XPS core-levels are characterised by the nomenclature nlj where n is the principal quantum number, l is the angular momentum quantum number and j = l + s (where s is the spin angular momentum number and equals ±½).

All orbitals except the s levels (l = 0) give rise to a doublet with the two possible states having different binding energies - This is spin-orbit splitting. The peaks will also have specific area ratios based on the degeneracy of each spin state, i.e. the number of different spin combinations that can give rise to the total j.

As an example, for 2p spectra, where n is 2 and l is 1, j will be 1/2 and 3/2, the area ratio for the two spin orbit peaks (2p1/2:2p3/2) will be 1:2 (corresponding to 2 electrons in the 2p1/2 level and 4 electrons in the 2p3/2 level).

A similar argument is made for the 3d peaks (3d5/2 and 3d3/2) , but here the peak aera ratio is 3:2 and so on. (see image attached)

Please go though the atomic shell model for the detail understanding. When we include spin–orbit interaction, the energies of states of the same level but with different j will no longer be identical. These levels represents the different energy level.

Please read the attached file. it is useful to understand this point.