The electronic structures of actinide systems are extremely complicated because of strong electron correlations and relativistic effects. The atomic actinide anion poses even more challenges for experimental and theoretical investigations due to the extra loosely bound electron. In this work, we report the electronic structure of ${\mathrm{U}}^{-}$ using the slow-electron velocity-map imaging method. The electron affinity of U was measured to be 314.97(9) meV. Two excited states of ${\mathrm{U}}^{-}$ were observed and their energies were determined to be 84(11) and 160(12) meV above its ground state, respectively. The present study provides convincing evidence that the transition between the first excited state and the ground state of ${\mathrm{U}}^{-}$ is electric dipole (E1) allowed.

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• Atomic & molecular clusters
• Cooling & trapping