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The liberation of electrons from a substance into vacuum. Since all substances are built up of atoms and since all atoms contain electrons, any substance may emit electrons; usually, however, the term refers to emission of electrons from the surface of a solid.
The process of electron emission is analogous to that of ionization of a free atom, in which the latter parts with one or more electrons. The energy of the electrons in an atom is lower than that of an electron at rest in vacuum; consequently, in order to ionize an atom, energy must be supplied to the electrons in some way or other. By the same token, a substance does not emit electrons spontaneously, but only if some of the electrons have energies equal to, or larger than, that of an electron at rest in vacuum. This may be achieved by various means, such as by heating, irradiation with light (photoemission), bombardment with charged particles (secondary emission), or use of a strong electric field (field, or cold, emission). See Field emission, Photoemission, Secondary emission, Thermionic emission
the liberation of electrons from the surface of a solid or liquid. Electron emission arises when part of the electrons in a body gain sufficient energy, under the influence of external factors, to overcome the potential barrier at the boundary of the body or when the surface potential barrier, under the action of an electric field, becomes transparent to part of the electrons that have the highest energies within the body.
Electron emission may arise when bodies are heated (seeTHERMIONIC EMISSION) or bombarded with electrons (seeSECONDARY ELECTRON EMISSION), ions (seeION EMISSION), or photons (seePHOTOEMISSION). Under certain conditions, for example, when a current is passed through a semiconductor with high electron mobility or when a strong electric field impulse is applied to a semiconductor, conduction electrons may be “heated” much more strongly than the crystal lattice, and consequently some electrons may escape from the body (hot-electron emission).
In order to observe electron emission, an electron-accelerating external electric field that “sucks off” electrons from the surface of the body (emitter) must be produced at the surface of the body. If this field is sufficiently great (≥102 volts/cm), it decreases the height of the potential barrier at the boundary of the body and accordingly the work function (the Schottky effect), as a result of which electron emission increases. In strong electric fields (~107 volts/cm), the surface potential barrier becomes very fine and tunnel “leakage” of electrons through it occurs (seeTUNNEL EFFECT), which is sometimes also called field emission. Thermo-field emission or field-enhanced photoemission may result from the simultaneous action of two or more factors. In very strong pulsed electric fields (~5 × 107 volts/cm), the tunnel effect leads to the rapid breakdown (explosion) of the micropeaks on the surface of the emitter and to the formation of dense plasma near the surface. The interaction of the plasma with the emitter surface produces a sharp increase in the electron emission current to 106amperes, with a current pulse length of a few tens of nanoseconds (explosive emission). During each current impulse, microquantities (~10–11 g) of matter are transported from the emitter to the anode.
REFERENCESDobretsov, L. N., and M. V. Gomoiunova. Emissionnaia elektronika. Moscow, 1966.
Bugaev, S. P., P. N. Vorontsov-Vel’iaminov, A. M. Iskol’dskii, S. A. Mesiats, D. I. Proskurovskii, and G. N. Fursei. “lavlenie vzryvnoi elektronnoi emissii.” In the collection Otkrytiia v SSSR 1976 goda. Moscow, 1977.
T. M. LIFSHITS