Photoelectric Emission: Definition, Theory and Applications

Photoelectric emission is defined as the release of electrons from the surface of a metal when electromagnetic radiation, such as light, is incident on its surface. The electrons that are released in the photoelectric emission are called photoelectrons. Photoelectric emission is also known as photoemission or the photoelectric effect.

What causes photoelectric emission?

Photoelectric emission can be explained by the quantum theory of light, which states that light consists of discrete packets of energy called photons. The energy of a photon is proportional to the frequency of the light and is given by the formula:

image 76

where E is the energy of the photon, h is Planck’s constant, and ν is the frequency of the light.

When a photon collides with an electron on the surface of a metal, it transfers its energy to the electron. If the energy of the photon is greater than or equal to the work function of the metal, which is the minimum amount of energy required to remove an electron from the metal surface, then the electron can escape from the metal. The excess energy of the photon becomes the kinetic energy of the photoelectron.

Photon energy and work function

The work function of a metal depends on its chemical composition and physical structure and varies from metal to metal. For example, the work function of potassium is about 2.3 eV, while that of platinum is about 6.3 eV.

The kinetic energy of a photoelectron can be calculated by subtracting the work function from the photon energy:

image 77

where KE is the kinetic energy of the photoelectron, E is the photon energy, and ϕ is the work function of the metal.

What are the factors affecting photoelectric emission?

The amount and characteristics of photoelectric emission depend on several factors, such as:

  • The frequency of the incident light: The frequency of the incident light determines whether photoelectric emission will occur or not. If the frequency is lower than a certain threshold frequency, which corresponds to the minimum photon energy required to overcome the work function of the metal, then no photoelectric emission will take place, regardless of the intensity or duration of the light. If the frequency is higher than or equal to the threshold frequency, then photoelectric emission will occur, and increasing the frequency will increase the kinetic energy of the photoelectrons.
  • The intensity of the incident light: The intensity of the incident light measures how many photons are hitting a unit area per unit of time. The intensity does not affect whether photoelectric emission will occur or not as long as it is above zero. However, increasing the intensity will increase the number of photoelectrons emitted per unit of time, which means that more current will flow through an external circuit connected to the metal.
  • The potential difference between the metal and an anode: If the metal is connected to a positive terminal (anode) of a battery through an external circuit, then the photoelectrons emitted from the metal will be attracted towards the anode and cause a current to flow in the circuit. This current is called the photoelectric current. However, if the potential difference between the metal and the anode is too high, then the electric field created by the battery will oppose the motion of the photoelectrons and prevent them from reaching the anode. This potential difference is called the stopping potential, and it corresponds to the maximum kinetic energy of the photoelectrons.

What are some applications of photoelectric emission?

Photoelectric emission has many applications in various fields of science and technology, such as:

  • Photocells: Photocells are devices that convert light energy into electrical energy by using photoelectric emission. Photocells consist of a metal cathode and an anode enclosed in a glass tube that is evacuated or filled with inert gas. When light strikes the cathode, photoelectrons are emitted and collected by the anode, creating a voltage difference between them. Photocells can be used for various purposes, such as measuring light intensity, detecting light signals, controlling electric circuits, and powering solar panels.
  • Photomultipliers: Photomultipliers are devices that amplify weak light signals by using photoelectric emission and secondary electron emission. Photomultipliers consist of a photocathode, a series of dynodes, and an anode enclosed in a vacuum tube. When light strikes the photocathode, photoelectrons are emitted and accelerated towards the first dynode, which is at a higher positive potential than the photocathode. When a photoelectron hits a dynode, it causes several secondary electrons to be emitted from the dynode. These secondary electrons are then accelerated towards the next dynode, which is at a higher positive potential than the previous one. This process repeats for several dynodes, resulting in a multiplication of electrons at each stage. The final output current at the anode, which collects all the electrons, is proportional to the number of photons hitting the photocathode but much larger in magnitude. Photomultipliers can be used for various purposes, such as detecting faint light sources, measuring radiation levels, and analyzing spectra.
  • Photoelectron spectroscopy: Photoelectron spectroscopy is a technique that uses photoelectric emission to study the electronic structure and chemical composition of materials. Photoelectron spectroscopy involves irradiating a sample with high-energy photons (such as X-rays or ultraviolet rays) and measuring the kinetic energy and angular distribution of the emitted photoelectrons. The kinetic energy of a photoelectron depends on the binding energy of the electron in its original orbital in the sample, which reflects its chemical environment. The angular distribution of a photoelectron depends on the shape and orientation of its original orbital in the sample, which reflects its symmetry properties. By analyzing these data, one can infer information about the atomic and molecular structure, chemical bonding, and electronic properties of the sample.

Summary

Photoelectric emission is a phenomenon where electrons are released from a metal surface when electromagnetic radiation (such as light) hits it. Photoelectric emission can be explained by a quantum theory that treats light as a stream of particles called photons with an energy proportional to their frequency. Photoelectric emission depends on several factors, such as a frequency and intensity of incident light, the work function and threshold frequency of a metal, and a potential difference between a metal and an anode. Photoelectric emission has many applications in various fields, such as photocells, photomultipliers, and photoelectron spectroscopy.

   
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