Hi John, I have a couple of comments to add to Bruce's reply: Feff is truly amazing. You can take a crystal structure for a Metal-oxide and apply it to a hydrated metal atom in solution with great success. -You can also do other non-physical things to the feff list of atoms and apply it to a real problem. There are a couple of thing to keep in mind. 1) Feff needs to have a list of atoms that extends well beyond the paths that are actually used in the fit. Always pad the feff.inp file by at least an Angstrom. If you are using feff calculation for the first time. Pad the feff list by one angstrom and then by two angstroms and check that the paths that you are going to use are the same. You do not want to use paths from the edges of the cluster that you give to feff in the feff.inp file. -Hence you need to use a structure like a metal-oxide to model a hydrated metal. 2) If you think that you have some unique geometry, linear multiple scattering paths then you need to be more careful to get these paths correct in the feff.inp file since they will have unique scattering amplitudes. 3) If the actually distance for the path included in the fit is different from the starting distance as given by the feff list, then you need to re-run the feff calculation. If you are just playing around you can move the paths by 0.5 Angstroms to get an idea that the path will work. In the final published fit, I like to see the change in the path length much less than this. If you are looking at solutions a great place to start is with the metal and the metal-oxide. Shelly
-----Original Message----- From: Bruce Ravel [mailto:ravel@phys.washington.edu] Sent: Tuesday, July 20, 2004 5:40 PM To: XAFS Analysis using Ifeffit Cc: John Butler Subject: [Ifeffit] Re: Artemis Question
Basically, the problem that I'm having is that the system I'm working on is a solution (to start I'm trying to look at a solution of Zinc Sulfate and then look at data with other components we're interested in). Unfortunately, the part of the manual dealing with solution analysis hasn't been written yet, and the people I know who use EXAFS don't do anything with solutions.
I was hoping that you could point me in the right direction for how to proceed with the analysis (e.g. is there anywhere I'm
On Sunday 18 July 2004 11:38 pm, John Butler wrote: likely to find
the FEFF files ... is there a 'standard' way to attack this kind of problem). Of course I assume you're a pretty busy guy and have neither the time nor inclination to give me a detailed step by step guide or anything ... I was just hoping you could point me in the right direction, give me a couple of tips, and/or show me where to look.
John,
As you can see, I have forwarded you question along with my answer to the Ifeffit mailing list.
You are correct to assume that I'm a busy guy. This week (like each of the other weeks this month) I am at NSLS collecting some data and helping our users. Fortunately, the mailing list is a solution to the "busy expert" problem. Enough people read and respond to the mailing list that someone should be available to answer just about every question.
I doubt, actually, that you problem is really so hard to get started with.. Lots of problems in XAFS do not fall into nice little pigeon holes. One knows how to deal with data on an iron foil because one knows what the structure of iron metal is. Most real research problems, obviously enough, are problems precisely because the structure is not perfectly well known.
In the case of a solvated complex, there is probably some sort of analogue which can be used as a starting place. Perhaps there is some similar compound whose structure *is* known. Perhaps your solvated complex has a crystalline form that is similar, at least in the local environment, to the solvated form. In any case, find something that is similar to your real material. Run feff and compare to the data. Perhaps your analogue structure will be close to your data. In that case, you should be able to parameterize the feff calculation in such a way that you can begin to understand the ways in which your complex differs from your analogue.
Perhaps the analogue structure is a very poor match for the data. That in itself is information, in that it tells you to go look for a different analogue.
The common theme to what I have said here (and indeed to any analysis problem) is that you make some educated guesses as to what you real sample looks like. You test those guesses. If they seem to be leading you in the right direction, super. If not, make new (and newly educated!) guesses.
My other piece of advice would be to spend some time fitting known spectra. If you are new to XAFS, then you are learning two things at the same time: how to interpret XAFS data and how to use my and Matt's screw-ball analysis programs! I consider it a good idea to spend some time fitting data for which you know the answer so that you can learn how to use the codes before plunging into the harder but more interesting problem of your own research.
Good luck and join the mailing list, B
-- Bruce Ravel ----------------------------------- ravel@phys.washington.edu Code 6134, Building 3, Room 222 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/
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Shelly said:
Feff is truly amazing.
Amen!
1) Feff needs to have a list of atoms that extends well beyond the paths that are actually used in the fit. Always pad the feff.inp file by at least an Angstrom. If you are using feff calculation for the first time. Pad the feff list by one angstrom and then by two angstroms and check that the paths that you are going to use are the same. You do not want to use paths from the edges of the cluster that you give to feff in the feff.inp file. -Hence you need to use a structure like a metal-oxide to model a hydrated metal.
Under what circumstances does FEFF actually need these extra atoms? When I take the copper example included by Artemis, for example, I can rerun the FEFF calculation with a cluster size of 3 angstroms, so that FEFF only has the nearest-neighbors to deal with. The path generated is identical to when a cluster size of 7 angstroms. Certainly if someone is using FEFF8 with a self-consistent field calculation it is necessary to have the cluster extend beyond the furthers paths, but with FEFF6L is this true? (Incidentally, for the novices out there, I do NOT recommend being satisfied with a single-shell fit when you have a reasonable guess as to the structure beyond the first-shell. One reason can be seen easily with Artemis: paths beyond the first-shell often have low-R "tails" that contribute to the signal even at the first-shell peak. Although the FEFF calculation for the first path may be the same whether or not you also calculated paths further out, the Ifeffit fit will be different depending on whether you include those "outer" paths or not.) --Scott Calvin Sarah Lawrence College
Hi all, I've been doing a literature search for an article I'm writing up, and I can't seem to find any examples of an XAS flow cell desgined for liquids. Sealed cells for liquids, yes; flow cells for passing gases over solids, yes; but no solution-chemistry flow cell. I feel like someone must have published work in that area before, but I can't seem to find it. So, does anyone know of any work done using a liquid-phase XAS flow cell? --Scott Calvin Sarah Lawrence College
Hi Scott,
I've been doing a literature search for an article I'm writing up, and I can't seem to find any examples of an XAS flow cell desgined for liquids. Sealed cells for liquids, yes; flow cells for passing gases over solids, yes; but no solution-chemistry flow cell. I feel like someone must have published work in that area before, but I can't seem to find it.
So, does anyone know of any work done using a liquid-phase XAS flow cell?
We (Penner-Hahn group) have done solution EXAFS measurement and time-resolved XAS at Bio-CAT at the APS. A Kapton tube is used for x-ray window and connected to PE tubes using epoxy glue. Tsu-Chien <Chien>001000</Chien>
Dear Scott, one more (late) addition, J.D. Grunwaldt et al. published a paper last year, in which they described the use of a flow cell for in-situ XAFS in supercritical CO2. These measurements were performed at HASYLAB beamline X1. (Catal. Lett. 90, 221-229 (2003) and Cat. Today, 91-92, 1-5 (2004)) Best regards, Edmund Welter HASYLAB at DESY
participants (5)
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Edmund Welter
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Kelly, Shelly D.
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Scott Calvin
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Scott Calvin
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Tsu-Chien Weng