[Ifeffit] Problem with XAS Viewer

Matt Newville newville at cars.uchicago.edu
Sat May 2 08:35:53 CDT 2020

Hi Xinyue,

On Fri, May 1, 2020 at 9:16 PM Xinyue Wang <xyw0719 at gmail.com> wrote:

> Hi,
> I am Xinyue, a student who interested in using XAS Viewer. My issue is
> when I tried baseline fitting, I followed the document. But there was no
> region, no centroid, and no fitting line shown. Here is a screenshot of my
> work. Could you please help me with this?
> Thanks for your help.

It's hard to tell from the screenshot alone.  Just to clarify, the idea for
the panel is to fit pre-edge peaks to known lineshapes (Gaussian,
Lorentzian, Voigt, etc).  The interface could probably be used for
general-purpose curve-fitting of many kinds of spectra, but right now it
definitely skews to pre-edge peak fitting.

Because of that emphasis, the idea is to first press the "Fit Baseline"
button -- this will do a crude fit of a Lorentzian + Line that are meant to
represent any residual background and the main absorption edge.  The fit
for this baseline is done over the energy range defined by "Fit Energy
Range" but ignoring the "Pre-edge Peak Range", which is meant to be the
portion of the XAS spectra where the pre-edge peaks are. That is, this
baseline fit should
look something like Figure 5.4.2 at
In that figure, the black dots show the ends of the Pre-edge Peak Range
that is ignored in this baseline fit.   For sure, the initial guess for
this range may not be very good.  But it is easy to refine this ignored
range.  It is also not actually critical to get this baseline fit perfect -
it's just a place to start the fits of the peaks themselves, and the
remaining fits will refine the baseline as well as the peaks over the full
energy range.     After a baseline fit is done, you can add peaks and then
use the "Fit Model", which fits over the whole "Fit Energy Range",
ignoring the "Pre-edge Peak Range".

>From your screenshot, I don't see much of a pre-edge and not many obvious
peaks.  The result is that the actual fit energy range for the baseline
(that is
"Fit Energy Range" - "Pre-edge Peak Range") is very short.  I can believe
the fit will fail under those conditions.  But I sort of think that failure
really comes because your data doesn't have pre-edge peaks.

Maybe you are trying to do something more complicated?

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