[Wien] Photon energies in XES ...

[Vladimir Timoshevskii]

Dear Wien2k users and developers,

I am working with experimentalists and try to simulate the XES measured by
soft x-ray detector, coupled with electron microscope. So, the ionization
source in this setup is the electronic gun of the TEM. The test compound is
hexagonal layered BN, which was quite well studied before, including the
similar setup. I would greatly appreciate if you could share your opinion
on the following 2 issues, which I am facing now:

i) Is it possible, in principle, to obtain correct photon energies instead
of shifting spectrum by hand to the EF position? I understand, that the
position of the core level (B-1s) is sensitive to the form of the potential
well, and the closer my potential is to the real one, the better is the
position of the core level. I tried different XC-functionals, and found
that actually the atomic-like Hartree-Fock gives the best results: the
whole spectrum (B K-edge) is shifted to higher energies, closer to
experiment, and the spectrum shape is also much better. However, there is
still ~10eV shift, relative to the experimental spectrum. So, the
XC-functional alone does not solve this problem ...

ii) This is a more fundamental question, and is actually related to the
first one. I guess, the main reason for the photon energy underestimation
is the presence of the core hole, which shifts the ionized core level to
lower energies. I did several test calculations of B-K spectrum using
supercells of diffferent sizes with a core hole in B 1s. Indeed, by playing
with fractional B 1s occupation (trying to catch the "transition state"),
it seems to be possible to shift the whole spectrum to experimental
position. But in this case, what about the "rule of the final state"?
According to this rule, the hole must be created in the valence band (and
screened out), and the core lavel must be filled. This is what we normally
assume ... Does that mean that the XES calculations with hole in the core
are unphysical, in spite of giving better photon energies? May be, the
situation here, especially when we use TEM electronic gun for core
ionization,  is more complicated? In my opinion, the valence-core
transitions are happening in the potential, already distorted by the
presence of the core hole. Am I right? Then, how it agrees with the "rule
of the final state"? Any thoughts on that would be highly appreciated!

Thanks a lot in advance!

Vladimir Timoshevskii
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://zeus.theochem.tuwien.ac.at/pipermail/wien/attachments/20150828/3e7120a6/attachment.html>