Observation of Bound-Bound Transitions in the Negative Ion of Lanthanum La$^{-}$

The negative ion of lanthanum has been investigated with tunable infrared laser photodetachment spectroscopy. The relative signal of neutral atom production was measured with a crossed laser-ion beam apparatus over the photon energy range 0.29 -- 0.49 eV. The spectrum reveals a number of sharp peaks due to bound-bound electric-dipole transitions in La$^{-}$, observed here through a two-step process of excitation followed by photodetachment of the upper state. The transitions responsible for four of the peaks are identified through comparison to the calculations of O'Malley and Beck [1]. The richness of the observed bound state spectrum is unprecedented for atomic negative ions, and it highlights the uniqueness of La$^{-}$ for applications such as laser cooling.\\[4pt] [1] S.M. O'Malley and D.R. Beck, \textit{Phys. Rev. A} \textbf{81}, 032503 (2010).