[Ifeffit] determining reasonable fitting parameters

[Matthew Marcus]

Oh, so it assumes that chi(k) = (mu(E(k))-spline(k))/const.?  Mine does (mu(E(k))-spline(k))/spline(k), which I guess has its own problems - any wiggles
driven in the spline will multiply the EXAFS.  I haven't seen any effect from this, though.  Maybe the right thing is (mu(E(k))-spline(k))/smooth(E(k))
where smooth is a smooth post-edge background such as a polynomial or spline with fewer knots than the subtraction spline or even a tensioned spline.  I'd
be willing to bet that this refinement will make no detectable difference, especially if you do the reference data the same way.  I've found that you can do
the most amazingly bogus things and get away with it as long as you do them equally to your reference and unknown, especially if the reference is very
similar to the unknown.  Of course, you can't rely on that.
	mam

On 7/27/2016 1:45 PM, Bruce Ravel wrote:
> On 07/27/2016 04:39 PM, Matthew Marcus wrote:
>> I'm not sure I get your point about ion-chamber response.  Shouldn't
>> that normalize out in post-edge spline?
>
> Athena edge-step normalizes (i.e. normalizes by a constant) by default. It does not do a functional (or energy dependent) normalization by default in the way you are suggesting.
>
>> If the I0 chamber gets less sensitive as a function of energy, then the
>> post-edge background rises by the same amount
>> as the wiggles get bigger, so it divides out.  Similarly for probe-depth
>> effects, depending on concentration.  Another
>> couple of possible sources of amplitude error:
>> 1.    The model for the pre-edge background is inaccurate, so that its
>> extrapolation to the EXAFS region is incorrect.
>>     For instance, if there's a lot of elastic scatter getting into the
>> fluorescence detector, then the pre-edge is curved,
>>     and if you model that with a straight line fitted near the edge,
>> them you'll be off far from the edge.  Similarly, in
>>     a long transmission scan, both pre- and post-edge are curved.  My
>> background-sub program has simple models for thse
>>     shapes which aren't perfect but help.  This effect is really hard to
>> control for because you have no real way to know
>>     what the 'right' answer is, most of the time.
>>
>> 2.    Overabsorption will definitely reduce the amplitude of the
>> wiggles, and I'm surprise Bruce didn't menntion it.  It looks
>>     much like pinhole effect.  Harmonics do similar things as well in
>> transmission.
>
> My brain is really tiny and it's getting to be late in the afternoon here.  It's amazing that many bullet points actually made it into the email!
>
> B
>
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