Electron Emission

Introduction

J.J. Thomson discovered electrons. To test his theory, he used a cathode-ray tube, an evacuated tube with gas inside, that he charged with electricity in 1897. The discovery of electrons was made possible thanks to this experiment. Electron emission refers to the release of electrons from a metallic surface. Emission from a surface requires a certain amount of energy; we learned about this in photoelectric emission. In addition to the light, electrons can also be emitted from any other surface.

Based on current theories, four distinct electron emission types exist, which we will learn in the later sections.

Electron emission from metals

The valence electrons of the atoms of metals have a loosely bonded outer shell. They are allowed to move freely within the metal, but they are not permitted to leave the metal’s surface. It is these electrons that are known as ‘free electrons.’ 

Electrons weigh 9.1 x 10-31 kg. At normal temperature, the metal’s free electrons are unable to freely move about inside the metal and leave its surface. An electron’s positive charge increases the minute it emerges from the metal’s surface, causing it to attract other electrons in its direction for electron emission from metals. As a result, restraining forces on the metal’s surface keep the free electrons bound to it. There is a possible barrier in the form of these attracting forces. If an electron cannot leave the surface because of this barrier, then some energy is offered for it to depart the surface.

 The ‘work function’ refers to this type of energy. So, what is the metal’s purpose? The minimum energy required by an electron to escape from a metal’s surface by defeating its attractive attraction is known as the work function. 

The factors on which work function depends are as follows :

• The metal’s qualities
• The metal’s purity
• The metal’s surface characteristics

A unit of measure for the work function is the electron volt (eV), which is indicated by the sign o. An electron’s energy to escape from a metallic surface is measured in eV. 

Types of electron emission

Only at energies greater than the metal’s work function may electrons be emitted from its surface. The following physical processes can supply the free electrons with the energy needed for the various types of electron emission:

• Thermionic field emission

• Secondary electron emission

• Electric field emission
• Photoelectric emission

Thermionic field emission

What does thermionic mean?

The terms “therm” and “ionic” refer to ions in a thermionic system. When it comes to thermionic field emission, what is it all about? Electrons are released when a metallic surface is heated. In this case, thermal energy is being used as the source of electron emission from the metallic surface. Thermal electrons, or thermions, are the electrons emitted by the surface. The amount of thermionic radiation emitted here is determined by the temperature of the metallic substrate.

Secondary electron emission

When fast-moving electrons termed primary electrons strike a metallic surface, a huge number of secondary electrons emissions occur.

When the electrons fall on the surface, they transmit their energy to the metal’s free electrons (bound by a strong attraction with the metal) so that their energy exceeds the metal’s work function and they release the electrons. The electrons emitted from the metal are called secondary electrons emissions, since they are emitted with the help of primary electrons.

Electric field emission

Cold cathode emission and field emission are two names for the same phenomenon. When a high electric field is applied to a metallic surface, electrons are emitted from it. 108 V/m is a strong electric field that causes the metal to emit electrons when free electrons leave the metal’s surface. When this occurs, the metal’s surface takes on a positive charge, which attracts more free electrons.

Photoelectric emission

The photoelectric effect was discovered by Albert Einstein, who won the Nobel Prize for his work. Free electrons are liberated from the metal surface in this sort of emission when light radiations of the appropriate frequency fall on the metal surface.

Light photons, or simply photons, are the packages of energy that provide the free electrons with the energy they need to emit light. Photoelectrons are the name given to the electrons emitted from the surface of an object. Due to the fact they are emitted by photons, the name has been given. Photoelectrons emitted from the surface are also affected by light intensity.

Conclusion

The release of electrons from a substance’s surface is known as electron emission. Free electrons are abundant in every metal block. Even at normal temperature, the metallic crystal has a large number of unbound electrons. Even though the metallic crystal’s free electrons can freely migrate from atom to atom, they cannot leave the metal’s surface to begin electron emission.

This is due to the fact that as the free electrons approach the metallic blocks’ edges, the positive nuclei behind them pull them back. Positive nuclei on all sides will pull the free electrons deep inside the metallic block. Because of this, they are able to move in any direction within the metallic block.

As soon as electrons exit the metallic block, there is no nucleus ahead of them, but they will be attracted by the nuclei behind them in the block. The positive nuclei act as a potential barrier for electrons trying to depart the metallic surface. The barrier that stops a free electron from escaping from the metallic surface is called the surface barrier. Free electrons can cross the surface barrier and escape from the metallic surface if they receive enough external energy.