Electron Emission

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What is Electron Emission?

Electrons were discovered by J.J Thomson. In 1897, he performed an experiment in which he took an evacuated tube, called the cathode-ray tube, filled the gas inside it, and passed the electric charge to it. This experiment led to the discovery of electrons. 

The emission of electrons from the metallic surface is called electron emission.

In Photoelectric emission, we learned the work function, which means the amount of energy required to emit electrons from the surface. The electrons can emit from the light and also from other surfaces.

In this article, we will learn electron emission and its types.


Free Electrons in Metals

In metals, the electrons in the outer shell called the valance electrons of the atoms are loosely bound. They are free to move easily within the metal but are not allowed to leave the surface of the metal. Such freely moving electrons are called free electrons. 

The mass of electrons is 9.1 x 10-31 kg. The free electron in the metal at room temperature cannot move about freely inside the metal and leave its surface. The moment an electron comes out of the metallic surface, it gains a positive charge and pulls electrons toward itself. Thus the free electrons remain stuck with the metal by attractive forces of the surface called the restraining forces.

These attractive forces lead to a potential barrier. So, this barrier doesn’t allow electrons to leave the surface, that’s why some energy is offered to force these electrons to leave the surface. This energy is called the ‘work function’. So, what is the work function of the metal?

The work function is the minimum or the lowest amount of energy required by an electron to escape from the metallic surface by overcoming the attractive force with the surface of the metal.

The work function is symbolized as o and measured in eV, where eV stands for electron volt. So, eV is the energy required by an electron to escape from the metallic surface. According to the theory of electron emission, there are four types of electron emissions. 

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Types of Emission of Electrons

The electron emission from the metallic surface is possible only when the energy supplied is greater than the work function of the metal. The energy required for the various types of electron emission can be supplied to the free electrons by the following physical processes:

  • Thermionic emission

  • Photoelectric emission

  • Secondary emission

  • Electric field emission


Thermionic Emission

What is thermionic? Therm in thermionic means ‘heat’ or ‘temperature’ and ionic means ions. 

So, what is thermionic emission in Physics?

In this phenomenon, a metallic surface is heated for the emission of electrons. Here, the energy being supplied for the emission of electrons from the metallic surface is the thermal energy. 

The electrons emitted from the surface are called the thermal electrons or thermions.

Here, the number of thermions emitted relies on the temperature of the metallic surface.

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Photoelectric Effect

Albert Einstein discovered the Photoelectric effect. Einstein was awarded the Nobel prize for discovering this phenomenon. In this type of emission, free electrons are released from the metallic surface when the light radiations of suitable frequency fall on the surface of the metal.                                   

The energy passed to the free electrons for their emission is being supplied in the form of packets called the light photons or simply photons. The electrons emitting from the surface are called the photoelectrons. The name provided is because they are emitted by photons. Also, the number of photoelectrons emitted from the surface relies on the intensity of the incident light.

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Electric Field Emission

Field emission is also called the cold cathode emission. In this phenomenon, the emission of electrons from the metallic surface occurs under the application of a strong electric field.  

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When free electrons leave the surface of the metal, the metallic surface acquires a positive charge and electrons get attracted to the positive charge because of which a strong electric field of about 108 V/m is applied to metal, it emits electrons.


Secondary Emission

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. 

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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. Since the electrons are emitted with the help of primary electrons, that’s why the electrons emitted from the metal are called the secondary electrons.


Point to Remember

According to the order of electron emission, the emission of electrons from Cesium requires the lowest energy, while Platinum requires the highest energy to lose electrons.

FAQ (Frequently Asked Questions)

1. List the work function of the following metals in the ascending order:

  1. Mo

  2. Na

  3. Ca

  4. Cs

The below table lists the 𝜑o of the following metals:

Metal 

𝜑o (in eV)

Cs

2.14

Na

2.75

Ca

3.20

Mo

4.17

2. Why is thermionic emission important?

Thermionic emission occurs with the help of thermal energy. It is an important application of electron tubes and fluorescent lamps. 

Also, these emissions create the energy required for a Fermi-level electron to escape in the order of eV, analogous to atomic ionization energies.

3. State four laws of photoelectric emission.

  1. The number of photoelectrons that emit in a second by a metallic surface is directly proportional to the intensity of incident light.

  2. Photoelectric emission is a fast process and the time lag between the electrons emission is negligible, i.e., 10-9 s.

  3. For an electron emission, the minimum frequency of the incident radiation is required, below which emission can’t take place, and that frequency is the threshold frequency.

  4. Above the threshold frequency, the maximum kinetic energy of the emitted photoelectrons depends only on the frequency/wavelength of the incident light.

4. What are the quick electron emissions called?

Electric field emissions are called quick electron emissions.