Infrared Absorption of Ce3+ in LaF3 and of CeF3

The infrared absorption spectra of Ce3+ in LaF3 and of CeF3 have been observed in the region of 3 to 6 μ. The four absorption lines for Ce3+ in LaF3 at 2160, 2240, 2635, and 2845 cm−1 that persist at liquid-helium temperature are identified as transitions originating on the ground Stark component of the 2F5/2 and terminating on the four Kramers-degenerate Stark levels of the 2F7/2. Observation of the temperature dependence of the spectrum has revealed two additional transitions beginning on a 2F5/2 Stark level 150 cm−1 above the ground state and ending on the two lowest levels of the 2F7/2. The results are essentially the same for CeF3 and are in substantial agreement with the results of temperature-dependent studies of the magnetic susceptibility and Faraday rotation of CeF3 and tysonite.