  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.
