While I understand the core electron binding energy of an element (XPS), e.g. Cr, will increase if some of its valance electrons participate in bonding formation, however, I am not sure about the followings:
(1) Whether there is any correlation between core electron binding energy and bond strength, say for a certain type of bond, the higher core electron binding energy indicates stronger bond and vice versa? Or, no such correlation? For example, in transition metal carbides or nitrides, Cr23C6, CrN etc.
(2) Or, is the electron binding energy only impacted by the number of valence electrons participating in bonding formation (outer shell electrons deviating from the nucleus, thus the nucleus will have a higher attraction towards the core electron)?
Aside questions:
(1) What're the experimental methods of evaluating a bond strength or bond energy, say, between Cr and nitrogen in different species of chromium nitrides?
(2) Is the bonding energy the same as the bond dissociation energy? Can the bond strength (energy) be presumably induced from the melting points or hardness values of compounds, e.g carbides and nitrides?
Thanks!
Zhe
Hello Zhe,
the core electrons are hardly influenced by bonding. The main influence can be detected for valence electrons. This can be maesured by different methods. A very sensitive method is the PES (Photo Electron Spectrometry). Here, the whole band structure of materials can be found.
I myself conducted XAS (X-ray absorption spectroscopy). Let me give an example for copper. Pure copper has 29 electrons, therein one 4s-electron. All d-states are occupied. In pure copper no excitation from a core state into a 3s-state can be found. In CuO, the 4s and a 3d state is empty - these electrons fill the oxygen shell. Now you find a transition from core state (2p) - what is a good reference energy - into a 3d-state. The oxygen spectrum (1s into 2p is changed compared to pure oxygen.
If you investigate CuO2 states (Cu-O-planes in superconductors), the copper lines 2p into 3d are very strong and shifted with respect to Cu in CuO. The same is valid for oxygen.
The differences in the line positions are due to the change of energies in the valence states.
With Regard
R. Mitdank
Rüdiger Mitdank Thanks very much for your response!
So, if I understand correctly, you said the shift observed in Cu from 2p to 3d states is more obvious in CuO2 as compared to CuO due to higher energy level of valence states, is that correct? I did not dig into the Cu-O bond strength in CuO and CuO2, but do you think the energy level of valence states can reflect the bond strength? In another word, is the Cu-O bond strength in CuO2 higher than that in CuO?
To expand a bit, if we compare the Cu-O bond in Cu2O, CuO, CuO2 with different valency (not sure about the valency in CuO2 btw), what is the order do we expect? And I assume the core electron binding energy of Cu is increasing from Cu2O to CuO to CuO2, correct?
My assumption may be totally wrong, but just want to understand the correlation between bond strength and core electron binding energy, though the interaction may be indirect.
Thanks ahead!
Zhe
@ Zhe:The energy of the core level is nearly unchanged because the interaction between the different valence electrons is screened by the remaining shell electrons. But the energies of the valence levels are changed due to the interaction of shell electrons of the different elements (in my example Cu and O). This leads to different transition energies (here between 2p and 3d and 1s to 2p for oxygen) but the energies for 2p and 1s remain and the outer (interacting) states 3d and 2p change in the order of some eV.
R. Mitdank
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