonable these values are in the fits I get. 1. Are phenomenological models like the correlated Debye model and Einstein model appropriate? It seems that Debye temperatures are hard to find for oxides. 2. Is there an equivalent value for ss2 in x-ray crystallography data that I can refer to for comparison in these materials? What should I look for in reviewing published x-ray crystallographic data? 3. Is the temperature dependence reported for the ss2 for some metals (like Cu and Al) similar to their respective oxides? For example, in Debye-Waller factor calculations reported for Al metal by R.C.G Killean (in J.Phys.F: Metal Phys., v. 4, pg. 1908, 1974), the ss2 changes by a factor of three between 25C and 300C. Would I expect a factor of three increase in crystalline zeolites (AlO4 structural units). 4. Specific to my work: I have studied Al containing oxides at temperatures between 25 and 300 C. I would like to quantify coordination changes by doing EXAFS fitting of the Al K-edge data. The EXAFS was taken at temperature and I observe no change in the broadening of the data. Likewise, the fitting shows no sensitivity in the Debye-Waller factor. It is nearly constant at 0.001 A over the temperatures of interest. Since ss2 and the coordination number (CN) are correlated, I would like a way to bind the error on the fitted CN by modeling real physical changes in the Debye-Waller factors. Do you have any suggestions? Looking forward to your thoughts. Ian Drake Graduate Student UC Berkeley

9 21

New Method for Normalization?
by Barton, David (DG) 30 Mar '09

30 Mar '09

XANES Aficionados, I know most users on this list are hard-core EXAFS analysts, but some of us use XANES analysis to decipher mixed phases or use it to get structural clues when data quality isn't sufficient for EXAFS analysis. Anyway, I have a question for those interested in quantitative analysis of XANES or those that are willing to offer an opinion on methods to normalize raw spectra to get reliable, repeatable comparisons of near-edge features. Has anyone considered using the methods of Penner-Hahn to normalize their data to get reliable intensities of near-edge features? The reference to their method of normalization is below: J. Synchrotron Rad. (2005). 12, 506-510 [doi:10.1107/S0909049504034193] "A method for normalization of X-ray absorption spectra" T.-C. Weng, G. S. Waldo and J. E. Penner-Hahn It is my understanding the Ifeffit uses a linear function to remove the pre-edge and a quadratic for the post-edge and that the Athena's flatten is a subtraction of these functions. This is an excellent method under most scenarios since the background is usually fairly smooth and can be well approximated by a quadratic equation. Personally, I have had only a few rare cases where the quadratic function on the post-edge was not sufficient to reliably normalize the data and in those cases a third-order polynomial was sufficient. Does anyone else have an opinion on using alternative normalization routines? Dave _________________________________ David Barton The Dow Chemical Company

