Electron emission

(electronics) The liberation of electrons from an electrode into the surrounding space, usually under the influence of heat, light, or a high electric field.

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 also Field emission; Photoemission; Secondary emission.