inside the metal, they cannot leave the surface of the metal. The reason is that when free electrons reach the surface of the metal, they are attracted by the positive nuclei of the metal. It is this attractive pull which will not allow free electrons to leave the metallic surface at room temperature. In order to leave the metallic surface, the free electrons must cross a potential barrier created by the positive nuclei of the metal.
The potential barrier which prevents free electrons from leaving the metallic surface is called surface barrier . Free electrons possess some kinetic energy and this energy is different for different electrons. The kinetic energy of the free electrons is not sufficient to overcome the surface barrier. Whenever an additional energy is given to the free electrons, they will have sufficient energy to cross the surface barrier and they escape from the metallic surface .
The liberation of electrons from any surface of a substance is called electron emission . - - - - Unit dual nature of radiation and matter Electrons Hot filament Figure . Thermionic emission from hot filament of cathode ray tube or x-ray tube ii) Field emission Electric field emission occurs when a very strong electric field is applied across the metal. This strong field pulls the free electrons and helps them to overcome the surface barrier of the metal (Figure .
). Examples: Field emission scanning electron microscopes, Field-emission display etc. Strong electric field Metal Electrons emitted – – – – – – – Figure . Field emission So the metal selected for electron emission should have low work function.
The electron emission is categorized into different types depending upon the form of energy being utilized. There are mainly four types of electron emission which are given below. i) Thermionic emission When a metal is heated to a high temperature, the free electrons on the surface of the metal get sufficient energy in the form of thermal energy so that they are emitted from the metallic surface (Figure . ).
This type of emission is known