A distributive mechanism for two-photon mean-frequency absorption

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Abstract
This paper describes a new mechanism for the concerted mean-frequency absorption process A+A+ℏω1+ℏω2→A*+A*. In contrast to a mechanism described previously, this effect can be subject to the normal selection rules for single-photon absorption, and is mediated by virtual photon coupling. Rate equations are given for molecules in a fluid, van der Waals molecules, and chromophore pairs in polyatomic molecules. The dependence of the absorption rate on the separation of the interacting pair is then examined in detail. An analogy is drawn between the long-range limit of the absorption rate and a process of hyper-Raman scattering followed by single-photon absorption. It is demonstrated that the two processes are equivalent where the molecular separation is large enough that the virtual photon can be considered as real and physically identifiable.