12 22

Re: [Ifeffit] Fwd: Re: why ss_2 is negative?
by hw26＠njit.edu 04 Mar '07

04 Mar '07

Dear all, Thanks million for your great advice for my EXAFS modeling! It helps me understand more about EXAFS and I really appreciate it! But I still have some questions to consult you. I took your suggestion and have the amp and e0 the same for each path. In addition, I fixed the number axial U-O as 2. For the equatorial ligand, since the total coordination number is between 4 and 6, I guess the number of U-F as 2(that is n1), and def the number of U-O as (5-n1)(that is n2). Then I did the fit. The result showed the ss_2 is still negative. So I set the ss_2 as 0.003, did the fitting again. I don't know if it is ok to fix the ss_2=0.003. It seems the dr_1 change a lot. In addition, I tried to do fitting in Kw=2 in stead of Kw=3. At this time, the ss_2 is positive, and from my understanding, it seems the modeling is more resonable for dr_1 and dr_2, but the R-factor is 0.023, a little bit larger than 0.02. How can I decide which one I should use, the Kw=3 or Kw=2? Is there anything I can do to improve the modeling? The attached is my modeling. I am sorry to put attachment in the email that my bother you. Best regards! Hao > Hao, > > A few more comments: > > 1. It is well advised for anyone starting to look at uranyl compounds > with XAS to read Shelly's various publications on that topic. > Geochem. Cosmo. Acta, 66(22) 3875-3891, (2002) is particularly > useful. Google scholar can help you locate more of her papers. > > 2. I am the first to admit that Artemis and Ifeffit are difficult > tools for the beginning exafs practitioner. These tools try to > provide a usable face to a difficult problem. Ifeffit was written > with enough flexibility and power to allow the so-called experts > to do the analysis they want to do. That means that Artemis has > enough freedom to allow the unsuspecting user to do ill-advised > things. > > You really need to think about what each of the parameters means. > For example, just because Artemis allows you to float e0 freely > for each path does not mean that doing so is a good idea. There > are situations where it is a very good idea to have more than one > e0 parameter in your fit, but I suspect that yours is not one of > them. Consequently, you should consider constraining all your > paths to have the same e0. > > What do I mean by "constrain"? Well, on the Guess,Def,Set page, > you can define a single e0 parameter as a guess parameter, then > use that parameter on each of the path pages. In that way, you > will have one floating parameter in your fit that is used to > describe the e0 of all your parameters. > > Similarly, you should think hard about the numbers of each kind of > path. A uranyl moiety is unlikely to have more or less than 2 > uranyl ligands. The uranyl bond is very strong, consequently it > is energetically unfavorable in a uranyl for those two bonds to be > over- or under-filled. Consequently, you can consider setting the > number of yl oxygens to 2. Furthermore, the equatorial plane is > likely to have between 4 and 6 oxygens, so you should think of > ways to constrain the total number of equatorial oxygens to be > something reasonable. Since the coordination number is highly > correlated with sigma^2 in a statistical sense, thinking well > about coordination number should help address your original > question. > > 3. Scott has made a very nice Artemis tutorial which can be found at > http://cars9.uchicago.edu/iffwiki/HoraeSoftware#contrib > It comes in the form of a series of Artemis projects. Newcomers > benefit by working through his tutorial before trying to analyse > their own data with Artemis. > > 4. The Ifeffit web site, my home page, and especialy http://xafs.org > are all filled with useful information for the XAS novice. If you > haven't already looked through the resources on those sites, you > really should budget some time to do so. > > > HTH, > B > > > -- > Bruce Ravel ---------------------------------------------- > bravel(a)anl.gov > > Molecular Environmental Science Group, Building 203, Room E-165 > MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007 > > Argonne National Laboratory phone and voice mail: (1) 630 252 > 5033 > Argonne IL 60439, USA fax: (1) 630 252 > 9793 > > My homepage: http://cars9.uchicago.edu/~ravel > EXAFS software: http://cars9.uchicago.edu/~ravel/software/ > > _______________________________________________ > Ifeffit mailing list > Ifeffit(a)millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit >

