[Ifeffit] fitting a specific k range

[Matt Newville]

Hi Matt,

On Wed, Dec 4, 2013 at 1:17 PM, Matt Frith <matt.frith at gmail.com> wrote:
> Dear All,
>
>
> I need some help in fitting an amorphous iron oxyhydoxide sample.  I am
> having difficulty producing a good fit, particularly in the k=4-6 range.
> Fitting this region well is very important for me, because if I add another
> metal(+3 oxidation state) into my system, this is where I observe the most
> quantifiable changes (The shoulder @ 5 A^-1 and the min @ 5.6 A^-1). Thus
> far I have been unable to fit the shoulder well enough to make meaningful
> comparisons.
>
>
>
> I have been fitting in kq with kmin=2.566 And kmax=10.877, and Rmin=1 and
> Rmax=3.5, and using the goethite O1.1, Fe.1, and Fe.3 paths. Attached is an
> Artemis file (P41_006_merge_norm_TRANS.fpj) for an amorphous Fe oxyhydroxide
> sample (Fe only, no other metals). The data was collected at the Fe K-edge.
>
>
> Is there a way to fit just this region (k~4-6 range) in k? If so what is the
> best method for doing this? If not, does anyone have suggestions as to how I
> can improve my fitting? Should I fit the data in k since the shoulder is
> less evident in kq?
>
> Thank you for your time.
>
> Sincerely,
>
> Matt Frith

I haven't looked at your project, but yes, you can restrict the fit k
range.  Just use kmin=4,kmax=6.   That said, I would not recommend
using such a short k-range.  For one thing, if you get a fit that does
match your data well in this region but doesn't match outside this
region, would you call it a good fit?   I sure hope not.

In general, any "features" that appear in chi(k) are a mixture of many
distances, and you may have to go out very far in R to accurately
reproduce them.   There are cases where you only have to consider a
split second or third shell to reproduce a diagnostic k space feature
for some mineral phase, but again, the better approach is to describe
this as a structural (R space) feature that translates into a
spectroscopic (k space) one.

Someone may be able to come up with a good counter-argument, but I
recommend not ever fitting in "k" space or "q" space -- just fit in R
space.

--Matt