Resonances in atoms and molecules

Observing resonances in cross-section-versus-energy curves has long been a way of life for nuclear and particle physicists, for whom this is often the only way to detect short-lived quantum states. Atomic physicists, on the other hand, have traditionally used other means for observing internal energy states of atoms and molecules. But sixteen years ago a resonance in the cross section for electrons scattering off helium changed the tradition, and by now there is a long catalog of observed resonances in atoms and molecules and an accompanying body of theoretical work that seeks to account for the newly discovered states and explain their behavior.