[Ifeffit] Fwd: Re: Re: problem about negative
ZHAN Fei
zhanfei at ihep.ac.cn
Wed Nov 12 03:21:55 CST 2014
Dear Bruce:
I am really sorry that I lost this mail you send,because the mechanism of my mail address.
Thanks a lot for your patience to give me detailed advise and strategy about my problems.I will
take a step back and gain a deeper understanding of my sample and method itself.
Thanks
Sincerely,
zhanfei
> -----原始邮件-----
> 发件人: "Bruce Ravel" <bravel at bnl.gov>
> 发送时间: 2014年11月11日 星期二
> 收件人: "XAFS Analysis using Ifeffit" <ifeffit at millenia.cars.aps.anl.gov>
> 抄送:
> 主题: Re: [Ifeffit] Fwd: Re: Re: problem about negative
>
> On 11/11/2014 10:19 AM, Bruce Ravel wrote:
> > Rbkg>1.1 can avoid the first peak,but the remaining curve is somewhat
> > not smooth.How to distinguish whether a long wavelength oscillation
> > appear as a real peak or false one?I am confused about it.
> > I find when I avoid the first scattering peak after white
> > line(kmin=4.8 maybe too large),the first peak of R space
> > disappeared.Because usually the first scatering peak is related to the
> > first coordiantion shell,can I think the first peak in R space less 1A
> > is a real peak instead of noiseï¼
>
> Zhanfei,
>
> As I said yesterday, I don't know anything about your sample, so it is
> hard for me to know what the "right" answer is to your problem. But I
> can make some observations and some suggestions.
>
> * Your data range is really short. As you seem to have noticed
> (judging from the very short k-range in your project file), your data
> has serious systematic problems starting at 9 or 10 inverse Angstroms.
> I cannot quite tell, but your data appear to be transmission data. Well
> it is possible that you have some instability or nonlinearity at the
> beamline, I would guess that your sample is not very homogeneous. It is
> possible that more disciplined sample preparation might help extend that
> range of the interpretable data.
>
> * You are right that slightly increasing the Rbkg value seems to make
> a positive difference in the extracted chi(k) data. Your data seem to
> be an example of the sort of data I was alluding to yesterday. The
> background subtraction is difficult because it is difficult to
> distinguish the Fourier components of the background function from the
> Fourier components of the data. I think that the background subtraction
> with Rbkg=1.1 looks much better than with Rbkg=1.0, but given how
> different they are, you have to be concerned about the correlations
> between the background and the parameters of the data.
>
> It is clear that your data are of the sort for which background
> subtraction is difficult. So how do you know what is an acceptable
> background subtraction?
>
> Well, using only Fourier methods, I think we have demonstrated that you
> cannot know the right answer without some kind of prior knowledge.
>
> So, how do you get that prior knowledge?
>
> Well, my advice is to first solve some simpler problems. Measure your
> standards.
>
> Measure the common forms of moly oxide and moly sulfide. Measure moly
> metal. Analyze all of them. The advantage of the standards is that you
> know what the answer should be. Do the data processing and data
> analysis. Make sure that, when you do the analysis, you get the right
> answers.
>
> Having done that exercise, you will then have a lot more knowledge about
> what the various forms of moly oxide look like and what the challenges
> are when doing the data processing and data analysis.
>
> Some questions:
>
> - Are there any forms of moly oxide for which the bond length is as
> short as 1.6? If so, do the conditions of formation exist in your
> system? Is your sample of a valence consistent with the valence of the
> Mo when it has such a short bond?
>
> - If you convince yourself that it is possible for moly to have an
> oxygen atom at 1.6 or 1.7, what did you have to do with your standard to
> get a sensible analysis of the EXAFS data? Hopefully, that will guide
> you to doing a sensible analysis of your unknown sample.
>
> - If none of your moly oxide standards have an O atom at 1.6 or 1.7,
> why do you think it is chemically reasonable for your unknown sample to
> have such a short bond? If, in fact, that short bond doesn't exist
> elsewhere in nature, why would your sample magically have such a short bond?
>
> - If you do not believe in such a short bond, then does your
> experience with the standards give you the confidence to increase Rbkg
> such that the low-R signal is removed by the background function?
>
>
>
> I completely understand that you have a compelling reason to understand
> your unknown sample and that I am suggesting that you spend a good chunk
> of time measuring and analyzing a bunch of standards that are not your
> actual research project. That might seem like time that takes you away
> from your real goal, but you have already demonstrated that you are
> stumped by your real goal. I am saying the only way to get over your
> current hurdle is to take a step back and gain a deeper understanding of
> the data, the physics of EXAFS, the methods of EXAFS analysis, and the
> intricacies of your current problem.
>
> B
>
>
>
> --
> Bruce Ravel ------------------------------------ bravel at bnl.gov
>
> National Institute of Standards and Technology
> Synchrotron Science Group at NSLS --- Beamlines U7A, X24A, X23A2
> Building 535A
> Upton NY, 11973
>
> Homepage: http://bruceravel.github.io/home/
> Software: https://github.com/bruceravel
> Demeter: http://bruceravel.github.io/demeter/
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