Photoionisation

Photoionisation is the physical process in which an atom or molecule absorbs a photon and loses one or more electrons, provided the photon energy exceeds the ionisation energy. In the simplest case, a single photon ejects one electron, leaving the residual ion in an excited or ground state. The excess energy of the photon, above the ionisation threshold, appears as kinetic energy of the ejected electron. If the radiation field is energetic enough or density is high, multiple ionisations can occur, either via multi-photon processes or by subsequent absorptions.

The rate of photoionisation depends on the photon flux and the ionisation cross-section, σ(ν), which is a function of photon frequency ν. The ionisation rate for a species is Γ = ∫ σ(ν) Fν / hν dν, where Fν is the specific photon flux. Near threshold, σ(ν) is relatively large and falls off with increasing ν, so softer ultraviolet photons are often efficient at ionising atoms, while X-rays penetrate and ionise deeper layers. The photoelectron energy distribution reflects hν − I, and the angular distribution depends on the photon polarization and the atomic state.

In astrophysics and atmospheric science, photoionisation is a dominant mechanism for producing and maintaining ionised gas. In H II regions and the intergalactic medium, ultraviolet and X-ray radiation from stars and quasars sets the ionisation state, balances by recombination, and drives radiative transfer calculations. In planetary ionospheres, solar radiation ionises constituents of the atmosphere, affecting conductivity and chemistry. Photoionisation is modeled with detailed balance equations and radiative transfer codes to predict spectra and ionisation fractions.