Observations of Multiple 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 -- 0.77 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 observed photodetachment spectrum is compared to theoretical calculations of energy levels and transition strengths by O'Malley and Beck [1]. The richness of the observed bound state spectrum is unprecedented for atomic negative ions, and it highlights the unique properties of La$^{-}$ for applications such as laser cooling.\\[4pt] [1] S.M. O'Malley and D.R. Beck, \textit{Phys. Rev. A} \textbf{79}, 012511 (2009); \textit{Phys. Rev. A} \textbf{81}, 032503 (2010).