Bound and Quasibound States of the Negative Ion of Lanthanum (La$^{-}$) Studied by Photodetachment Spectrsocopy

The negative ion of lanthanum, La$^{-}$, has the richest bound state spectrum ever observed for an atomic negative ion [1], and it has been proposed as perhaps the best candidate for laser cooling of a negative ion [2,3]. In the present experiments, La$^{-}$ is investigated using tunable infrared spectroscopy. The relative signal for neutral atom production was measured with a crossed ion-beam--laser-beam apparatus over the photon energy range 520-900 meV to probe the continuum region above the La neutral atom ground state. The spectrum shows multiple resonance peaks due to transitions to quasibound excited states of La$^{-}$ which subsequently autodetach. In addition, photodetachment thresholds are observed to excited states of La. The measured spectrum is consistent with the recently reported revised electron affinity for lanthanum [4]. \\[4pt] [1] C.W. Walter, N. D. Gibson, D. J. Matyas, C. Crocker, K. A. Dungan, B. R. Matola, J. Rohlen, \textit{Phys. Rev. Lett.} \textbf{113}, 063001 (2014); [2] S.M. O’Malley and D.R. Beck, \textit{Phys. Rev. A} \textbf{81}, 032503 (2010); [3] E. Jordan, G. Cerchiari, S. Fritzsche, A. Kellerbauer, \textit{Phys. Rev. Lett.} \textbf{115}, 113001 (2015); [4] L. Pan and D.R. Beck, \textit{Phys. Rev. A} \textbf{93}, 062501 (2016).