A Promising Candidate for Laser Cooling of Negative Ions: Observations of Bound-Bound Transitions in La$^{-}$

The negative ion of lanthanum, La$^{-}$, has been previously proposed as perhaps the best candidate of any atomic anion for laser cooling based on theoretical analysis [1]. In the present experiments, transitions between bound states of La$^{-}$ are investigated using tunable infrared photodetachment spectroscopy. The relative signal for neutral atom production was measured with a crossed ion-beam--laser-beam apparatus over the photon energy range 290--580 meV. The spectrum reveals at least ten sharp resonance peaks, some of which are interpreted as due to bound-bound electric dipole transitions in La$^{-}$ observed through resonant two-photon detachment. 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{81}, 032503 (2010).