Despite the tremendous advances in laser cooling of neutral atoms and positive ions, no negatively charged ion has been directly laser cooled. The negative ion of lanthanum, ${\mathrm{La}}^{–}$, has been proposed as the best candidate for laser cooling of any atomic anion [ and , Phys. Rev. A 81, 032503 (2010)]. Tunable infrared laser photodetachment spectroscopy is used to measure the bound-state structure of ${\mathrm{La}}^{–}$, revealing a spectrum of unprecedented richness with multiple bound-bound electric dipole transitions. The potential laser-cooling transition (${}^3{F}_2^e\to {}^3{D}_1^o$) is identified and its excitation energy is measured. The results confirm that ${\mathrm{La}}^{–}$ is a very promising negative ion for laser-cooling applications.

• Received 14 April 2014

DOI:https://doi.org/10.1103/PhysRevLett.113.063001