Photoexcitation of He- hollow-ion resonances: observation of the 2s2p2 4P state.

Highly correlated states are studied in He-, a fundamental 3-electron system and prototypical negative ion. The 2s2p(2) 4P state is observed for the first time. This state is detected in a resonant simultaneous double-Auger decay of unprecedented strength. In addition, the first measurements of photodetachment cross sections, positions, widths, and shapes of triply excited resonances in He- are reported. These measurements provide a sensitive test for several sophisticated ab initio calculations, and indicate differences in the position and shape of some structures.