Electronic Configuration Assignments for UO from Electric Dipole Moment Measurements

Diatomic UO has more than 48 bound states within 10000 cm^-1 of the ground state. This electronic state congestion has been attributed to interleaved states from the electronic configurations U²⁺(5f³7s)O^2- and U²⁺(5f²7s²)O^2-, respectively. Ligand field theory predicts that each electronic configuration will exhibit states with distinguishable, characteristic vibrational and rotational constants. However, vibronic state mixing modifies the observed vibration-rotation constants, leading to uncertainty in the configurational assignments. The permanent electric dipole moment (μ) of an electronic state should also manifest a value that is characteristic of the parent electronic configuration. μ and other electrostatic and magnetostatic properties should be less influenced by the vibronic state mixing, providing more robust indicators for configurational assignments. In the present study, we have measured the μ values for four electronic states of UO. The results clearly demonstrate that the ground state (X(1)4) and the first electronically excited state ((2)4) are derived from the U²⁺(5f³7s)O^2- and U²⁺(5f²7s²)O^2- configurations, respectively.