[Ifeffit] Vanadium pre-edge peak fitting

[Will Bennett]

Hi everyone,

I've recently collected some V K-edge XANES spectra and am in the process
of analysing the data. The pre-edge region has been shown to be quite
useful for exploring the oxidation state and coordination geometry of
vanadium in various samples. This approach requires the pre-edge peak area
and centroid of unknowns to be compared with those of a suite of standards,
with these parameters typically determined by fitting a peak (or peaks) to
the pre-edge feature. The attached paper by Chaurand et al shows an example
of how this is often done, but it raised a question in my mind that I
wanted to clarify before proceeding to analyse my data in this way...

Is it necessary to justify a particular selection of peak fitting
parameters based on physical attributes? For example, Chaurand et al fits
multiple pseudo-voigt peaks to the V pre-edge feature, with the Lorentzian
contribution to this pseudo-voigt function constrained to be the same as
the core-hole lifetime width at the V K-edge (i.e. 0.8 eV). The Gaussian
contribution is allowed to vary, but the Lorentzian to Gaussian intensity
ratio is set at 1:1. After justifying these parameters, the authors go on
to say that *"It should be noted that width and height of the modeled
pseudo-Voigt functions have little physical significance, being a
convolution of two functions with significantly different width."* If this
is the case, does it really matter what specific peak fitting parameters
are used? If the goal is to simply obtain an accurate peak area and
centroid of the pre-edge feature, wouldn't an empirical peak fitting
approach provide comparable data? I'm not against fitting a series of
pseudo-voigt functions constrained in a similar way, to my data, but I'd
like to have a good justification for doing so.

I'm quite new to the world of peak fitting in the context of XAS data
analysis, so I'm looking forward to hearing the views of the mailing list.

Thanks in advance.

Will
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