The ejection on one or more electrons from an atom, molecule, or positive ion following the absorption of one or more photons. The process of electron ejection from matter following the absorption of electromagnetic radiation has been under investigation for over a century. The earliest measurements involved the irradiation of metal surfaces by ultraviolet radiation. The theoretical interpretation of this phenomenon, known as the photoelectric effect, played an important role in establishing quantum mechanics. It was shown that, contrary to classical ideas, energy exchanges between radiation and matter are mediated by integral numbers of photons. The ejection of electrons from a metal requires that the photon energy exceed the binding energy of electrons in the metal (that the wavelength of light be sufficiently small), whatever the photon flux.