Fluctuations and local symmetry in single-molecule rhodamine 6G Raman scattering on silver nanocrystal aggregates

Surface enhanced Raman scattering (SERS) from Rhodamine 6G adsorbed on small, optically resolvable aggregates of silver nanoparticles is detectable at the single-molecule level. The signal fluctuates on a second time scale, presumably due to movement of the molecule in to and Out of the SERS active hot spots. The depolarization of the signal is found to be independent of the fluctuations, to vary from one spot to the next, and to be highly dependent on the direction of the incident electric field. The SERS continuum dominates the signal and is assumed to arise from electronic Raman processes in the metal that are triggered by chemisorption of the molecule in the hot spot. The source of the Raman signal is therefore highly localized around the hot spot. The local structure determines the depolarization properties and is often extremely anisotropic. Conversely, the Rayleigh scattering does not depend on the molecule or the hot spot and is delocalized over the whole aggregate, which is always found to be highly isotropic. The Rayleigh scattering therefore does not fluctuate and is always highly polarized.