Core hole effect plays an important role in determining X-ray absorption spectra (XAS) from first principles. Sometimes core hole effect is negligible as compare to spectra where core-hole is absent in the calculation. What are the physical reason behind this? Simply, what are the factors affecting the core-hole effects in any material?
Any help will be appreciated.
Hi Soumyadeep,
Your question is very interesting, and I guess you are a theoretician.
I will try to shed some light on this issue as a experimentalist. But before that I am assuming that you know the reason behind the large core-hole effect in photo-electron spectroscopy (e.g. XPS) and negligible core hole effect in XAS.
Consider the case of L egdes XAS, for 2p6 3d9 system (spin orbit splitted 3d3/2 & 3d5/2 initial states). In the final state after excitation, in such system, you will end up with the spin-orbit splitted 2p3/2 & 2p1/2 states, and the usual selection rule will give you the features in XAS. This description comes from totally atomic point of view. In case of some oxide materials (for example with strong hybridization TM-O), it is energetically possible to transfer one electron from the O 2p band to the above mentioned atomic states of TM. Because of this so called "charge transfer effects" the additional contribution appears i.e. 2p6 3dn+1 L(bar) in the initial state and 2p5 3dn+2 L(bar) in the final state. And now the additional multiplets will appear in the atomic spectra. Similarly, other distortion and field (e.g. crystal field) will further modified the initial and final state and hence the spectra.
Finally, (your concern), because of this charge transfer the effect of 2p core hole (in this example) is screened well.
I hope this will provide you some information.
Regards
A. Ahad
Dear Abdul, thank you very much for the answer.
So according to your explanation core-hole effect in XAS depends on factors like (i) charge transfer and (ii) crystal field splitting.
Crystal field splitting does not affect core-hole inetraction. It just lifts up the degeneracy in d-orbitals in case of TM compounds. So, additional final states are possible.
XAS spectrum in this case will contain additional features corresponding to modified final states (for 2p to 3d transitions), which will be absent if there were no crystal field splitting. So, CF results into modification of final states, but does not influence core-hole interaction.
Dear Kiranjot, thanks for the answer.
First of all, the physical reason behind crystal field degeneracy lifting is the hybridization between TM compound's d-orbitals with surrounded ligand's p-orbital.
If CF results modifies the final state/add some additional final states then the matrix element in Fermi-Golden rule modifies, which in-turn modify the spectrum.
But I completely agree with Abdul Ahad and Kiranjot Dhaliwal that creation of core-hole does not influence CFS. Only modify the partial DOS of constituent atoms, hence the spectra. I just misinterpret Abdul's answer.
Dear Soumyadeep,
These are the factors which affect the core hole potential in any material:
1) screening by the excited photoelectron valid in the case of XAS.
2)charge transfer effect---related to covalency.
3)distortion or presence of a shorter bond length also results in the increase of core hole interaction.
4)co-ordination no. & geometry: octahedral or tetrahedral---octahedral geometry generally has more screening as compared to tetrahedral one which affects the core hole potential.
Hope it helps?
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