3 2

Re: [Ifeffit] EXAFS analysis with FEFF
by Bruce Ravel 27 Feb '07

27 Feb '07

Hi Gemma, I am not sure that I have the whole answer for you. I too have had trouble at times getting feff8 to run with hydrogen atoms in the cluster. I am CCing my response to the mailing lists for Feff and Ifeffit in hopes that someone else might have some more wisdom for you. The feff.inp file you sent me is appended to the end of this email, in case any of the brains on the mailing lists need to see it. On Tuesday 27 February 2007 11:27, Gemma GUILERA wrote: > I am currently running FEFF8.2 for one of my systems which include some H > atoms (as hydrides). These are actually very important in the structure and > I want to consider them in the run. > For this I used the FOLP function in POT with an overlapping value of 0.8 > for H as suggested in the manual. Also, since I think that the AFOLP card > gets deactivated, I also added these cards for the rest of the potentials > keeping them at 1.15. > I obviously do something completely wrong because the program stops and > appears an error message of the type: > > FOLP for POTENTIAL type 0 is too big. > Reduce overlap using FOLP and rerun. > > I do not understand what I need to do, because the potential 0 is not the > one I am modifying with respect to the default values. > I would really appreciate if you could have a look at the feff.inp file and > tell me what did I do wrongly...and sorry for wasting your time. Well, my understanding is that Feff's various overlapping methods do have effects that are not restricted to the ipots that you are explicitly modifying. That is, if you restrain the size of one muffin tin, other muffin tins can get a bit bigger or smaller to compensate. It depends on the geometry and how the atom you are modifying is represented in the cluster. My first thought is that you need to play around with the FOLP value for the absorber. Try smaller values to see if you can get Feff to run to completion. My second thought is to abandon the FOLP keywords and try using AFOLP. Another thought is that you may need to use a larger cluster. Some atom types -- B for example -- have few representatives in your cluster and so their muffin tin radii might be improperly constrained. Does anyone else have any good suggestions for Gemma? B TITLE FeBH6Fe complex EDGE K S02 1.0 * pot xsph fms paths genfmt ff2chi CONTROL 1 1 1 1 1 1 PRINT 1 0 0 0 0 3 * r_scf [ l_scf n_scf ca ] SCF 2.90160 0 15 0.1 * ixc [ Vr Vi ] EXCHANGE 0 0 0 FOLP 0 1.15 FOLP 1 0.8 !Use 0.8 muffin-tin radii overlapping factor for H FOLP 2 1.15 FOLP 3 1.15 FOLP 4 1.15 FOLP 5 1.15 EXAFS RPATH 6 * kmax [ delta_k delta_e ] *XANES 4.0 0.07 0.5 * r_fms [ l_fms ] *FMS 2.90160 0 * *RPATH 0.10000 * emin emax resolution *LDOS -20 20 0.1 POTENTIALS * ipot z [ label l_scmt l_fms stoichiometry ] 0 26 Fe -1 -1 0 1 1 H -1 -1 6 2 5 B -1 -1 1 3 15 P -1 -1 6 4 6 C -1 -1 46 5 26 Fe -1 -1 2 ATOMS 0.00000 0.00000 0.00000 0 Fe1 0.00000 -0.71680 -1.08741 -0.20797 1 H42 1.31891 -1.33000 0.40717 0.04190 1 H41 1.39156 -0.20933 -0.03121 -1.56977 1 H40 1.58398 -1.50973 -0.47766 -1.07196 2 B1 1.91221 0.07929 -0.26849 2.16349 3 P3 2.18152 0.78129 2.03650 0.10939 3 P2 2.18397 2.03623 -0.76920 -0.20440 3 P1 2.18625 -2.30265 0.13209 -1.93594 1 H42 3.01123 -2.81012 -0.92412 -0.57414 1 H40 3.01338 -1.68946 -1.36249 -2.18581 1 H41 3.08033 3.15055 -0.21476 1.16774 4 C22 3.36686 0.19242 -2.02477 2.69082 4 C25 3.37301 2.87884 -0.31857 -1.75462 4 C13 3.38643 1.52559 0.58481 2.96848 4 C24 3.38840 2.32803 2.20201 1.14003 4 C49 3.40121 -0.35734 3.30346 0.81773 4 C37 3.42188 1.28771 2.89227 -1.43362 4 C43 3.47544 2.68114 0.97735 2.02827 4 C23 3.50109 -1.29194 0.30124 3.24246 4 C31 3.50334 2.42741 -2.57443 -0.17395 4 C1 3.54263 1.08472 2.33572 -2.68697 4 C48 3.72183 -2.31005 1.13130 2.75352 4 C36 3.76803 -3.01946 -0.95533 -2.14391 5 Fe1 3.82441 -0.91239 -2.83347 2.42108 4 C26 3.83701 1.44841 -3.56032 -0.07829 4 C3 3.84446 2.35340 -0.84952 -2.93358 4 C15 3.85565 -1.36594 3.79736 -0.00896 4 C38 4.03557 -0.32880 3.71800 2.14367 4 C42 4.30429 1.32048 -2.62656 3.24943 4 C30 4.38193 3.96885 0.55229 -1.84177 4 C21 4.41009 1.89033 4.15594 -1.34798 4 C44 4.76049 -1.27608 0.01031 4.60350 4 C32 4.77710 3.77855 -2.96794 -0.18652 4 C11 4.80842 3.85573 1.40280 2.91662 4 C50 5.03400 -4.10418 -3.29104 0.54306 4 C48 5.28868 -3.80075 -2.99182 -2.25330 3 P2 5.33611 -3.09875 -0.68684 -4.30740 3 P3 5.35048 -2.10707 1.87815 -4.56499 4 C26 5.36715 -0.70940 -2.08663 -4.89743 4 C36 5.37049 -5.05569 -0.18613 -1.93951 3 P1 5.41815 -5.37286 -0.10580 0.78966 4 C15 5.43161 -1.65351 -4.75268 -2.13495 4 C38 5.46627 -4.46787 2.60499 -2.06562 4 C3 5.56908 -1.72752 -1.25656 -5.38637 4 C31 5.79450 -4.30717 -3.84760 -0.71029 4 C43 5.81895 -2.66211 -4.25879 -2.96164 4 C37 5.83056 -3.21187 1.06944 -4.83473 4 C25 5.90207 -5.89830 -0.63676 -0.38929 4 C13 5.94533 END -- Bruce Ravel ---------------------------------------------- bravel(a)anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/

