autoionize

Autoionization (also autoionisation) is the spontaneous ejection of an electron from an atom, ion, or molecule after internal excitation, when the energy of the excited state lies above the ionization threshold. In this process, a discrete excited state—often a doubly excited or Rydberg-like state—couples to the ionization continuum through electron-electron interactions, and decays to a continuum state without absorbing a photon. This nonradiative decay competes with radiative decay and produces a free electron and an ion.

The coupling to the continuum gives the excited state a finite lifetime, typically ranging from femtoseconds to nanoseconds. Autoionization manifests as resonances in photoabsorption and photoionization spectra, with line shapes described by Fano interference, which arise from the mixing of direct ionization pathways with autoionization pathways. The Fano profile is characterized by an asymmetry parameter that controls the degree and direction of the skew in the resonance. The phenomenon reflects the underlying electronic structure and the interaction between bound and continuum states.

Autoionization occurs in a wide range of systems, including helium and other noble gases, various atoms and ions, and molecular species, and it plays a role in plasmas and astrophysical environments. It is a nonradiative decay channel that competes with radiative decay. It is related to, but distinct from, Auger decay, which involves relaxation of a core vacancy with emission of an additional electron.

Historically, autoionization contributed to the development of quantum collision theory and the understanding of electron correlation. In practice, it provides a diagnostic tool in spectroscopy and a factor in accurately modeling ionization processes in laboratory and astrophysical plasmas.