Molecular proximity effects in two-photon absorption

Under laser irradiation, the mutual interaction between molecules is shown to lead to a cooperative nonlinear process which results in entirely new features in the absorption spectrum. In the quantum electrodynamical formalism the coupling between molecular centres, induced by molecular proximity rather than collision, is mediated by means of virtual photon coupling and allows transitions which are forbidden in the absence of such interactions. The manifestation of circular dichroism, potential methods of observation and specific molecular examples are discussed. Attention is drawn to a number of possible resonance enhancement mechanisms and to the dependance of the absorption rate of molecular separation.