Photon-electron interaction

In photon-electron interactions, a photon can either be absorbed by an electron or scattered by it. During the interaction, the photon transfers some or all of its energy and momentum to the electron.

  • One of the most common interactions used to explain the link between electromagnetism and visible light is the photon-electron interaction.
  • The specific outcome of a photon-electron interaction depends on the photon’s energy and the electron’s state. For example, if the photon has enough energy, it can knock the electron out of its orbit. This is known as the photoelectric effect. If the photon does not have enough energy to knock the electron out of its orbit, it can be scattered by the electron. This is known as Compton scattering.
    • On absorption of a photon by an electron, the electron gains energy and transitions to a higher energy level – a higher orbit around the nucleus of an atom.
    • When an electron scatters a photon, the photon alters its trajectory and might lose some energy. However, the electron’s energy level remains unaffected.
  • A photon can transfer all of its energy to an electron, even if the photon has more energy than the electron’s binding energy. In this case, the electron will be ejected from the atom with excess kinetic energy.
  • The amount of energy a photon can transfer to an electron is capped by its energy. A photon cannot impart more energy to an electron than it possesses.
  • The likelihood of a photon being absorbed by an electron hinges on both the photon’s energy and the electron’s energy level. A higher photon energy increases the absorption likelihood, while a higher electron energy level decreases the absorption probability. (The likelihood of absorption also depends on the polarization of the photon and the orientation of the electron’s orbital.)
  • The likelihood of a photon being scattered by an electron is similarly influenced by photon energy and electron energy level.  (The likelihood of scattering also depends on the angle of photon-electron collision and the spin of the electron). \
  • Examples of a photons-electron interaction include:
    • Photoelectric Effect: When a photon with adequate energy impacts an electron, the electron can be expelled from the atom. Called the photoelectric effect, the photon’s energy must exceed the electron’s binding energy within the atom.
    • Compton Scattering: In a collision between a photon and an electron, the photon can scatter. The photon might relinquish energy during the collision, and the electron can acquire a portion of the photon’s energy.
    • Chromophore excitation: The photon-chromophore interaction accounts for the observable colour of objects.
      • The interaction between photons and chromophores is more intricate than photon-electron interaction. It encompasses energy redistribution among all electrons in a molecule and can result in various outcomes such as fluorescence, phosphorescence, and energy transfer.
      • The interaction between photons and chromophores is generally referred to as molecular excitation or chromophore excitation, distinct from photon-electron interaction but can still be considered a type of photon-electron interaction since it involves the interaction of photons with the electrons in chromophore molecules.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References
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Summary