|A Hollow-Ion Resonance of Unprecedented Strength| A so-called
hollow ion is formed when core electrons are removed or excited
to higher energy levels, leaving an empty inner shell. Such
states can be produced in He-, a fundamental three-electron
system and prototypical negative ion. The nuclear Coulomb
attraction is efficiently screened in negative ions, greatly
enhancing the effects that the electrons have on each other and
providing an ideal opportunity to verify and further motivate
theoretical models of electron correlation. Our understanding of
these basic interactions can elucidate processes of importance
in many fields, from the interpretation of cosmic spectra to
x-ray lasing efforts using inner-shell ionization and hollow-ion
formation. At the Ion-Photon Beamline at the ALS, researchers
have detected in negative helium ions a resonant simultaneous
double-Auger decay of unprecedented strength, evidence of a
triply excited hollow-ion state that has eluded observation for
25 years. In contrast to valen