Energy level analysis of Np3+:LaCl3 and Np3+:LaBr3

Abstract

The polarized absorption and fluorescence spectra of 0.01–5% Np³⁺ doped into single‐crystal LaCl₃ were measured at moderate and high resolution in the range to 50 000 cm⁻¹ at 298, 77, and 4 K. The 150 crystal field components identified were fit by a parametrized model which has previously been shown to provide an excellent account of the energy level structures observed for lanthanides doped into LaCl₃. This constitutes the first successful evaluation of the crystal field interaction for an actinide ion in LaCl₃ in which J mixing effects were explicitly included. The crystal field parameters for Np³⁺:LaCl₃, B²₀=163, B⁴₀=−632, B⁶₀=−1625, and B⁶₆=1028, all in cm⁻¹, were determined to be approximately twice as large as those for the analogous lanthanide, Pm³⁺:LaCl₃. From analysis of Zeeman patterns in the fluorescence spectra, the ground state was found to be doubly degenerate, having crystal quantum numbers ±2 and a parallel splitting factor 0.17 LU (Lorentz unit).