The spectrum of Tm III in CaF2

The crystal field of CaF2 is evaluated on a point-charge model, and from comparison with the crystal field constants derived from the optical spectrum (Kiss 1962) of Tm III in CaF2, empirical values of < r4>= 4.3 a.u., < r6> = 36 a.u. are found. These are considerably larger than the estimates of Freeman & Watson (1962). The crystal field data reveal a small discrepancy with the paramagnetic resonance data of Hayes & Twidell (1961), which can be resolved by the introduction of an orbital k-factor which is slightly less than unity (1 — k — 0009 ± 0-001). Comparison with the spectrum of Tm I measured by atomic beams (Ritter 1962) shows that the magnetic hyperfine constant for a free Tm III ion is (2.2 ± 0.5 )% greater, while the spin-orbit coupling is 1.2% greater, assuming a core polarization correction of the same magnitude as that measured for Eu III in CaF2. Similar effects are shown to occur for Ho III in CaF2. The results suggest that the values of < r-3 > and the spin-orbit coupling parameter ζ are only about 1 to 2 % greater for the free divalent 4 f ions than for the atoms.