Local Symmetry and Unstability in Single-Molecule Rhoda Mine 6G Raman Scattering on Silver Nano Crystal Aggregates
Surfaceenhanced Raman scattering (SERS) from Rhodamine 6G adsorbed on small, opticallyresolvable aggregates of silver nanoparticles is detectable at thesingle-molecule level. The signal fluctuates on a second time scale, presumablydue 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 theunstabilitys, to vary from one spot to the next, and to be highly dependent onthe direction of the incident electric field. The “SERS continuum” dominatesthe signal and is assumed to arise from electronic Raman processes in the metalthat are triggered by chemisorption of the molecule in the hot spot. The sourceof the Raman signal is therefore highly localized around the hot spot. Thelocal structure determines the depolarization properties and is often extremelyanisotropic. Conversely, the Rayleigh scattering does not depend on themolecule or the hot spot and is delocalized over the whole aggregate, which isalways found to be highly isotropic. The Rayleigh scattering therefore does notfluctuate and is always highly polarized.