Laser cooling is a well-established technique for the creation of ensembles of ultracold neutral atoms or positive ions. This ability has opened many exciting new research fields over the past 40 years. However, no negatively charged ions have been directly laser cooled because a cycling transition is very rare in atomic anions. Efforts of more than a decade currently have ${\mathrm{La}}^{-}$ as the most promising candidate. We report on experimental and theoretical studies supporting ${\mathrm{Th}}^{-}$ as a new promising candidate for laser cooling. The measured and calculated electron affinities of Th are, respectively, $4901.35(48){\mathrm{cm}}^{-1}$ and $4832{\mathrm{cm}}^{-1}$, or 0.607 690(60) and 0.599 eV, almost a factor of 2 larger than the previous theoretical value of 0.368 eV. The ground state of ${\mathrm{Th}}^{-}$ is determined to be $6d^{3}7s^2{{}^{4}F}_{3/2}^e$ rather than $6d^{2}7s^{2}7p{{}^{4}G}_{5/2}^o$. The consequence of this is that there are several strong electric dipole transitions between the bound levels arising from configurations $6d^{3}7s^2$ and $6d^{2}7s^{2}7p$ in ${\mathrm{Th}}^{-}$. The potential laser-cooling transition is ${}^2{S}_{1/2}^o\leftrightarrow {{}^{4}F}_{3/2}^e$ with a wavelength of $2.6\mu \mathrm{m}$. The zero nuclear spin and hence lack of hyperfine structure in ${\mathrm{Th}}^{-}$ reduces the potential complications in laser cooling as encountered in ${\mathrm{La}}^{-}$, making ${\mathrm{Th}}^{-}$ a new and exciting candidate for laser cooling.

We report a combined experimental and theoretical investigation of [XAuCN]⁻ (X = F, Cl, Br, I) to examine the chemical bonding in the mixed cyanide halide Au(I) complexes. Photoelectron spectra are obtained for [XAuCN]⁻, yielding electron affinities of 5.38 ± 0.05, 5.14 ± 0.05, and 4.75 ± 0.05 eV for XAuCN (X = Cl, Br, I), respectively. Relativistic quantum chemical calculations based on wavefunction theory and density functional theory are carried out to help interpret the photoelectron spectra and elucidate the electronic structures and chemical bonding in the [XAuCN]⁻ complexes. Spin–orbit coupling is found to be important in all the complexes, quenching the Renner–Teller distortion in the neutral molecules. Ab initio calculations including spin–orbit effects allow quantitative assignments of the observed photoelectron spectra. A variety of chemical bonding analyses based on the charge population, bond orders, and electron localization functions have been carried out, revealing a gradual transition from ionic behavior between F–Au in [FAuCN]⁻ to relatively strong covalent bonding between I–Au in [IAuCN]⁻. Both relativistic effects and electron correlations are shown to enhance the covalency in the gold iodide complex.

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.