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Electron Emission

The liberation of electrons from the surface of a metal is known as Electron Emission. If a piece of metal is investigated at room temperature, the random motion of the electrons will be shown in Fig. However, these electrons are free to the extent that they may transfer from one atom to another within the metal but they cannot leave the metal surface to provide electron mission. It is because the free electrons that start at the surface of metal find behind them positive nuclei pulling them back and none pulling forward. Thus at the surface of the metal , a free electron encounters forces that prevent it to leave the metal. In other words, the metallic surface offer a barrier to free electrons, their kinetic energy increases and is known as surface barrier.

However, if sufficient energy is given to the free electrons, their kinetic energy increases and thus the electrons will cross over the surface barrier to leave the metal.
Equipotential Surface
    Work function (W0): The minimum energy required by an electron to just escape (i.e. with zero velocity) from metal's surface is called Work function (W0) of the metal. The work function of pure metals varies (roughly) from 2eV to 6eV. Its value depends upon the nature of the metal, its purity and the conditions of the surface. We selected those metals for electron emission which have low work function.
Types of Electron Emission:
The electron emission from the surface of a metal is possible only if sufficient addition energy (equal to work function of the sources such as heat energy, energy stored in electric field, light energy or kinetic energy of the electric charges bombarding the metal surface. Accordingly; there are following four principal method of obtaining electron emission from the surface of a metal:
(I) Thermionic emission: In this method, the metal is heated to a sufficient temperature (about 2500oC) to enable the free electrons to leave the metal surface. The number of electrons emitted depends upon the temperature. The higher the temperature, the greater is the emission of electrons. This type of emission is employed in vacuum tubes.
(II) Field emission: In this method, a strong electric field (i.e. a high positive voltage) is applied at the metal surface which pulls the free electrons out of the metal because of the attraction of positive field. The strong the electric field, the greater is the electron emission.
(III) Photoelectric emission: In this method, the energy of light falling upon the metal surface is transferred to the free electrons within the metal to enable them to leave the surface. The greater the intensity of light beam falling on the metal surface, the greater is the photoelectric emission. Photoelectric emission is utilized in photo tubes which from the basis of television and sound films.
(IV) Secondary emission: In this method, a high velocity beam of electrons or other out. The intensity of secondary emission depends upon the emitter material, mass and energy of bombarding particles.
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