The UV-visible absorption bands in the spectra of lanthanoid ions are 'X', probably because of the excitation of electrons involving 'Y'. The 'X' and 'Y', respectively, are:

In lanthanoids, the $4f$ orbitals are well shielded from the surroundings by the outer $5s$ and $5p$ electrons, so the $f$-$f$ transitions are only weakly perturbed by the ligand environment, producing narrow (sharp) absorption bands due to excitation of electrons within the $f$ orbitals.