What is Electron Emission?
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.
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 that prevents a free electron to liberate from the metallic surface is called surface barrier. When 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.
Types of Electron Emission
The types of electron emission include:
Each of these types of electron emission will be explained below.
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 to 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.
Field emission (or field electron emission) is defined as the electron emission caused by an electric field. A strong external electric field nearer to the surface of the emitter can affect the emission of electrons. A free electron at the extreme surface of the metal can’t escape the surface due to back pull from the positive nuclei in the emitter body. This back pull on a free electron causes a potential barrier on the electron.
An electron has to overcome this potential barrier to get emitted from the emitter surface. In more details we can say, a free electron well inside the emitter body feels alteration force from all sides due to the presence of positive nuclei surrounding it. But at the extreme edge of the surface of the emitter body, the free electron feels only the alternative force from the nuclei behind it. Since there is no nucleus ahead of the free electron to attract it in the outward direction.
Due to this electrostatic alteration force on a free electron, it has to acquire sufficient kinetic energy to overcome this free for escaping from the surface of the emitter. In other words, there will be a potential barrier for that electron created on the surface of the emitter. This potential barrier is also known as the surface barrier. A free electron has to overcome this surface barrier for getting emitted.
But when a sufficiently high positive charge is placed in front of the emitter surface, due to the strong electrostatic force of the created electric field, the free electron can get sufficient energy to overcome the surface barrier and can get emitted from the surface of the emitter body. As this type of electron emission is caused by the electric field present in the space, it is called field emission.
When we place a metal surface in front of a conductor of high positive potential, with respect to the potential of the metal surface, the strong electric field exerts a force on the free electrons. As the force applied to the free electrons is strong enough, the free electrons overcome the restraining force (i.e. surface barrier) and come out from the metal surface.
In the emission process, a very intense electric field needs to be created in the space. A million volts per centimeter would be the voltage gradient of the field. In the field emission process, the emitter may not need any extra heating for the field emission and that is why the process is also referred to as cold cathode emission.
Light is the flow of photons. Each photon possesses energy in it. The energy of the photons depends upon the frequency or wavelength of the light ray. On striking on the metal surface, some of the photons transfer their energy to the free electrons. Hence, 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.
Secondary Electron Emission
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 kinetic energy to overcome the surface barrier and start electron emission. This type of emission is known as secondary electron emission. Secondary emission is undesired in electron-beam tubes such as Klystron tubes, and hence efforts are made to suppress secondary emissions.