High resolution electronic spectroscopy of uranium mononitride, UN.

The isoelectronic molecules UN and UO⁺ are known to have Ω = 3.5 and Ω = 4.5 ground states, respectively (where Ω is the unsigned projection of the electronic angular momentum along the internuclear axis). A ligand field theory model has been proposed to account for the difference [Matthew and Morse, J. Chem. Phys. 138, 184303 (2013)]. The ground state of UO⁺ arises from the U³⁺(5f³(⁴I_4.5))O²⁻ configuration. Owing to the higher nominal charge of the N³⁻ ligand, the U³⁺ ion in UN is stabilized by promoting one of the 5f electrons to the more polarizable 7s orbital, reducing the repulsive interaction with the ligand and rendering U³⁺(5f²7s(⁴H_3.5))N³⁻ the lowest energy configuration. In the present work, we have advanced the characterization of the UN ground state through studies of two electronic transitions, [18.35]4.5-X(1)3.5 and [18.63]4.5-X(1)3.5, using sub-Doppler laser excitation techniques with fluorescence detection. Spectra were recorded under field-free conditions and in the presence of static electric or magnetic fields. The ground state electric dipole moment [μ = 4.30(2) D] and magnetic g_e-factor [2.160(9)] were determined from these data. These values were both consistent with the 5f²7s configurational assignment. Dispersed fluorescence measurements were used to determine vibrational constants for the ground and first electronically excited states. Electric dipole moments and magnetic g_e-factors are also reported for the higher-energy electronically excited states. [ABSTRACT FROM AUTHOR]