Energy Levels ofCe2+in CaF2

The absorption spectrum of cerium-doped Ca${\mathrm{F}}_{2}$ has been observed at several temperatures. When the cerium is reduced to the divalent state at room temperature, the absorption spectrum of ${\mathrm{Ce}}^{2+}$ 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 Ca${\mathrm{F}}_{2}$: ${\mathrm{Ce}}^{2+}$ at low temperature is seen to consist of transitions to levels of the $4{f}^{2}$ configuration. This identification is confirmed by a conventional crystal-field calculation of the energy levels and wave functions of the $4f5d$ and $4{f}^{2}$ electronic configurations of ${\mathrm{Ce}}^{2+}$ occupying a cubic site in Ca${\mathrm{F}}_{2}$, which shows that while the ground state of the free divalent cerium ion is a level of the $4{f}^{2}$ configuration, the ground state of this ion in a sufficiently strong crystal field (${D}_{q}g1000$ ${\mathrm{cm}}^{\ensuremath{-}1}$) is a ${T}_{2}$ level of the $4f5d$ configuration. Observations of the Zeeman effect have been made which confirm the ${T}_{2}$ character of the ground state. The energies and relative intensities of the allowed transitions between this ground state and the levels of the $4{f}^{2}$ 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.