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The rare-earth ions may exist in the divalent state in suitable host crystals such as CaF2. All of the trivalent ions from Ce to Yb (and probably also La) are reduced in situ to the divalent state in CaF2 by gamma irradiation. The spectra of most of these ions show that the ground and first few excited states derive fromfn configurations, but the weak absorption due to these is masked at higher energies by strong broad bands of the parity-permitted f"->fn-'d transitions. The excitation energy of these spectra have been calculated in a first approximation as the energy difference between the "Hund rule," single-determinant states of the configurationsr-'d andf". This procedure satisfactorily accounts for the remarkable variations in the excitation energy in passing from one ion to the next in the series with the exception of Gd++ Ce++ and Tb++. Gd++ probably has Fa for its ground configuration, while Ce++ and Tb++ are borderlin~ cases: !he spectral str:uc~ure probably arises chiefly from the crystal-field splitting of the a orbital, since each ion m CaF2 has a Similar spectrum, and the spectra change drastically in sites of other than cubic symmetry.

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