[Ifeffit] Any coorelation about 'white line" in K-edge XAS with oxidation states?

[Robert Gordon]

Hi Marco,

Yes, a s - d transition is dipole forbidden, but the quadrupole 
transition can occur, as can a transition
s - mixed p/d. The edge position in XAFS for metals like Ni and Cu is 
taken at the inflection point leading to
the first main absorption feature, but is considered to have s - d 
character. For NiO, features near this edge
indicator in the figure would be considered pre-edge. For NiO, the edge 
position would be considered
shifted from that of Ni foil.

The white line for a K-edge generally corresponds to a noticeable number 
of unoccupied states with p character
on the absorbing atom in the presence of the core hole (i.e. 
intermediate states) with "small" energy dispersion.
Look at the energy difference between the edge indicator and "white 
line" indicator. That is about 18 eV.
For most materials, the work function is 4 - 6 eV between the highest 
occupied states and the continuum
in the ground state. Edge position is sensitive to charge transfer 
between target atom and ligands, not specifically
a formal oxidation state. What you see is not a measurement of the 
ground state Density of States, but an influence
of the core hole on intermediate states, and of the emitted 
photoelectron during the lifetime of the corehole.

Much of this terminology also arises from consideration of bulk 
materials, not nanoscale.
Features in the XANES can arise from long range interactions, which are 
lessened as the scale
gets considerably reduced.

The figure shows a clear transition from something with a pronounced 
white line feature
to something with a less-pronounced feature and increased intensity near 
the foil edge energy, which I would
interpret as a loss of coordinating oxygen yielding something more 
metallic, or at least more covalent,
in character on the substrate. I am curious as to what Ni2Mo3N Ni K-edge 
would reveal in comparison
to their results. It is not unreasonable for the authors to conclude a 
reduction of the Ni is occurring by loss
of coordinating oxygen - "fully reduced to Ni(0)"...well, that is a bit 
of an overstatement - "extent of electron transfer
from the Ni reduced to that of a more metallic/covalent environment" 
would be more exact, but I understand
what is meant and would not argue with another author's writing style if 
it is not an egregious overstatement.

So, to answer your question:
"how should we correctly interpret the “white line” and edge energy from 
XAS spectrum of metals’ K-edge and L-edge?"

We do this by a careful consideration of known standards and knowledge 
of the influence of bonding
environment and extent of electron transfer between absorber and ligands

The concept of formal oxidation state (oxidation/reduction) is a 
convenient means of discussing electron
transfer in more ionic systems but less useful as the degree of 
covalency increases.


-R.





On 2021-12-11 6:11 a.m., Marco Wang wrote:
>
> Dear everyone:
>
> Ihave some questions about the interpretation of "white line", from 
> anarticle (https://doi.org/10.1038/s41467-021-27116-8) and the figure 
> was attached. In the article, it is said that _“the sample was fully 
> reduced to Ni (0) state at ~480 °C judging from the edge position and 
> ‘white-line’ profiles”_.
>
> First, we assumed that the metal wascoordinated with the same anion.
>
> We all know that /K/-edge transition in 3d and 4d metalis' spin 
> forbidden, so it may be inappropriate to conclude that the intensity 
> of the "white line" is correlated with metals' oxidation states. But 
> in otherwords, the probability for s→d transition may be proportional 
> to thenumber of empty d orbitals, so it seems likeif the theintensity 
> of "white line" is higher, the oxidation state is higher.
>
> So, how should we correctly interpret the “white line” and edge energy 
> from XAS spectrum of metals’ /K/-edge and /L/-edge?
>
>
>
> Thank you in advance for your time.
>
> Marco Wang
>
>
>
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