1 0

Re: [Ifeffit] Ifeffit Digest, Vol 48, Issue 14
by Roque, J (Josep) 27 Feb '07

27 Feb '07

The statement "Feff 6.02L doesn't work" is false also in Josep's computer. It works correctly.. Thanks. J. -----Original Message----- From: ifeffit-bounces(a)millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of ifeffit-request(a)millenia.cars.aps.anl.gov Sent: 26 February 2007 18:00 To: ifeffit(a)millenia.cars.aps.anl.gov Subject: Ifeffit Digest, Vol 48, Issue 14 Send Ifeffit mailing list submissions to ifeffit(a)millenia.cars.aps.anl.gov To subscribe or unsubscribe via the World Wide Web, visit http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit or, via email, send a message with subject or body 'help' to ifeffit-request(a)millenia.cars.aps.anl.gov You can reach the person managing the list at ifeffit-owner(a)millenia.cars.aps.anl.gov When replying, please edit your Subject line so it is more specific than "Re: Contents of Ifeffit digest..." Today's Topics: 1. Re: E0 for bulk crystals and nanoparticles (Leandro Araujo) 2. Fwd: Re: E0 for bulk crystals and nanoparticles (Bruce Ravel) 3. FEFF 6L.02 (Roque, J (Josep)) 4. Re: FEFF 6L.02 (Bruce Ravel) 5. Re: FEFF 6L.02 (Anatoly Frenkel) ---------------------------------------------------------------------- Message: 1 Date: Mon, 26 Feb 2007 14:52:10 +1100 From: "Leandro Araujo" <leandrolangie(a)gmail.com> Subject: Re: [Ifeffit] E0 for bulk crystals and nanoparticles To: "XAFS Analysis using Ifeffit" <ifeffit(a)millenia.cars.aps.anl.gov> Message-ID: <5f59b3760702251952y535fae5axb9fd91d63789564f(a)mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Anatoly and Scott, thanks for the quick and valuable answers! Sorry I didn't specify the system in more detail; this is a point that Matt always stresses when replying messages in this list (i.e., there is no "one fits all" answer, so a detailed description of the system is needed) and by now I should have assimilated it already... Anyway, I see no changes whatsoever in the XANES spectra of the T-dep measurements, so I think it will be worth the try. Since I'm dealing with Ge NCs buried about 1micron deep into a SiO2 matrix, I reckon effects like hydrogen chemisorption, charge exchange with support, ligand desorption and oxidation will not take place, specially at such low T's. Thanks once more for the help, Leandro ------------------------------ Message: 2 Date: Mon, 26 Feb 2007 09:04:16 -0600 From: Bruce Ravel <bravel(a)anl.gov> Subject: [Ifeffit] Fwd: Re: E0 for bulk crystals and nanoparticles To: XAFS Ifeffit <ifeffit(a)millenia.cars.aps.anl.gov> Message-ID: <200702260904.16718.bravel(a)anl.gov> Content-Type: Text/Plain; charset="iso-8859-15" On Sunday 25 February 2007 20:44, Leandro Araujo wrote: > I don't know if it is just me, but in my short time working with EXAFS > I got the strong impression that it is very easy to fall into the > temptation of sophistry to get to the "expected" results from a fit. That is very well put. I hope everyone reading the list today appreciates your wise words. > In this respect, I consider the mailing list an invaluable resource to > help overcoming such a temptation. That's great -- thanks. Kind words go a long way with those of us who invest time on the mailing list. As for your question -- I really liked Scott's point about looking at the XANES as a clue about whether e0 should be changing in the EXAFS. Use all the information (spectroscopic or otherwise) available to you. B -- Bruce Ravel ---------------------------------------------- bravel(a)anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/ ------------------------------ Message: 3 Date: Mon, 26 Feb 2007 16:29:49 -0000 From: "Roque, J $Josep$" <josep.roque(a)diamond.ac.uk> Subject: [Ifeffit] FEFF 6L.02 To: <ifeffit(a)millenia.cars.aps.anl.gov> Message-ID: <DC69C90BD4A39249BF59689B2E06427803B04E42(a)exchange33.fed.cclrc.ac.uk> Content-Type: text/plain; charset="us-ascii" I am trying to run FEFF 6L.02 from version of IFEFFIT 1.2.9 on a Windows platform but it doesn't seems to work.. How can I fix this problem?? Cheers Dr Josep Roque Post Doctoral Research Associate: Microfocus Spectroscopy (I18) Diamond Light Source Ltd Diamond House Chilton Didcot Oxfordshire OX11 0DE Tel: 01235 778735 Email: josep.roque(a)diamond.ac.uk

