The liberation of electrons from any surface of a substance is called electron emission. 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.
On reaching the extreme surface of the metallic block, the free electrons do not get any positive nucleus ahead of them, but they will have attractive force by the nuclei behind them inside the metallic block. So, to leave the metallic surface an electron must cross a potential barrier offers by the positive nuclei. This potential barrier which prevents a free electron to liberate from the metallic surface is called surface barrier. When a sufficient external energy is given to the free electrons, it can cross the surface barrier and liberate from the metallic surface.
A free electron possesses some kinetic energy. But this kinetic energy is not sufficient to overcome the surface barrier. The extra energy required to just overcome the surface barrier of any metal by an electron is called work function.
Suppose a free electron required total 5 ev (Electron Volts) to overcome the surface barrier of a metal. If the electron does have 1 ev kinetic energy previously in it, then the extra energy required as work function is 5 – 1 = 4 ev.
Electron emission from a metal surface depends upon the work function of the metal. The work function may vary from metal to metal. It is a typical property of metal. Smaller work function comes more effective electron emission since in that case the extra energy required by free electrons at metal surface to emit from it is smaller. So the metal used for electron emission should be of low work function depending upon the nature of energy observed by free electron during emission, electron emission can be categories in different types. There are mainly four types of electron emission commonly used.
When the metal is heated sufficiently, the free electrons at the extreme surface of the metal get sufficient energy to emit from the metal. This emission is referred as thermionic emission. The intensity of Thermionic emission depends on the metal used for the emission as well as the temperature of the metal. This emission is mainly used in vacuum tubes devices.
Here high positive field is applied in front of the metal surface. The influence of this high electric field pulls the free electrons and help them to cross the surface barrier of the metal.
Photo Electric Emission
Light is the flow of photons. Each photon possesses energy in it. The energy of the photons depends upon the frequency or wave length of the light ray. On stricking on the metal surface same of the photons transfer its energy to the free electrons. Hence, the free electrons can get sufficient energy to overcome the surface barrier and starts electron emission. The intensity of photoelectric emission depends on the intensity of falling light.
When a beam of high-velocity electrons stricks on the metal surface, the kinetic energy of high velocity striking electrons, transferred to the free electrons on the metal surface. Thus the free electrons may get sufficient kinatic energy to overcome the surface barrier and start electron