What is electron emission and how electrons emitted?

We know that metals have electrons that are loosely bound to the nucleus in the outermost shells. The temperature difference changes the number of free electrons. At room temperature, there are large numbers of free electrons that are randomly moving inside the metal.

Even though electrons are freely moving inside the metal, they cannot leave the surface of the metal. Because the free electrons are attracted by positive nuclei of the metal. These attractive electrons will not leave the nuclei at room temperature.

If the free electrons want to leave the metallic surface, that free electrons must cross a potential barrier that is created by the positive nuclei of the metal. This potential barrier is called as a surface barrier because these potential barriers prevent free electrons from leaving the metallic surface.

All kinds of free electrons possess kinetic energy and that energy is different for different electrons. Electron kinetic energy is not sufficient to overcome the surface barrier.

Suppose an additional energy is given to those free electrons, they will have sufficient energy to cross the surface barrier. The liberation of electrons from any surface of a substance is called electron emission.

The minimum energy needed for an electron to escape from the metal surface is called the work function of that metal. That work function is function is different for different metals. Like copper work function energy is 4.65 eV, aluminium work function is 4.28 eV.

Types of electron emission

These are the four types of electron emission,

1.     Thermionic emission

2.     Field emission

3.     Photo electric emission

4.     Secondary emission

Thermionic emission

When a metal is heated to a high temperature, the free electrons get sufficient energy to leave the metal surface. So that free electrons are emitted from the metallic surface. 

This thermionic emission of electrons is used in cathode-ray tubes, X-ray tubes and electron microscopes.

Field emission

When a very strong electric field is applied across the metal surface, the free electrons overcome the surface barrier of the metal.

Field emission scanning electron microscope and field emission display are using this field emission method.

Photoelectric emission

When electromagnetic radiation of suitable frequency is incident on the surface of the metal, that energy is transferred from the radiation to the free electrons.

Photodiodes and photoelectric cells are made with this photoelectric emission.

Secondary emission

When a beam of fast-moving electrons strikes the surface of the metal, the kinetic energy of the striking electrons is transferred to the free electrons on the metal surface.

Image intensifiers and photomultiplier tubes are made up of this secondary emission method.

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