Energy Levels of Np3+ in LaBr3

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Abstract
The polarized absorption and emission spectra of single crystals of LaBr3 doped with nominally 0.15% and 0.75% 237Np have been recorded between 5000 and 25 000 cm−1 at temperatures of liquid helium and nitrogen. Fluorescence excitation spectra and both parallel and perpendicular Zeeman spectra were also recorded. Based on the number, polarization, and magnetic behavior of Stark components appearing in distinct groups, as well as on preliminary 5f-hydrogenic calculations, 14 absorption groups were identified as levels of the ground electronic configuration 5f4 of Np3+. The assumption of a pure 5f4 electronic configuration does not lead to a satisfactory interpretation of the spectrum; however, the inclusion of electrostatic configuration interaction permits a satisfactory fitting of the observed levels with an rms deviation in energy of 86.4 cm−1 using the parameters: E1=3394.3±29.1, E2=14.19±0.22, E3=317.5±1.9, ζ5f=1969.2±10.8, α=35.0±2.2, and β=−802.4±11.3 cm−1. Using 25 crystalline-field-split Stark components of the 5I4,5,6,7 levels of the ground multiplet, a first-order crystal-field perturbation calculation was performed. An rms deviation in energy of 8.0 cm−1 is obtained using the parameters: B20=−22.8±8, B40=−86.7±2.4, B60=−70.8±0.9, B66=+857.1±12.0 cm−1. The negative value obtained for B20 represents a crystal-field shielding in excess of 100% and is discussed in some detail. Parallel and perpendicular Zeeman interactions are also discussed in some detail.