A force that may "override" the intrinsic dispersion force (see London Force) is known as optical binding. This involves the irradiation of molecules by a moderately intense laser, in a similar mechanism to laser-assisted resonance energy transfer (see All-optical Switching).
The optical binding mechanism is shown on the left:
- Region I (before interaction i) - the right and left molecules are unexcited (blue)
- Region II (between i and j) - a laser photon is absorbed by the right molecule and it moves into an intermediate state (green)
- Region III (between j and k) - a photon is emitted by the right molecule and it returns into the unexcited state (blue)
- Region IV (between k and l) - this photon is absorbed by the left molecule and it moves into an intermediate state (green)
- Region V (after l) - a laser photon is stimulated from the left molecule and returns into the unexcited state (blue)
The energy shift (similar to the matrix element) for optical binding involves two alpha tensors - due to two interactions at each molecule - and one V tensor (since the molecules interact via one photon).
The energy shift is already a measurable quantity and, therefore, does not need to be squared to find the transfer rate.