We can use XPS to investigate/determine chemical identity of metal ions in complexes. For example, we expect the binding energy of Fe3+ to be higher than a neutral Fe atom and Fe2+ because of the increased effective nuclear charge. I have also seen some references for pure organic compounds in which the binding energy of a particular atom of reference (say carbon) increases as the electronegativity of an atom directly bonded to the carbon increases. However, these references do not give any reason why this trend.
I justify this on the basis that the electron to be knocked off is now experiencing a higher nuclear pull from both the parent atom as well as the neighbouring atoms leading to the observed increase in the binding energy.
However, if we have a complex in which the coordination number varies, say a square planar Ni2+ and an octahedral Ni2+, do we expect the binding energy of Ni in both complexes to be same?
Also, if for a specific geometry, we have nitrogen coordinated to nickel on one hand and oxygen on the other hand, how would these donor atoms with different electronegativity affect the binding energy of Ni?
Your contributions will be appreciated.
The position of a peak in XPS is affected by the initial binding energy state of the electron in the orbital. When something external to the atom changes the binding energy state of the electron in the orbital, the result will be a shift in the XPS peak position. Ligands pull or push electrons from/to an atom. That changes the effective charge on the atom. That changes the binding energy of the electron in orbitals. That shifts the XPS peak. Other factors can work in the same way. The coordination number effect ... I cannot say for certain whether it will or will not affect the initial state. Others may have greater insight.
The position of a peak in XPS is also affected by factors called final state effects. These have to do with what happens as the hole that is left when the electron leaves relaxes back to its ground state. I can image that coordination geometry has some subtle final state effect. This is equivalent to the changes that happens as one goes from individual atoms to atomic clusters to bulk materials. The final state is dissipated differently in each system.
Finally, metallic Ni and oxidized Ni have distinctly different peak positions.
Hello,
on the website http://www.xpsfitting.com/
you can get some hints. Look for the example "Copper". There it is shown the complexity of the photoionization prozess.
Dear David,
I would suggest to read the following articles: http://www.sciencedirect.com/science/article/pii/S0020169300806052
http://chempedia.info/info/34025/
They might help you with the "understanding". There are plenty of other articles dealing with this phenomenon, especially:
http://www.xpsdata.com/frmain.htm
Ask for this handbook at your library it would help you definitely. This is probably the largest collection of spectra.
Pavel
Hi David
In terms of electronegativity shifts etc, you seem to have a grasp on that and also Jeffery's reply is useful for you.
In terms of Different environments for BE shifts this can happen, although the magnitude can be small. Other things to look out for are changed in multiple splitting, especially for high-spin vs low-spin complexes for example. SOme examples drealing with such phenomena are (albeit for Fe and Co compounds)
https://doi.org/10.1016/0009-2614(76)80360-0
https://doi.org/10.1016/0368-2048(83)80028-0
http://www.surfacesciencewestern.com/pdf/04-006_apg.pdf
http://iopscience.iop.org/article/10.1088/0022-3719/14/25/010/pdf
A (non Ni Oxide) references for Ni which may be a good start for you is:
http://dx.doi.org/10.1063/1.446359
Dear Jeffrey J Weimer, Dietmar Hirsch, P. Dytrych and David J. Morgan ,
Indeed I appreciate your respective contribution to the question raised. I now have almost unlimited information on the subject. Thank you all for your time and contributions.
David, I wish I see Professor Jeffrey. He is a distinguished scholar.
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