Photoionization is the physical process in which an incident photon ejects one or more electrons from an atom, ion or molecule. This is essentially the same process that occurs with the photoelectric effect with metals. In the case of a gas or single atoms, the term photoionization is more common.
The ejected electrons, known as photoelectrons, carry information about their pre-ionized states. For example, a single electron can have a kinetic energy equal to the energy of the incident photon minus the electron binding energy of the state it left. Photons with energies less than the electron binding energy may be absorbed or scattered but will not photoionize the atom or ion.
For example, to ionize hydrogen, photons need an energy greater than 13.6 electronvolts (the Rydberg energy), which corresponds to a wavelength of 91.2 nm. For photons with greater energy than this, the energy of the emitted photoelectron is given by:
where h is the Planck constant and ν is the frequency of the photon.
This formula defines the photoelectric effect.
Not every photon which encounters an atom or ion will photoionize it. The probability of photoionization is related to the photoionization cross-section, which depends on the energy of the photon and the target being considered. For photon energies below the ionization threshold, the photoionization cross-section is near zero. But with the development of pulsed lasers it has become possible to create extremely intense, coherent light where multi-photon ionization may occur. At even higher intensities (around 10 - 10 W/cm of infrared or visible light), non-perturbative phenomena such as barrier suppression ionization and rescattering ionization are observed.