Vibronic Raman scattering at the quantum limit of plasmons.

We record sequences of Raman spectra at a plasmonic junction formed by a gold AFM tip in contact with a silver surface coated with 4,4'-dimercaptostilbene (DMS). A 2D correlation analysis of the recorded trajectories reveals that the observable vibrational states can be divided into subsets, by virtue of the symmetry of DMS (C2h). The first set comprises the totally symmetric vibrations of DMS (ag) that are neither correlated with each other nor with the fluctuating background, assigned to the signature of charge-transfer plasmons mediated by DMS. The second set consists of bu modes, which are correlated both with each other and with the background. Our findings are rationalized on the basis of the charge-transfer theory of Raman scattering and illustrate how current carrying plasmons modulate the vibronic coupling terms from which the intensities of the bu states are derived. In effect, this study identifies gateway molecular modes for mediating charge shuttling across a plasmonic gap.