Electronic Spectra of Cesium Fluoride Complexes of Pentavalent Neptunium

Abstract

The absorption spectra of magenta‐colored CsNpF₆ both as a mull of the crystalline solid and as a solution in CsF·2HF were obtained under various conditions. The room‐temperature spectra were unique to the electronic energy levels of the seven Russell–Saunders states for the ${\text{5f}}^2$ configuration, split by spin–orbit coupling into 13 free‐ion levels; this confirms the chemical evidence for the presence of pentavalent neptunium. At liquid‐nitrogen temperatures the spectra of CsNpF₆ mulls were interpreted in terms of the free‐ion $\mathrm{f–f}$ transitions of Np⁵⁺ split by an $O_h$ crystal field. Term assignments to the following experimental energy levels obtained at room temperature were made on the basis of the observed Stark splitting in the low‐temperature spectra, and agreement with calculated energy levels (in cm⁻¹): ${}^3{F}_2{\text{\hspace{0.17em}=\hspace{0.17em}4337,}}^3{H}_5{\text{\hspace{0.17em}=\hspace{0.17em}8425,}}^3{F}_3{\text{\hspace{0.17em}=\hspace{0.17em}8797,}}^3{F}_4{\text{\hspace{0.17em}=\hspace{0.17em}9701,}}^3{H}_6{\text{\hspace{0.17em}=\hspace{0.17em}13 738,}}^3{P}_0{\text{\hspace{0.17em}=\hspace{0.17em}16 129,}}^1{D}_2{\text{\hspace{0.17em}=\hspace{0.17em}16 736,}}^1{G}_4{\text{\hspace{0.17em}=\hspace{0.17em}18 265,}}^3{P}_1{\text{\hspace{0.17em}=\hspace{0.17em}19 531,}}^1{I}_6{\text{\hspace{0.17em}=\hspace{0.17em}20 619,}}^3{P}_2\text{\hspace{0.17em}=\hspace{0.17em}26 667}$ . All levels above the ${}^3{H}_4$ ground state were observed except ${}^1{S}_0$ . A least‐squares fit of these experimental energy levels was made to the Racah Coulomb interaction parameters, $E^1\text{\hspace{0.17em}=\hspace{0.17em}2807}{\mathrm{cm}}^{\text{−1}}{\mathrm{,\; E}}^2\text{\hspace{0.17em}=\hspace{0.17em}19.98}{\mathrm{cm}}^{\text{−1}}{\mathrm{,\; E}}^3\text{\hspace{0.17em}=\hspace{0.17em}284.1}{\mathrm{cm}}^{\text{−1}}$ ; the spin–orbit coupling parameter $\text{ζ\hspace{0.17em}=\hspace{0.17em}2316}{\mathrm{cm}}^{\text{−1}}$ ; and the configuration‐interaction parameters, $\text{α\hspace{0.17em}=\hspace{0.17em}2.7}{\mathrm{cm}}^{\text{−1}}$ and $\text{β\hspace{0.17em}=\hspace{0.17em}52.9}{\mathrm{cm}}^{\text{−1}}$ . The position of the ${\text{5f}}^2$ levels of Np⁵⁺ on the intermediate coupling diagram compared to the $f^2$ levels of the isoelectronic series Pr³⁺, Nd⁴⁺, Th²⁺, U⁴⁺, and Pu⁶⁺ (in PuF₆) was intermediate between U⁴⁺ and Pu⁶⁺ as expected.