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The surface-enhanced Raman excitation profiles (REPs) of rhodamine 6G (R6G) on Ag surfaces are studied using a tunable optical parametric oscillator excitation source and versatile detection scheme. These experiments afford the ability to finely tune the excitation wavelength near the molecular resonance of R6G (i.e., approximately 500-575 nm) and perform wavelength-scanned surface-enhanced Raman excitation measurements of a single molecule. The ensemble-averaged surface-enhanced REPs are measured for collections of molecules on Ag island films. The relative contributions of the 0-0 and 0-1 vibronic transitions to the surface-enhanced REPs vary with vibrational frequency. These results highlight the role of excitation energy in determining the resonance Raman intensities for R6G on surface-enhancing nanostructures. Single-molecule measurements are obtained from individual molecules of R6G on Ag colloidal aggregates, where single-molecule junctions are located using the isotope-edited approach. Overall, single-molecule surface-enhanced REPs are narrow in comparison to the ensemble-averaged excitation profiles due to a reduction in inhomogeneous broadening. This work describes the first Raman excitation spectroscopy studies of a single molecule, revealing new information previously obscured by the ensemble.

We present here a detailed study of the specific nanoparticle structures that give rise to single-molecule surface-enhanced Raman scattering (SMSERS). A variety of structures are observed, but the simplest are dimers of Ag nanocrystals. We chose one of these structures for detailed study using electrodynamics calculations and found that the electromagnetic SERS enhancement factors of 10(9) are easily obtained and are consistent with single-molecule SERS activity.