Photodetachment Spectroscopy of Quasibound States of the Negative Ion of Lanthanum

The negative ion of lanthanum, La$^-$, has one of the richest bound state spectra observed for an atomic negative ion and has been proposed as a promising candidate for laser-cooling applications. In the present experiments, La$^-$ was 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 590 - 920 meV (2100 - 1350 nm) to probe the continuum region above the La neutral atom ground state. Eleven prominent peaks were observed in the La$^-$ photodetachment cross section due to resonant excitation of quasibound transient negative ion states in the continuum which subsequently autodetach. In addition, thresholds were observed for photodetachment from several bound states of La$^-$ to both ground and excited states of La. The present results provide information on the excited state structure and dynamics of La$^-$ that depend crucially on multielectron correlation effects.