On Friday 23 February 2007 21:41, hw26@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@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/