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).
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).