Hi Brandon,
I don't find this terribly surprising if your samples are mixtures of
a metal and a metal oxide.
If you look at the derivative of the XANES spectra of a pure
transition metal and its oxides, you'll generally find that the
biggest difference is not in the position of the derivative peaks, but
in their relative size; for the metal the lower energy derivative peak
is much larger than it is for the oxide. A mixture of oxide and metal,
therefore, will be primarily evident in XANES through the relative
heights of these peaks; sometimes the apparent shift can be very
subtle indeed. For example, consider what happens if the first peak in
the first derivative is 20 times lower in the oxide than in the metal,
and is also 1.5 eV higher in energy. If you have one sample which is
70% metal, and you compare it to a sample which is 40% metal, the tiny
contribution of the oxide to the first peak will hardly be visible in
either case, and you might see no measurable shift in energy in the
XANES for that peak. (You'll see a big change in its amplitude,
though.) Likewise, the white line may be dominated by the oxide, and
show little shift in the XANES (but once again, a big change in
amplitude).
An EXAFS fit, however, doesn't suffer from weighting issues in the
same way, and may indeed show an E0 shift when the mixture of
oxidation states changes.
For my description to apply to your system, though, I'd expect you to
be seeing substantial changes in the amplitudes of XANES features,
even if their position doesn't shift much. Is that the case for your
data?
--Scott Calvin
Sarah Lawrence College
On Jun 5, 2011, at 11:56 PM, Brandon Reese wrote:
> Hi all,
>
> I am looking at EXAFS of thin film metal oxides. I am varying both
> metal content and the oxygen content of the films. I aligned the
> scans with a metal reference foil collected simultaneously. In
> Artemis, I have noticed that when changing between films with no
> extra oxygen versus those with extra oxygen there is a shift in the
> fitted E0 of ~1.5 eV (after aligning to the foil). I tried setting
> the E0 in Athena to the peak of the 1st derivative and the peak of
> the white line with the same result (~7 eV difference). I was a
> little surprised by the offset because in Athena the E0 values
> varied by <0.5 eV. I am not sure if the argument could be made that
> this shift is a result in a changing oxidation state because it
> doesn't show up in the XANES (at least qualitatively). Are there
> other experimental effects that could cause a shift like this, or is
> this likely something real in my material? If anyone want to see a
> representative group of data, let me know.
>
>
>
