Resonance energy transfer

Resonance energy transfer (RET) is a mechanism involving the transfer of energy from one molecule to another. It is mediated by dipole-dipole interactions; a dipole is often contained within a molecule.

Quantum electrodynamical calculations determined that radiationless and radiative energy transfer are the short- and long-range regions of a single unified mechanism (given below).

Feynman diagram for resonance energy transfer

The Feynman diagram that represents energy transfer involves two vertical lines (the two molecules) and one photon travelling between them. Starting from the bottom:

  • Region I (before interaction i) - the left molecule is unexcited (blue) and the right excited (red)
  • Region II (between i and j) - a photon is emitted by the right molecule and it moves into the unexcited state (blue)
  • Region III (after j) - this photon is absorbed by the left molecule and it moves into an excited state (red)

Resonance energy transfer involves two molecules with a photon-molecule interaction at each. The latter relates to a mu vector, which represents a dipole within a molecule. Due to the presence of two interacting molecules via a photon, a V tensor is also required. The transfer rate is therefore proportional to the square of (mu multiplied by V multiplied by mu).

Recent research has determined that this transfer rate can be altered by the input of a laser beam (see the All-optical Switching sub-section for details).

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