[Ifeffit] k-range question & R-factor

Scott Calvin scalvin at sarahlawrence.edu
Tue Jan 15 08:39:36 CST 2013


Hi Chris,

I don't see a reason to think that data is a glitch. For one thing, it's not consistent across datasets. The features also look smooth, and not so glitch-like. The spike around 8.2 inverse angstroms in some of the datasets looks a bit more like a glitch, but it's fairly modest and narrow enough not to mess you up too much.

The spacing of those features look OK--there's a double feature in some of the datasets around 6-7 inverse angstroms; it's plausible there would be another reature like that above it. In fact, I can make an argument that there's some kind of beating going on that gives a shoulder at 3.5-5, a double peak at 5-7, and two peaks at 7-8 inverse angstroms.

So I would recommend including that data and seeing what it does to your fits. If that range is garbage, your fits will probably reject it.


As for your second question, R-factors are always a kind of average across the data, by definition. So "total" mismatch doesn't really make sense. Off-hand, though, I don't recall how ifeffit weights the data for the purposes of calculating R-factors for multiple datasets, and that may be your question.

--Scott Calvin
Sarah Lawrence College

On Jan 15, 2013, at 9:21 AM, Christopher Patridge wrote:

> Hello Users,
> 
> I was looking for an opinion about the chi(k) signal in a set of data I 
> am analyzing.  Brief background, this is a set of in-situ XAS data 
> collected at the Fe K edge from a working electrochemical cell at a 
> range of potentials during charge; I did not collect the data. I suspect 
> the feature at ~ 8 angstroms-1, although present in all the spectra is 
> noise or glitch and wondered if I am being overly cautious?
> 
> My conservative range ( k = 2-7 and R = 1-2) really constrains the model 
> Nidp = 3.31.  Luckily, multiple datasets ( 8 ) to the rescue to give me 
> some flexibility.  In a multiple dataset fitting, is the R-factor of the 
> whole set just the average or total mismatch across all the datasets or 
> it calculated another way?
> 
> Working towards happiness,
> 
> Chris Patridge
> 
> -- 
> ********************************
> Christopher J. Patridge, PhD
> NRC Post Doctoral Research Associate
> Naval Research Laboratory
> Washington, DC 20375
> Cell: 315-529-0501
> 
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