[Ifeffit] Re[2]: a question about background refinement

[Matt Newville]

Hi Stanislav,

Sorry I missed the point that the different E0's were 
for two different data sets!

>> > Can you get decent results with
> >> >a lower Rbkg/Rmin?
> >> No, I can not. 1.5A is around the minimum value in Chi(R) magnitude
> >> where the first peak can be separated from the others. However, the peak
> >> is a result of superposition of background, oxygen path and also
> >> the alloy structure.
> 
> MN> I think I don't understand this.  What are the "others" that the
> MN> first peak can't be separated from??
> 
> Sorry for the confusion that I have introduced.
> I am doing two-data set simultaneous fit (Bruce already mentioned that in the
> list) where the two absorbers are Ru and Pt atoms
> The model structure looks like this:
> 1. Ru data
> FEFF0(Ru-Ru):feff0001
> FEFF1(Ru-Pt):feff0001
> FEFF2(Ru-O):feff0001
> 2. Pt data
> FEFF3(Pt-Pt):feff0001
> FEFF4(Pt-Ru):feff0001
> 
> I was talking about the first part of the model
> that is related to Ru data. What I meant was that around 1.0-1.5A I saw a
> peak. According to the model that peak was a result of overlap of background and
> contributions from different scatterers, such that I could not say
> that the peak was primarily because of oxygen or any other scatterer.
> 
> I think the above was what you described as:
> >'first shell peak', which is sometimes difficult
> >to define for the asymmetric peak common with short metal-oxygen
> >distances
> 
> Now if I set rmin=1.5A, the peak is not included in the R-range(it is
> below rmin). At lower values of rmin a part of the peak is in the range.
> That could be the reason why I was unable to get decent results with lower
> rmin.
 
Hmm, it does seems a little troubling, but it might be OK.  I guess
that's why you asked in the first place. Sorry I can't give a more
definitive answer without looking at the data and/or fit.  

Does the Ru-Ru/Ru-Pt contribution overlap with Ru-O in this 'low-R
peak'?

Related to that: Is there a good understanding of why the Ru would
partially oxidize while the Pt appears to not oxidize? This seems
perfectly reasonable to me, but I'll ask anyway: you see Ru-O but
not Pt-O right?

If you see that the Pt edge background is OK and there's no need to
add oxygen to the Pt model, that does make the need to include a
'background+oxygen peak' at the Ru edge more believable. But I'd
still suggest looking at the different fit contributions in both k-
and R-space.

Hope that helps, even though you've probably tried all these things
and more....

--Matt