Electron Emission

Introduction

The release of electrons from a metal surface is known as electron emission. The nucleus is at the center of an atom, and electrons revolve around it. Electrons move in a variety of orbits. The electrons in the first orbit are strongly drawn to the nucleus. However, electrons in the outermost orbit are attracted to the nucleus. As a result, these electrons have a proclivity to leave the orbit if they receive enough energy. If we give this energy to the outermost electron in the form of heat, the electrons will absorb it and leave the metal surface.

Emission

A cathode has a higher electron concentration than the beam of electrons radiated. It is verified through the fact that the concentration of conduction band electrons in metals varies from $\mathrm{10^{22}\:to\:10^{23}electrons/cm^{3}}$. As a result, the transition from restrained to loose decreases electron density by 8 to 9 orders of magnitude. Moreover, the thermal power, of the electrons within the cathode is small.

At room temperature, the power spread close to the Fermi energy in copper is $\mathrm{K_{B}T}$ or 0.02 eV. (3000$\mathrm{^{\circ}K}$). To discharge these cold, limited electrons, however, the cathode has to be heated to around 25000$\mathrm{^{\circ}K}$, in all likelihood to result in a beam of thermal power of 0.20 eV. In summary, the simple decrease limit of the beam emittance is described by using the electron temperature of the emission method.

Because of the (MB) distribution, this ultimate emittance is frequently referred to as thermal emittance. In summary, elementary particles are categorized as bosons whether they've integers or $\mathrm{\frac{1}{2}}$ integer spin. Bosons follow MB statistics, whereas fermions use FD statistics.

• The MB distribution applies to particles of any variety that can share the same energy state.

• FD distributions are used for particles with only one energy state per particle and cannot share the same energy state.

Fig:1 Electron emission

Types of Electron Emission

There are, however, 3 essential emission processes that can be modeled based on thermal emittance 1. thermionic emission, 2. photo-electric emission, and 3. field emission.

Thermionic Emission

The release of electrons from a metal surface is known as electron emission. In an atom, the nucleus is at the center and electrons revolve around it. The electrons revolve around many orbits. The electrons in the first orbit are very highly attracted to the nucleus. However, the electrons in the outermost orbit are very weekly attracted to the nucleus. So these electrons tend to leave the orbit if at all they get sufficient energy. If we provide this energy to the outermost electron in form of heat these electrons absorb this energy and leave the metal surface. This emission of electrons leaving the metal surface because of the gain of thermal energy is called thermionic emission.

Photoelectric Emission

The three steps of the Spicer model [Spicer] can be used to describe photoelectric emission from a metal −

• Electron absorption of photons

• Surface electron transport

• Break through the barrier.

When a massive external field is implemented to the cathode, this Schottky energy grounds for the lower barrier, which is a few tens of an eV for fields of one hundred MV/m. The Schottky effect increases quantum efficiency significantly for a metal cathode like copper, whose work function is 4.6 eV.

The photo-emission process can be understood if all electrons absorbed by photons in step 1 escape. Accordingly, the quantum efficiency is directly proportional to electrons excited above the barrier by a photon. The photo-electric emittance also varies with the photon energy due to the energy spread. In step 2, electron-electron scattering redistributes energy across the lattice through the electron-phonon interaction, whereas in step 3, electron-electron scattering redistributes energy to the surface. Electron-phonon interaction is important for semiconductor cathodes, whereas electron-electron scattering is dominant for metals.

Field Emission

Field emission occurs when extremely high fields of 109 V/m or greater are present. The quantum mechanical tunneling of electrons through the barrier requires high electric fields to lower the barrier sufficiently so that useful emission currents can be achieved. Thermal Field emission takes place when electron temperatures exceed 1000 degrees Celsius. These types of emissions are often used in thermionic RF guns, which are used to inject third-generation storage rings with light. A low field and a high temperature (+100000$\mathrm{^{\circ}K}$) have the greatest influence on the current density, with the latter increasing by over an order of magnitude over the ambient (3000$\mathrm{^{\circ}K}$) temperature. Any significant current in field emission requires fields greater than 109 Volt per meter since electron yield is exponentially sensitive to external fields. To create high fields, pulses of high voltage are used in conjunction with field-enhancing, sharp emitters.

Work Function

It is the minimum energy required by an electron to escape from the metal surface. It is denoted by $\mathrm{\phi _{0}}$. Its unit is eV

$$\mathrm{1eV=1.6\times 10^{-19}J}$$

It depends upon nature of metal i.e. it varies from one metal to another

For example, Na = 2.75 eV

Pt = 5.65 eV

Uses of Electron Emission

• The electrical bulb

• Cathode Ray tube

Conclusion

The release of electrons from a metal surface is known as electron emission. The nucleus is at the heart of an atom, and electrons orbit it. Electrons move in various orbits. In the first orbit, electrons are strongly attracted to the nucleus. Electrons in the outermost orbit, on the other hand, are drawn to the nucleus. As a result, if they receive enough energy, these electrons have a proclivity to leave the orbit. If we apply this energy in the form of heat to the outermost electron, the electrons will absorb it and leave the metal surface. In this tutorial, we have learned about electron emission, types of electron emission, work function, uses of electron emission, and some FAQs.

FAQs

Q1. What factors influence electron emissions?

Ans. Electron emission is defined as the liberation of electrons from the surface caused by temperature increase, radiation, or a powerful electric field.

Q2. Why doesn't electron emission happen at room temperature?

Ans. These electrons travel randomly in the arrangement of atoms at room temperature, but they cannot leave the metallic surface. Ordinary metals do not lose electrons at room temperature. This implies the existence of a force that prevents electrons from leaving the metallic surface permanently.

Q3. What happens when electrons are emitted?

Ans. When an electron shifts levels, it loses energy and the atom radiates photons. The photon is emitted when an electron moves from a higher energy level to a lower energy level. The photon's energy is the exact energy lost by the electron as it moves to its lower energy level.

Q4. What are the advantages of using metals for electron emissions?

Ans. Because the free electrons in metals lack sufficient energy to leave the metal. The loose electrons that try to leave the metal are drawn back by the positive electric force of the nucleus. As a result, without sufficient energy, free electrons cannot escape from metal.

Q5. What do quick electron emissions go by?

Ans. Field electron emission is the emission of electrons caused by an electrostatic field. It is also known as field emission (FE) and electron field emission.