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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