ENERGY LEVELS OF CE2+ IN CAF2

The absorption spectrum of cerium-doped CaF2 has been observed at several temperatures. When the cerium is reduced to the divalent state at room temperature, the absorption spectrum of Ce2+ is found to be unstable against thermal decay or bleaching by visible light; furthermore, this spectrum is found to comprise one state of a photochromic system. The absorption spectrum of CaF2: Ce2+ at low temperature is seen to consist of transitions to levels of the 4f2 configuration. This identification is confirmed by a conventional crystal-field calculation of the energy levels and wave functions of the 4f5d and 4f2 electronic configurations of Ce2+ occupying a cubic site in CaF2, which shows that while the ground state of the free divalent cerium ion is a level of the 4f2 configuration, the ground state of this ion in a sufficiently strong crystal field (Dq>1000 cm-1) is a T2 level of the 4f5d configuration. Observations of the Zeeman effect have been made which confirm the T2 character of the ground state. The energies and relative intensities of the allowed transitions between this ground state and the levels of the 4f2 configuration which were calculated using these wave functions were found to be good agreement with our optical and near-infrared absorption data, the agreement being especially good for the near-infrared portion of the spectrum. © 1969 The American Physical Society.