Re: [Ifeffit] Question about using a proper model

Hi, I may as well chime in since I actually have some expertise with trying to get XAS of organometallic compounds on silica surfaces (mainly pertaining to quesion#1). These are the papers I've previously published with my group for inorganic and organometallic compounds grafted on silica surfaces: http://www.pnas.org/content/111/32/11624 http://onlinelibrary.wiley.com/doi/10.1002/adsc.201400125/abstract http://onlinelibrary.wiley.com/doi/10.1002/anie.201308983/abstract the methodology we've had the most success with consists of, synthesize inorganic/organometallic precursor, and get XRD crystal structure, then grafting the compound onto the silica surface. We then take XAS measurements of both the precursor and the grafted species. From other characterization methods (NMR, IR) we have a good idea that the compound hasnt changed too much upon grafting to the silica surface. Then we take XAS measurements of both of these samples, fit the structure of the molecular precursor to the XRD data, then use some of the parameters of the fit of the molecular structure to fit the surface species. So in the previously described method we went around the problem of not knowing the structure of the species on the surface by using complementary characterization methods in conjunction with XAS to get a good fit. now let's say the structure on the surface starts to deviate more from the molecular analog (ie we do a heat treatment on the surface species on silica), then it gets trickier and I have to add more variables to the structure, but I was usually successful in fitting the surface species by playing around with the molecular model. Now, in some cases (not yet published) we havent been able to crystallize the molecular compound, and it's structure is not published. In those cases we had to dig through crystallographic databases to find structures that are similar (at least first shell coordination, and distances) to the one we have on the surface, and try to fit it. I've usually managed to get a reasonable fit by doing this. but if you cant synthesize a molecular analog, and you cant find a crystallographic structure reasonably close to what you hope to find on the surface (based on other characterization methods), then I'd say you're out of luck. best of luck, georges On Tue, Oct 28, 2014 at 2:57 AM, pushkar shejwalkar < pshejwalkar2004@gmail.com> wrote:

Dear Matt thank you very much for the comments. I will try to change the E0 value in athena accordingly. It appeared to me from the lecture notes that we need to do some saving file in artemis and then opening it in athena or something like that. but may be I read it all wrong. Thanks again Pushkar

On Tue, Oct 28, 2014 at 10:29 AM, Matt Newville < newville@cars.uchicago.edu> wrote:

...

Pushkar,

On Mon, Oct 27, 2014 at 7:57 PM, pushkar shejwalkar wrote:

...

Dear Matt, Thank you very much for your reply. Yes it generates list of XYZ cordinates. I am wondering if its ok to use such information. The reason is that while converting the XYZ file into .inp file, the format is different and hence I am simply typing the coordinates into it, however I am not sure as to should I change nleg values, Rmax values and other mphase mfeff values and all. Is it same for all the program? just changing the coordinates is enough?

Please understand that we only know as much about what you are trying to as you actually tell us. While we can give you advice, we cannot actually solve your problems for you.

I think that what you are trying to do is create a feff.inp file from atomic coordinates generated by some other program called Spartan (coincidentally, Greek-themed!). A feff.inp has several parts to it, one being a list of X, Y, Z, IPOT, where X, Y, Z are atomic coordinated (to be clear -- in Angstroms) and IPOT is an integer index of one of the Potentials listed elsewhere in the file. There must be between 2 and 7 IPOTs, number sequentially starting with 0. There is exactly 1 IPOT '0' -- this is the absorbing atom. Each scattering element must have its own IPOT (including any other elements that are the same Z as the absorber).

Starting with an existing feff.inp file and modifying the list of atomic coordinates and, if necessary, the list of potentials, is highly recommended. It's not that hard.

...

About lower E0 value, I read in Shelly's paper that mostly ppl do not report E0 value

People do report E0.

...

and if they and if it is >10 then there may be problem with modelling or so.

An E0 shift > 10 (or < -10) is a warning that something may not be right. It's not necessarily wrong, but should cause you to pause and try to understand such a large discrepancy between what Feff thought E0 should be and what the maximum of the first derivative is.

...

one of the method to reduce such chances of high E0 is by properly aligning spectra in athena. She mention in this a method to use first artemis for getting first shell fitting using E0=0 and then coming back to the athena and using this file as reference. But I am not sure how to open artemis file in athena because if I open .fpj file in athena it doesnt recognise and says that the format is not readable if you continue then athena shutdown. In artemis I cannot save file as .prj. is there a way around? OR IT IS JUST THAT I AM DOING IT ALL WRONG?

Just change E0 in Athena, perhaps copying a Group first, and use that chi(k) with the updated E0 as the data to fit. Usually Feff wants E0 further up the edge than the maximum of the first derivative, but not always.

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

-- Best Regards, Pushkar Shejwalkar. Post-doctoral -Researcher,JSPS Fellow Hokkaido University, Sapporo, Japan

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