Abstract
The crystal field of CaF
2
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 CaF
2
, empirical values of <
r
4
>= 4.3 a.u., < r
6
> = 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 CaF
2
. Similar effects are shown to occur for Ho III in CaF
2
. 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.
Reference18 articles.
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