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
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!