The three groups of fluorescent line spectra of Tb<sup>3+</sup> in CaF<sub>2</sub> " spectrum I" between the crystal-field split levels of the <sup>5</sup><i>D</i><sub>4</sub> and the <sup>7</sup>F<sub>3</sub>, <sup>7</sup>F<sub>4</sub>, and <sup>7</sup>F<sub>5</sub> free-ion states have been combined into energy-level schemes which tend to support a cubic crystal field symmetry and which give information on the crystal-field split-leve representations and energies to 5 cm<sup>-1</sup> for a thin crystal.
NORMAN RABBINER, "Fluorescence of Tb3+ in CaF2," J. Opt. Soc. Am. 55, 436-438 (1965)