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Photoionization is the process by which an atom or molecule absorbs one or more photons and is subsequently ionized. This occurs when the energy of the absorbed photons exceeds the binding energy of an electron to the atom or molecule. In simpler terms, light energy is used to knock an electron off. The minimum energy required for photoionization is called the ionization energy or ionization potential. This phenomenon is crucial in various scientific fields, including astrophysics, where it plays a role in the ionization of interstellar gas, and in plasma physics, where it is used to generate and control plasmas. Photoionization can be a single-photon process, where one photon provides all the necessary energy, or a multi-photon process, where the absorption of multiple photons is required to overcome the binding energy of the electron. The ejected electron is called a photoelectron, and its kinetic energy is related to the photon energy and the ionization energy. The study of photoionization provides valuable insights into the electronic structure and properties of atoms and molecules.