Each rare earth ions has its own characteristic transitions depending upon allowed or forbidden transitions. For example Ce3+, Eu2+, Sm2+ have allowed f-d transitions where as ions like Sm3+, Eu3+, Nd3+, Tb3+, Pr3+, Dy3+, Er3+, Yb3+ have f-f forbidden transitions. Dieke Energy level diagram will help you to calculate the transitions for each rare earth ion.