Free Ion 4fn Levels of the Tetravalent Lanthanides. Fluorescence and Absorption Spectra of Cesium Dysprosium (IV) Heptafluoride

When a mixture of chloride or sulfate salts of cesium and dysprosium in a 3:1 mole ratio was treated with fluorine gas at 10‐60 psig and 200°‐350°C, the white Dy(III) starting material was converted to an orange compound with an empirical formula approaching Cs₃DyF₇. Fluorescence and infrared absorption spectra of fluorocarbon mulls of the orange fluorination product, measured at liquid nitrogen temperature, were compared with experimental and literature values of the energy levels for the ${\text{4f}}^8\mathrm{Tb\hspace{0.17em}(III)}$ and the ${\text{4f}}^9\mathrm{Dy\hspace{0.17em}(III)}$ . All of the components of the ⁷F ground‐state multiplet for the ${\text{4f}}^8\mathrm{Dy\hspace{0.17em}(IV)}$ were identified as was the ${}^5{D}_4$ resonance fluorescence level. The $\text{4f}$ energy levels of Dy(IV) were higher than those of the isoelectronic Tb(III) as would be expected from the less effective screening of the nuclear charge. Using the free‐ion approximation, the ${\text{4f}}^n$ energy levels were calculated by diagonalization of the energy matrices with the adjustment of two parameters, the coulomb integral $F_2$ and the spin‐orbit coupling constant ζ, until reasonable agreement was obtained with the experimental Dy(IV) levels. By interpolation, the parameters $F_2$ and ζ for the largely unexplored Nd(IV) and Tb(IV) systems were estimated and the corresponding ${\text{4f}}^n$ levels calculated.