Define and explain electron emission. Give different types of electron emissions.
The phenomenon of emission of electron from a metal surface by supplying external energy is called electron emission.
Explanation: In metals, the electrons in the outermost orbit of an atom are loosely bound and can move freely in all possible directions within the metal even at room temperature. These electrons in the metal are known as free electrons. These free electrons are responsible for the conductivity of a metal. However, these free electrons cannot leave the surface of the metal of their own.
As soon as an electron tends to leave the metal surface, a positive charge is developed on the surface of the metal. This positive charge pulls back (or attracts) the free electron tending to leave the metal surface. This attracting force acts like a surface barrier for free electrons. The free electron can leave the metal surface only if an external energy is available to it to overcome the surface barrier.
Types of electron emission :
The external energy needed to emit electrons from a metal surface can be supplied in different ways leading to different types of electron emission as discussed below:
(i) Thermionic emission:
The process of emission of electrons from a metallic surface when the metal surface is heated is known as thermionic emission. The free electrons in the metal absorb the heat energy and can overcome the surface barrier. As a result, the free electrons are emitted from the metal surface. The electrons emitted in this way are sometimes referred to as thermions because they are emitted due to thermal energy.
(ii) Photo-electric emission:
The process of emission of electrons from a metallic surface, when light of suitable frequency is incident on the metal surface is known as photo-electric emission. The free electrons in the metal absorb the energy of the incident light and cross over the surface barrier. Hence, the free electrons are emitted from the metal surface. The electrons emitted so are known as photo-electrons.because they are emitted due to light (photo) energy.
(iii) Secondary emission:
The process of emission of electrons from a metallic surface, when highly energetic electron beam is incident on the metal surface is called secondary emission. The electrons emitted so are known as secondary electrons.
(iv) Field emission (cold cathode emission):
The process of emission of free electrons from a metallic surface, when a strong electric field ($\approx 10^8 V {m}^{-1}$) is applied across the metal surface is known as field emission. This type of emission is also known as cold emission or cold cathode emission.