Separation of Electromagnetic and Chemical Contributions to Surface-Enhanced Raman Spectra on Nanoengineered Plasmonic Substrates

Ramansignalsfrommoleculesadsorbedonanoblemetalsurfaceare enhanced by many orders of magnitude due to the plasmon resonances of the substrate.Additionally,theenhancedspectraaremodifiedcomparedtothespectra ofneatmolecules;manyvibrationalfrequenciesareshifted,andrelativeintensities undergo significant changes upon attachment to the metal. With the goal of devising an effective scheme for separating the electromagnetic and chemical effects, we explore the origin of the Raman spectra modification of benzenethiol adsorbed on nanostructured gold surfaces. The spectral modifications are attrib- uted to the frequencydependence of the electromagnetic enhancement and to the effect of chemical binding. The latter contribution can be reproduced computa- tionally using molecule-metal cluster models. We present evidence that the effect of chemical binding is mostly due to changes in the electronic structure of the molecule rather than to the fixed orientation of molecules relative to the substrate. SECTION Kinetics, Spectroscopy