1 0

FEFF 6L.02
by Roque, J (Josep) 26 Feb '07

26 Feb '07

I am trying to run FEFF 6L.02 from version of IFEFFIT 1.2.9 on a Windows platform but it doesn't seems to work.. How can I fix this problem?? Cheers Dr Josep Roque Post Doctoral Research Associate: Microfocus Spectroscopy (I18) Diamond Light Source Ltd Diamond House Chilton Didcot Oxfordshire OX11 0DE Tel: 01235 778735 Email: josep.roque(a)diamond.ac.uk

3 2

Fwd: Re: E0 for bulk crystals and nanoparticles
by Bruce Ravel 26 Feb '07

26 Feb '07

On Sunday 25 February 2007 20:44, Leandro Araujo wrote: > I don't know if it is just me, but in my short time working with > EXAFS I got the strong impression that it is very easy to fall into > the temptation of sophistry to get to the "expected" results from a > fit. That is very well put. I hope everyone reading the list today appreciates your wise words. > In this respect, I consider the mailing list an invaluable resource > to help overcoming such a temptation. That's great -- thanks. Kind words go a long way with those of us who invest time on the mailing list. As for your question -- I really liked Scott's point about looking at the XANES as a clue about whether e0 should be changing in the EXAFS. Use all the information (spectroscopic or otherwise) available to you. B -- Bruce Ravel ---------------------------------------------- bravel(a)anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/

1 0

E0 for bulk crystals and nanoparticles
by Leandro Langie Araujo 25 Feb '07

25 Feb '07

Dear all, could you please let me know your thoughts on this question that has been consuming much of my time lately? It goes like this: When we perform first shell analysis of elemental crystals and nanoparticles (like bulk Ge and Ge NPs, or bulk Pt and Pt NPs, for example), should both bulk and NP samples have the same E0 shift relative to the theory (or, fit-wise, the same deltaE0 when fitting in Artemis)? And, even more important to know, why so? I've been reading some papers and I can find people who do set the NPs deltaE0 to be the same as the crystalline foil as well as people who report different deltaE0s for NPs relative to the foil. This confuses me because by setting (or not) deltaE0 we directly change the correlated variables, delR and C3. Intuitively I would think that for Ge, in particular, NPs could have a different deltaE0 from the bulk since they present smaller band gaps and different electronic structure, causing differences in the occupied/unoccupied states (that's why they show some PL while the bulk doesn't). Would you say this is reasonable? I have used Athena to calibrate (maximum of energy derivative = atomic value), align edges, background subtract (with same parameters) and Fourier transform (with same parameters) both bulk and NP spectra (at the same temperature, from same run/measurement conditions), which I then fit in Artemis to the FEFF8-generated crystalline Ge standard. And I do see a significant change in the odd cumulants (delR and C3) when I let deltaE0 run free or when I set it to the bulk value (the difference is between 1 and 2 eV). Would you say this is physically reasonable or more likely to be a fit artifact? Thanks, Leandro

