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I was trying to figure out the different peaks in a XPS spectrum for Molybdenum, but I could not understand them. About XPS, I know that this technique is based on the binding energy of the electrons of different elements

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you are quite right, XPS does measure the 'binding energy' of electrons. More specifically, XPS bombards a surface with X-rays and when those X-rays have sufficient energy they are able to sufficiently excite electrons to be emitted from the sample. Each peak in an XPS spectrum corresponds to the energy necessary to emit a given electron.

So what would you expect to see for Mo?

  • Outer lying electrons (for Mo, we are talking about electrons in the 4p and 4s orbitals) will emit at a lower energy. You should observe low-energy peaks that correspond to these emissions (approx. 30 - 60 eV).
  • you should see a cluster of peaks at approx. 230 eV that correspond to the d-orbital electrons.
  • Finally, you might see discrete peaks at very high energy, approx. 2600 - 2800 eV that correspond to emission from the 2s and 2p orbitals

In general, outer orbitals emit at lower energy, and inner orbitals emit at higher energy.

Hope this helps!

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