What is Electron Emission?
In an evacuated space many electronics devices depend on the movement of electrons for their operation. To satisfy this purpose the free electrons must be ejected from the surface of a conductor by supplying the sufficient energy from the external source. which known as electron emission. The emitted electrons can be a move in vacuum under the influence of an electric field.
Electron emission is made by metals because of metals have many free electrons. As shown in the above figure a piece of metal is investigated at room temperature in the above figure you can see the random motion of the free electron. These electrons are free only to the extent to which they may transfer from one atom to another atom within the metal, they can not leave the metal surface. Because of the free electrons at the surface of a metal find behind them positive nucleus pulling them back and no one pulling forward.
At the surface of the metal, electrons are prevented to leave the metal. In technical words, you can say that the metallic surface offers a barrier to free electron and is known as the surface barrier. However, when sufficient energy is given to the free electron, its kinetic energy is increased and electron crosses the surface barrier. This additional amount of energy required by the electron to cross the surface barrier is known as work function.
If The total energy required to liberate an electron from metal is 4 eV then energy possessed by the electron is 0.5 eV then additional energy means work function id 4.0-0.5=3.5 eV. The work function depends on the nature of the metal.
Types of Electron Emission
The electron emission is only possible if a sufficient amount of additional energy is supplied from some external source. The external source may come from heat energy, light energy or in form of kinetic energy. There are four methods of electron emission.
Now let’s understand all the methods one by one.
The process of electron emission from a metal surface by supplying the thermal energy is known as thermionic emission. At ordinary temperatures, the energy possessed by a free electron in the metal is sufficient to cause them to escape from the surface. When heat is applied to the metal, heat energy is converted into the kinetic energy. Which cause the motion of free electrons.
When temperature increase which is equal to the work function of the metal they overcome the surface barrier and leave the metal surface. Metals with the lower work function will require less amount of energy and will emit electrons at a lower temperature. Common materials used for electron emission are tungsten, thoriated tungsten and metallic oxides of barium and strontium. It may be noted that tungsten must be heated at 2300°C to get electron emission.
The process of electron emission by the application of strong electric field at the surface of the metal is known as field emission. When the surface of the metal is placed closed to the high voltage conductor which is positive with respect to the metal surface. If the positive potential is great enough it succeeds in overcoming the metal surface and free electrons will be emitted from the metal surface as shown below figure.
Field emission can be obtained at temperatures much lower than the thermionic emission. It is sometimes called cold cathode emission or auto electron emission.
Electron emission from the bombarding high-speed electrons or other particle is known as secondary emission.when the high-speed electrons suddenly strike the metallic surface they may give some or all their kinetic energy in the metal. If the energy of the striking electron is sufficient which cause free electrons to escape the metal of the surface. This is called secondary emission. The electrons which strike the metals are called primary electrons.
The Emitted electrons are called secondary electrons. The intensity of the secondary emission depends upon the emitter material mass and energy of the bombarding particles. The effects of the secondary emission are very undesirable in many electronics devices.
Photo Electric Emission
When Electron emission is done by the application of the light is known as photoelectric emission. When a beam of light strikes the surface of certain metals the energy of the photons of light is transferred to the free electrons within the metal. If the energy of the striking photon is greater than the work function of the metal then the free electrons will be leaving the surface of the metal. Emitted electrons are known as photoelectrons and this phenomenon is known as photoelectric emission.
The amount of the photoelectric emission depends on the intensity of light falling on the emitter and frequency of radiations. The frequency and intensity are the greater, The greater is the photoelectric emission. Photoelectric emission is used in phototube which is a basis of television and sound films.