[Ifeffit] white line intensity v. edge position indeterminingoxidation state of Re

[Anatoly Frenkel]

I agree with Bruce on this, and would like to add that in nanoparticles the
very Fermi energy may be shifting around - as a function of size,
metallicity, support-particle interaction, etc. For example, in bulk
materials, the change in the oxidation state of an absorber by, say, -1, due
to reduction of an oxide, for example,  is accompanied by the shift to the
lower energies of characteristic XANES features (peak position, max
derivatives, or the energy at the half edge step, - these are most popular
"reference" points for the chemical shift determination). The reason for
this "chemical" shift is screening of the coure hole potential by the
"extra" electron and the concomitant decrease in the ionization potential.

However, in nanoparticles, the changes in the charge state of the absorber
also noticeably change Fermi energy, which a few extra electrons per
particle _increase_ Fermi energy compared to the initial sample. That, of
course, causes the increase of the ionization potential. Which of this
trends prevails is a very interesting question - we have seen both positive
and negative shifts in nanoparticles, which means that these effects are at
least the same order of magnitude.

What is important, however, is that the energ shift in nanoparticles does
not scale simply with the formal oxidation state.

Anatoly


-----Original Message-----
From: ifeffit-bounces at millenia.cars.aps.anl.gov
[mailto:ifeffit-bounces at millenia.cars.aps.anl.gov]On Behalf Of Bruce
Ravel
Sent: Monday, May 21, 2007 6:59 PM
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] white line intensity v. edge position
indeterminingoxidation state of Re


On Monday 21 May 2007, Sven L.M. Schroeder wrote:
> Edward
>
> Another method to get information on oxidation states from near-edges was
> used relatively commonly in the 1980s and early 1990s, but has gone out of
> fashion since (can someone tell me why? Is the underlying physics
flawed?).
>
> You determine the ionisation potential (IP) by fitting the arctan step
> background function simultaneously with Gauss/Lorentz lines for all the
> near-edge resonances. The inflection point of the arctan corresponds to
the
> IP. Variations in the IP can be interpreted using the same considerations
> as for binding energy shifts in XPS...
>
> For example, for Au there is an almost perfectly linear relationship
> between IP and oxidation state, from Au(-1) via Au(0) and Au(+1) to Au(+3)
> ! In this case the correlation with oxidation state is a lot stronger than
> for white line intensities and/or the now commonly used 'edge inflection
> points'...
>
> Sven

My understanding of the physics behind the assumption that you can fit
a XANES spectrum by an arctan and some peak-y functions is that you
are assuming that the system is a perturbation from a simple,
Drude-like gas of electrons.  At zero kelvin, the electron density is
a well defined, sharp step function with the step at the Fermi energy.
At finite temperature, that sharp edge step is smeared into a Fermi
function -- essentially an arctan.  As a perturbation, there is some
structure to the electron density that is handled by the peak-y
functions.  To the extent that you can point to the location of the
Fermi energy on a XANES spectrum, the underlying physics of using an
arctan is well-founded.

In the sequence of pure-materials you describe, the arctan method
should work well assuming you are talking about pure phase materials,
even in the case of Au+3 where the "perturbation" (also known as a
rather enormous white line) is not small.

However, I don't think the use of one arctan is well suited to a mixed
phase situation, which is a possibility for what Edward is seeing in
his electrochemistry experiments.  That is, at some intermediate
voltage, a fraction of the sample might be oxidized and a fraction
metallic.  In that case, the XANES spectrum needs two arctans mixed in
some ratio.  Then the fitting of lineshapes has to include a mixing
term to describe the amount of each species.  The line shape fitting
thus inherits some of the systematic uncertainty of the linear
combination approach, along with all the rest of its sources of error.

In the case of mixed species, tracking a feature in the XANES
(inflection, white line position, what have you) can usually be shown
to be as accurate as a linear combination solution assuming the
correct feature can be identified.  And in some systems, fitting a
single arctan might work well also -- but that would not be
generalizable to all systems.

My US$0.02 worth,
B


--
 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

 Molecular Environmental Science Group, Building 203, Room E-165
 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007

 Argonne National Laboratory         phone and voice mail: (1) 630 252 5033
 Argonne IL 60439, USA                                fax: (1) 630 252 9793

 My homepage:    http://cars9.uchicago.edu/~ravel
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/


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