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(also photoelectric effect), the ejection of electrons by a substance when the substance is exposed to electromagnetic radiation, or photons.
Photoelectricity was discovered by H. Hertz in 1887. The first fundamental investigation of photoelectricity was performed in 1888 by A. G. Stoletov, who found that the exposure of the negative electrode to light plays a substantial role in the generation of a photoelectric current in a circuit containing metal electrodes and a voltage source and that the intensity of the photoelectric current is proportional to the intensity of the light. In 1899, P. Lenard demonstrated that electrons are ejected from metals exposed to light. A. Einstein presented the first theoretical explanation of the laws of photoelectricity in 1905. The theory of photoelectricity was developed more consistently by I. E. Tamm and S. P. Shubin in 1931. A. F. Ioffe (1907) and P. I. Lukirskii and S. S. Prilezhaev (1928) made important contributions to the experimental study of photoelectricity.
Photoelectricity is a quantum effect. Its discovery and investigation played an important role in the experimental verification of quantum theory, which provides the only basis for explaining the laws that govern photoelectricity. A free electron cannot absorb a photon because the laws of conservation of energy and momentum cannot be simultaneously satisfied. Photoelectricity from an atom, a molecule, or a condensed medium is possible because an electron in such a particle or medium is bound. The coupling of a bound electron is characterized in an atom by the ionization energy and in a condensed medium by the work function. The law of conservation of energy in the case of photoelectricity is expressed by the Einstein photoelectric law ℰ = ħω – ℰi, where ℰ is the kinetic energy of a photoelectron, ħω is the photon energy, ħ is Planck’s constant, and ℰi, is the ionization energy of an atom or the electronic work function of a solid. When ħω <ℰi, photoelectricity cannot be generated.
Photoelectricity may be observed as the photoionization of gases, which consist of isolated atoms or molecules. In this case, the primary event is the absorption of a photon by an atom and the ionization of the atom, which is accompanied by the ejection of an electron. We may assume, with a high degree of accuracy, that the entire photon energy, less the ionization energy, is transferred to the ejected electron. In condensed media, the photon absorption mechanism depends on the photon energy. When ħω is equal to or not very much greater—that is, greater by a factor of ten to several hundred—than the work function, the radiation is absorbed by conduction electrons in metals or by valence electrons in semiconductors and dielectrics; in a solid, valence electrons become conduction electrons. Such absorption may result in photoemission, which is also called the external photoelectric effect, or in an internal photoelectric effect, such as photoconductivity. In the case of photoemission, the photoelectric threshold energy is equal to the work function; in the case of an internal photoelectric effect, it is equal to the width of the forbidden band.
When the photon energies ħω are many times greater than the bond energy in a condensed medium, as in the case of gamma rays, photoelectrons may be ejected from the innermost electron shells of an atom. In this case, the effect of the medium on the primary photoelectric event is negligible in comparison with the binding energy of an electron in an atom, and photoelectricity is generated in the same manner as in isolated atoms. The effective photoelectric cross section σp first increases with ω and then decreases when ħω becomes greater than the binding energy of the electrons in the innermost shells of an atom. The dependence of σp on ω may be explained qualitatively by observing that the greater ħω is in comparison with ℰi, the more negligible is the coupling of an electron to an atom and that the generation of photoelectricity is impossible for a free electron. Since the electrons in the K-shell are the most strongly bound electrons in an atom and since the coupling increases with atomic number Z, σp has the largest value for K-electrons and increases rapidly (~Z5) as the elements become heavier. When ħω is of the same order as the atomic binding energies, the photoelectric effect is the predominant gamma-ray absorption mechanism for atoms; at higher photon energies, the role of the photoelectric effect becomes less important in comparison with other mechanisms, such as the Compton effect and electron-positron pair production.
The absorption of a gamma-ray photon by an atomic nucleus that accompanies the rearrangement of the nucleus is called a photonuclear reaction.
Photoelectricity is widely used in studies of the structure of matter—that is, the structure of atoms, atomic nuclei, and solids (see) —and in photoelectric devices.
REFERENCESHertz, H. “Über einen Einfluss des ultravioletten Lichtes auf die electrische Entladung.” Annalen der Physik und Chemie, 1887, vol.31.
Stoletov, A. G. Izbr. soch. Moscow-Leningrad, 1950.
Einstein, A. Sobr. nauchn. tr., vol. 3. Moscow, 1966.
Tamm, I., and S. Schubin. “Zur Theorie des Photoeffektes an Metallen.” Zeitschrift für Physik, 1931, vol. 68.
Lukirskii. P. I. O fotoeffekte. Moscow-Leningrad, 1933.
Starodubtsev, S. V., and A. M. Romanov. Vzaimodeislvie gammaizlucheniia s veshchestvom, part 1. Tashkent, 1964.
T. M. LIFSHITS