Cooperative single-photon absorption at interfaces

The pairwise absorption of light by chemically distinct molecules in close proximity can result in their cooperative single-photon excitation. It is shown how new characteristics emerge where the two molecular species concerned lie on opposite sides of a phase boundary or interface, as for example with two immiscible liquids, one polar and the other non-polar. Under such circumstances it is shown that if the polar phase has a component with a strong intramolecular charge transfer transition, it can thereby display significant absorption outside its normal absorption band. Such behaviour, which uniquely characterises the interface, occurs at a rate that is quadratically dependent upon the difference between the permanent dipole moments of the corresponding molecular ground and excited states of the polar species.