Electron Emission

Table of Contents

• What is electron emission?
• Different methods of electron emission
  • 1.   Thermionic emission:
  • 2.   Field emission or cold cathode emission:
  • 3.   Photoelectric emission:
  • 4.   Secondary emission:
• Some Important Points Regarding Electron Emission

What is electron emission?

The phenomenon of emission of electrons from a metal surface is called electron emission.

Different methods of electron emission

For the emission of electrons from the metal surface, the energy of electrons must be higher than the work function of the metal. For their release from the metal surface, the electrons may be supplied the required amount of energy by any one of the following methods :

1.   Thermionic emission:

When a metal is heated, its free electrons get sufficient thermal energy, and they can overcome surface barrier. This method of removal of electrons is called thermionic emission and the emitted electrons are called thermions or thermo-electrons.

2.   Field emission or cold cathode emission:

When a metal surface is subjected to very high electric fields, of the order of 10³ to 10⁸ Vm^-1, electrons are emitted from it. This is known as field or cold cathode emission. This method of electron emission is dangerous and less efficient.

3.   Photoelectric emission:

It is the process in which electrons are emitted from a metal surface when electromagnetic radiations of sufficiently high frequency are incident on it. The emitted electrons are called photoelectrons, but their rate of emission is very low.

4.   Secondary emission:

When fast moving electrons strike a metal surface, they transfer some of their energy to the free electrons of the metal. As a result, the energy of the free electrons becomes more than the work function of the metal and they get ejected from the metal surface. The emitted electrons are called secondary electrons and this method of removal of electrons is called secondary emission.

Some Important Points Regarding Electron Emission

• Free electrons in a metal are free in the sense that they move inside the metal in a constant potential. They are not free to come out of the metal. Additional energy is needed to eject them out from the metal surface.
• All the free electrons in a metal do not have the same energy. At a constant temperature, these electrons have a certain energy distribution.
• Because of the energy distribution of free electrons in a metal, the energy required to eject an electron is different for different electrons. An electron with higher energy requires less additional energy to come out of the metal than that with lower energy. Work function is the minimum energy required by an electron to get out of the metal surface.