Single-photon pair excitations in weakly interacting systems

A conventional perturbation treatment of intermolecular forces gives a physically motivated formula for single-photon absorption by a weakly interacting pair. The transition dipole matrix element arises from a generalised induction mechanism and is large for transitions where one or both monomers undergoes a significant charge shift on excitation. Whilst intrinsic low intensity and lack of spectral windows at higher energies render observation of single-photon pair excitations difficult in the electronic spectrum, the prospects are better for observation of the vibrational analogue.