London dispersion force
In terms of QED, this force involves two molecules with two photons travelling between them.
- Region I (before interaction i) - both molecules are unexcited (blue)
- Region II (between i and j) - a photon is emitted by the right molecule and it moves into an intermediate state (green)
- Region III (between j and k) - this photon is abosrbed by the left molecule and it moves into an intermediate state (green)
- Region IV (between k and l) - a second photon is emitted by the right molecule and it returns into the unexcited state (blue)
- Region V (after l) - this second photon is absorbed by the left molecule and it returns into the unexcited state (blue)
This mechanism may be confusing since, in this case, an unexcited molecule is able to emit a photon. This is due to Heisenberg Uncertainty Principle; a full explaination is beyond the scope of this website.
The energy shift (similar to the matrix element) for the London force involves two alpha tensors - due to two interactions at each molecule - and two V tensors (since the molecular interactions are mediated by two photons).