photoionization

Photoionization is the process by which an atom or molecule absorbs a photon with sufficient energy to remove one or more electrons, producing a positively charged ion. The minimum photon energy required is the ionization energy or threshold energy of the species.

The probability of ionization by a photon of frequency ν is described by the photoionization cross section σ(ν). The ionization rate per target atom is R = ∫ σ(ν) Φ(ν) dν, where Φ(ν) is the photon flux spectrum. The energy of the ejected electron is Ekin = hν − I, with I the ionization energy. The cross section generally rises above threshold and depends on atomic structure.

In astrophysics, photoionization by ultraviolet, extreme ultraviolet, and X-ray photons ionizes gas in the interstellar and intergalactic medium, creating H II regions around hot stars and contributing to cosmic reionization. It also governs the ionospheres of planets and comets and affects laboratory and laser-produced plasmas.

Photoionization is distinct from collisional ionization, which requires high-temperature particle collisions; and from the photoelectric effect on solids, which involves emission from surfaces with different physics and typically lower photon energies.

Applications include spectroscopic diagnostics of astrophysical plasmas, modeling of ionization balance, and interpretation of emission and absorption lines in galaxies and quasars.