Electron Emission

In the present scenario of science and technology we have lots of electronic devices whose operating principles depend on the flow of electron beams through an evacuated space. In addition to such electronic systems, the electron beam is also required in majority of the experimental purposes in different fields of science and engineering. So to fulfill these purposes the free electrons must be ejected from the surface of substances (generally conductors) by supplying the required energy by some outer means. This process of liberation of free electrons is what we call as electron emissionThe movement of these ejected free electrons can be controlled by applying an external electric field. These electrons move in vacuum under the influence of applied electric field, and thus constitute electric current in vacuum.

The phenomenon of escaping of electrons from the surface of substances is known as electron emission.

Out of all substances, metals are preferred for electron emission because they have large number of free electrons. These free electrons are free from the influence of their respective parent nuclei and move randomly inside the volume of the metal piece due to thermal agitation. However these randomly wandering free electrons don't have that much energy which can enable them to leave or escape from the metal-surface. Whenever a free electron at surface tries to leave the surface, it finds behind a positive ion pulling it back. Thus the metal-surface acts like a barrier for a free electron tending to leave out. This barrier offered by the surface to free electrons is known as surface barrier.

Thus if a free electron has to escape from the surface it must have sufficient energy to overcome the surface barrier. As stated earlier, normally at room temperature a free electron cannot get that energy by thermal agitation. So additional energy from some external sources must be supplied to the electrons to initiate the process of electron-emission. This additional energy demanded by a free electron to overcome the surface barrier of metal is called work function of the metal.

The amount of additional energy required by a free electron to escape from a metallic surface is called work function of the metal.

The value of work function is different for different metals and depends on the nature of metal, its purity and the surface conditions. We express the work functions of metals in small units of energy like electron volt (eV). For pure metals, work function varies roughly varies from 2eV to 6eV. It should be noted that primarily metals having lower value of work functions are preferable so that even a supply smaller energy could cause electron-emission.

Types of Electron Emission

The electron emission can take place only if sufficient amount of additional energy (equal to work function of the metal) is supplied from some external means. This additional energy may be provided from a variety of sources like heat energy, light energy, energy stored in electric field or kinetic energy of charged particles bombarding the metal surface. Depending upon the nature of the external energy source, we have following four major types of electron emission methods.

1) Thermionic Emission

When a metal piece is heated or in other words the metal piece is supplied by heat energy, the kinetic energy of the free electrons increases. If we keep on supplying heat, the energy of free electrons keeps increasing and finally a time comes when their energy becomes sufficient to overcome the surface barrier. In this case, some of the free electrons may leave the metallic surface. This type of electron emission from a metallic surface by supplying thermal energy is known as thermionic emission. This type of electron emission is employed in vacuum tubes.

2) Photo-electric Emission

In this method, when light waves of sufficient frequency fall on a metallic surface electrons are instantly emitted by the surface. The photons of light strike with free electrons on the surface and impart their energies to those electrons, in this way some of the surface electrons may get the required additional energy equal to work function and may come out. Electrons emitted this way are known as photo-electrons and the phenomenon is called Photo-electric Effect.

3) Field Emission

In this method, the metal piece is subjected to a strong external electric field. This applied field accelerates the inner conduction electrons and this acceleration may cause them to overcome the surface barrier and escape out.

4) Secondary Emission

In this method, high energy electrons or other charged particles are bombarded on the metal surface and this causes the free electrons to be knocked out from the surface.
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