Photoionisation is a physical process in which a photon strikes an atom, ion or molecule, resulting in the ejection of an electron.

The ejected electron is known as a photoelectron, and has a kinetic energy equal to the energy of the photon minus the electron binding energy . Photons which have energies less than the electron binding energy may be scattered but will not photoionise the atom or ion.

For example, to ionise hydrogen, photons need an energy greater than 13.6 electron volts, which corresponds to a wavelength of 91nm. For photons with greater energy than this, the energy of the emitted photoelectron is given by:

1/2mv² = hν - 13.6eV

where h is Planck's constant and ν is the frequency of the photon.

Not every photon which encounters an atom or ion will photoionise it. The probability of photoionisation is related to the photoionisation cross-section , which depends on the energy of the photon and the ion or atom being considered. For photon energies below the ionisation threshold, the photoionisation cross-section is zero. Above the threshold, the cross section decreases as the inverse cube of the photon energy.