Abstract

There is a wide range of phenomena resulting from the cooperative absorption, emission, or scattering of photons by molecules, atoms, ions, etc. For instance, two molecules (atoms) initially in their ground states can both be prompted to their excited electronic states through the concerted absorption of one¹ or two² laser photons, as: Such processes, (1) or (2), are characterised by absorption at wavelengths where neither component A nor B displays intrinsic absorption, so that the cooperative effect unequivocally signifies the interaction. In terms of the quantum electrodynamical (QED) formulation, the excess (or deficit) of energy carried by the incident photon annihilated at A is transferred to another molecule B through a virtual photon. Virtual photon coupling is a common feature of all bimolecular photophysical processes, other examples being resonance transfer of excitation energy,^3,4 bimolecular Raman scattering,⁵ bimolecular three-photon absorption, etc.⁶ Within such a classification, resonance energy transfer, appears as the lowest order process, as it involves no incident photons.

© 1994 IEEE