What is Electron Emission? | Lucent for GK - UPSC PDF Download

Introduction

Metals harbor a unique characteristic: the presence of loosely bound valence electrons in their outer shell. These electrons have the freedom to move effortlessly within the metal, but they are typically confined to its surface. Such mobile electrons are referred to as free electrons. With a mass of 9.1 x 10^(-31) kg, these free electrons cannot wander freely within the metal at room temperature without leaving its surface. When an electron manages to escape the metallic surface, it acquires a positive charge, attracting other electrons towards itself. Consequently, these free electrons remain tethered to the metal's surface by restraining forces arising from these attractive interactions, creating a potential barrier. The presence of this barrier impedes electrons from leaving the surface, necessitating the provision of additional energy to liberate them. This energy is known as the "work function," denoted by the symbol Φ, and measured in electron volts (eV). The work function represents the minimum energy required by an electron to overcome the attractive forces and escape from the metallic surface.

Types of Electron Emission

According to the theory of electron emission, four main types of emission can occur when the energy supplied exceeds the work function of the metal. These types include thermionic emission, photoelectric emission, secondary emission, and electric field emission. Let us delve into each of these types and gain a deeper understanding of their mechanisms.

Thermionic Emission: Harnessing Heat
Thermionic emission involves the emission of electrons from a metallic surface through the application of heat. The thermal energy supplied to the metallic surface facilitates the liberation of electrons, which are commonly referred to as thermions. The number of thermions emitted depends on the temperature of the metallic surface, with higher temperatures leading to increased emission.

Photoelectric Emission: Illuminating the Path
The photoelectric effect, discovered by Albert Einstein, showcases the emission of free electrons from a metallic surface when it is illuminated by light radiation of a suitable frequency. In this process, the energy required for electron emission is supplied in the form of discrete packets known as photons. These photons transfer their energy to the free electrons, prompting their release from the surface. Consequently, the liberated electrons are termed photoelectrons. The number of photoelectrons emitted relies on the intensity of the incident light.

Electric Field Emission: Unleashing the Power of Fields
Electric field emission, also known as cold cathode emission, involves the emission of electrons from a metallic surface under the influence of a strong electric field. When free electrons depart from the metal surface, it acquires a positive charge, subsequently attracting electrons due to the electrostatic force. By applying a robust electric field of approximately 10^8 V/m to the metal, electron emission is stimulated.

Secondary Emission: Amplifying the Phenomenon
Secondary emission is a captivating phenomenon wherein a significant number of electrons are emitted when highly energetic primary electrons collide with a metallic surface. These energetic primary electrons transfer their energy to the bound electrons within the metal, boosting their energy beyond the work function threshold and causing their release. Given that the emitted electrons originate from the influence of primary electrons, they are aptly termed secondary electrons.