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.