5 7

Ubuntu binary package problem
by M. Rovezzi 25 Feb '07

25 Feb '07

Dear all, I'm sending to the list a question mainly directed to Carlo, anyway. I've installed the ifeffit/horae packages on a Ubuntu 6.10 "Edgy Eft" distribution from Carlo repositories. Here the problems: mauro@stalker3:~$ ifeffit Killed mauro@stalker3:~$ athena Can't load '/usr/lib/perl5/auto/Ifeffit/Ifeffit.so' for module Ifeffit: /usr/lib/perl5/auto/Ifeffit/Ifeffit.so: cannot map zero-fill pages: Cannot allocate memory at /usr/lib/perl/5.8/DynaLoader.pm line 225. at /usr/bin/athena line 66 Compilation failed in require at /usr/bin/athena line 66. BEGIN failed--compilation aborted at /usr/bin/athena line 70. mauro@stalker3:~$ artemis Can't load '/usr/lib/perl5/auto/Ifeffit/Ifeffit.so' for module Ifeffit: /usr/lib/perl5/auto/Ifeffit/Ifeffit.so: cannot map zero-fill pages: Cannot allocate memory at /usr/lib/perl/5.8/DynaLoader.pm line 225. at /usr/bin/artemis line 52 Compilation failed in require at /usr/bin/artemis line 52. BEGIN failed--compilation aborted at /usr/bin/artemis line 56. It seems related to memory allocation and I have to say that I'm having these problems only on low memory machines (192 Mb of RAM). Anyway I don't understand why Ifeffit is killing itself and how to obtain more information on what is happening with the libraries... well, I wait for your interesting answers. Mauro -- Mauro Rovezzi <mauro.rovezzi(a)esrf.fr> PhD Student - GILDA BM08 at ESRF (Grenoble, FR) ------------------------------------------------- This mail sent through IMP: http://horde.org/imp/

3 3

Fwd: Re: why ss_2 is negative?
by Bruce Ravel 24 Feb '07

24 Feb '07

On Friday 23 February 2007 21:41, hw26(a)njit.edu wrote: > Yes, I am modeling the uranyl ligand and two equatorial ligands, one of > which is hypothesized as U-F (from BF4 anion), and the other as U-O (from > hydration),because in the uranyl nitrate standard sloution(without > MOEMIMBF4), there is only one peak that represents the U-O equatorial > ligand, but with the MOEMIMBF4, the equatorial peak is split to two peaks. > So I guess the other equatorial U-F formed. Hao, A few more comments: 1. It is well advised for anyone starting to look at uranyl compounds with XAS to read Shelly's various publications on that topic. Geochem. Cosmo. Acta, 66(22) 3875-3891, (2002) is particularly useful. Google scholar can help you locate more of her papers. 2. I am the first to admit that Artemis and Ifeffit are difficult tools for the beginning exafs practitioner. These tools try to provide a usable face to a difficult problem. Ifeffit was written with enough flexibility and power to allow the so-called experts to do the analysis they want to do. That means that Artemis has enough freedom to allow the unsuspecting user to do ill-advised things. You really need to think about what each of the parameters means. For example, just because Artemis allows you to float e0 freely for each path does not mean that doing so is a good idea. There are situations where it is a very good idea to have more than one e0 parameter in your fit, but I suspect that yours is not one of them. Consequently, you should consider constraining all your paths to have the same e0. What do I mean by "constrain"? Well, on the Guess,Def,Set page, you can define a single e0 parameter as a guess parameter, then use that parameter on each of the path pages. In that way, you will have one floating parameter in your fit that is used to describe the e0 of all your parameters. Similarly, you should think hard about the numbers of each kind of path. A uranyl moiety is unlikely to have more or less than 2 uranyl ligands. The uranyl bond is very strong, consequently it is energetically unfavorable in a uranyl for those two bonds to be over- or under-filled. Consequently, you can consider setting the number of yl oxygens to 2. Furthermore, the equatorial plane is likely to have between 4 and 6 oxygens, so you should think of ways to constrain the total number of equatorial oxygens to be something reasonable. Since the coordination number is highly correlated with sigma^2 in a statistical sense, thinking well about coordination number should help address your original question. 3. Scott has made a very nice Artemis tutorial which can be found at http://cars9.uchicago.edu/iffwiki/HoraeSoftware#contrib It comes in the form of a series of Artemis projects. Newcomers benefit by working through his tutorial before trying to analyse their own data with Artemis. 4. The Ifeffit web site, my home page, and especialy http://xafs.org are all filled with useful information for the XAS novice. If you haven't already looked through the resources on those sites, you really should budget some time to do so. HTH, B -- Bruce Ravel ---------------------------------------------- bravel(a)anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/

1 0