Electronic Spectra of the 5f3 Actinides: U3+, Np4+, Pu5+, and AmO2++. The f3 Intermediate Coupling Diagram

The absorption spectra of fluoride salts of 5f3Np4+ obtained at room temperature in various environments were compared with the known spectra of f3Nd3+ and U3+. Correlation with known spectra allowed assignment of levels to Np4+, and based on these, the complete 41-level f3 intermediate spin–orbit coupling diagram was constructed. The diagram was used to interpret the absorption spectra and to make spectroscopic assignments to 12 low-lying electronic energy levels for Pu5+. Ten low-lying levels for Am(VI) obtained from published AmO2++ aqueous solution spectra were interpreted also. Spectroscopic parameters obtained from least-squares fits to the observed spectra were: Np4+, F2 = 232.4 cm−1, ζ = 2178 cm−1; Pu5+, F2 = 230.5 cm−1, ζ = 2295 cm−1; AmO2++, F2 = 260.3 cm−1, ζ = 3070 cm−1. Only the lowest-lying energy levels were found to be in substantial agreement with the f3 intermediate coupling diagram.