Electron emission is defined as the liberation of electrons from any surface of a substance. Any piece of a metal block consists of plenty of free electrons. Even at room temperature, there are plenty of free electrons moving randomly inside the metallic crystal. Although the free electrons move randomly from one atom to another inside the metallic crystal, they can not leave the surface of the metal to initiate electron emission.
This is because the free electrons reach the extreme boundary of the metallic blocks are pulled back by the positive nuclei behind them. The free electrons well inside the metallic block will be attracted by the positive nuclei from all sides. So they can move freely in any direction inside the metallic block.
Types of Electron Emission
The electron emission is possible only if sufficient energy (equal to the work function of the metal) is supplied to the metal in the form of heat energy, light energy, etc.
Depending on the source of energy, electron emission can be of the following types:
- Thermionic Emission: In this type, the metal is heated to a sufficient temperature to enable the free electrons to come out of its surface.
- Field Emission: In this type, a very strong electric field is applied to the metal which pulls the electrons out of the surface due to the attraction of the positive field.
- Photoelectric Emission: In this type, the light of a certain frequency is made to fall on the metal surface which leads to the emission of electrons.
Secondary emission is a phenomenon in which a large number of electrons emit when fast moving electrons called the primary electrons strike the metallic surface.
The fast-moving electrons are highly energetic, and when they fall on the surface, they transfer their energy to the free electrons (bonded by a strong attractive force with the metal) of the metal by which their energy becomes more than the work function of the metal and they release the electrons.