Divalent lanthanide ions ground state

In a vast majority of these applications, these ions are introduced into crystalline solids in their stable trivalent state. However, there are a few lanthanide ions whose divalent state is also stable, e.g., Eu ", Tm ". One of the main differences between the trivalent and divalent lanthanides (apart from the electrical charge) is that the charge-transfer transitions and the parity allowed 5d-4f transitions of the divalent ions are located at lower energies than those of the trivalent ions because of this they can more easily studied experimentally. Regarding Tm " ions, it should be kept in mind that the 4f electron shell has only two energy levels the ground state p7/2 and the excited state located at approximately 9500 cm [2]. As a result, the 4f-4f and 4f-5d transitions of the Tm ions do not overlap and can be easily studied separately. 

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