Raman scattering is most commonly associated with a change in vibrational state within one molecule, with signals in the
corresponding spectrum widely used to identify material structures. When the corresponding theory is developed using
quantum electrodynamics, the fundamental scattering process is described by a single photon of one radiation mode being
annihilated with the concurrent creation of another photon; the two photon energies differ by an amount corresponding to
the transfer of vibrational energy within the system. Here, we consider nanoscale interactions between neighboring
molecules to mediate the process, by way of a virtual photon exchange to connect the evolution of the two molecular states.
We consider both a single and pair of virtual photon exchanges. Our analysis deploys two realistic assumptions: in each
pairwise interaction the two components are considered to be (i) chemically different and (ii) held in a fixed orientation
with respect to each other, displaced by an amount equivalent to the near-field region; resulting in higher order dependences
on displacement R becoming increasingly significant, and at the limit the short-range R-6 term can even dominate over R-3
dependence. In our investigation one center undergoes a change in vibrational energy; each neighboring molecule returns
to the electronic and vibrational state in which it began. For the purposes of providing results, a Stokes transition has been
assumed; analogous principles hold for the anti-Stokes counterpart. Experimentally, there is no change to the dependence
on the intensity of laser light. However, the various mechanisms presented herein lead to different selection rules applying
in each instance. In some cases specifically identifiable mechanisms will be active for a given transition, leading to new
and characteristic lines in the Raman spectrum. A thorough investigation of all physically achievable mechanisms will be
detailed in this work.
Mathew D. Williams, David S. Bradshaw, and David L. Andrews, "On the emergence of Raman signals characterizing multicenter nanoscale interactions," Proc. SPIE 9884, Nanophotonics VI, 98840N (Presented at SPIE Photonics Europe: April 05, 2016; Published: 18 April 2016); https://doi.org/10.1117/12.2227719.
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