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 $4f^2$ configuration. This identification is confirmed by a conventional crystal-field calculation of the energy levels and wave functions of the $4f5d$ and $4f^2$ 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 $4f^2$ configuration, the ground state of this ion in a sufficiently strong crystal field ($D_q>1000\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$) 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 $4f^2$ 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.

• Received 4 March 1969

DOI:https://doi.org/10.1103/PhysRev.186.276