[crt]

I would have a question to XPS spectrum. Why the peaks do not follow the Aufbau principle? For example I found the spectrum of Sn in a book and the order of peaks is 3s 3p 3d 4s etc. Why isn't it 3s 3p 4s 3d etc?

Thanks a lot.

[Mitch]

It is a tricky concept. XPS measures the binding energy of the electron, and 4s electrons are more tightly bound than a 3d electron.

X   X⁺ + e^-

Aufbau's principle is more subtle, it is about the lowest energy configuration of electrons in a neutral atom. Weird I know, I didn't really get it until graduate school.

[crt]

Thanks a lot for the explanation.
You say 4s electrons are more tightly bound. But then I would expect the signal of 4s electrons in higher regions of the spectrum. Isn't it then the other way around? 3d electrons are more tight than 4s?

So this means if an orbital is on lower energy level the electrons are not necessarily bound more tightly? Is this right? It would be really weird for me.

[crt]

Problem solved. I have got the answer.

So this is because the Koopmans' theorem is only a rough approximation. The IE is not exactly the binding energy with negative sign. The theorem does not take into account relaxation and perturbation processes which result in swapping of orbitals on similar energy level.

The other thing is that IE is defined for isolated atoms and molecules but XPS is done on solid samples. The environmental effect cannot be neglected.

That's it...

[Mitch]

Thanks for the follow up.