title: Surface-enhanced Raman excitation spectroscopy of a single rhodamine 6G molecule.
abstract: 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.
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA.
authors.forename: Jon A
authors.initials: JA
authors.lastname: Dieringer
authors.forename: Kristin L
authors.initials: KL
authors.lastname: Wustholz
authors.forename: David J
authors.initials: DJ
authors.lastname: Masiello
authors.forename: Jon P
authors.initials: JP
authors.lastname: Camden
authors.forename: Samuel L
authors.initials: SL
authors.lastname: Kleinman
authors.forename: George C
authors.initials: GC
authors.lastname: Schatz
authors.forename: Richard P
authors.initials: RP
authors.lastname: Van Duyne