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Overtones and combinations in single-molecule surface-enhanced resonance Raman scattering spectra.

The observation of overtones and combinations in the SERRS spectra of single molecules dispersed in Langmuir-Blodgett monolayers is confirmed for a family of molecules. The detection of fundamentals, combinations, and overtones in single-molecule spectra of a series of perylenetetracarboxylic diimides (PTCD) is achieved with spatially resolved surface-enhanced resonance Raman scattering (SERRS). The Langmuir-Blodgett technique is used to create monomolecular thick films on metal islands containing on average one probed molecule within the field of view of the Raman microscope. The enhancement needed for single-molecule detection is achieved through the multiplicative effects of electromagnetic enhancement by metal nanostructures and resonance Raman enhancement by excitation into molecular electronic absorption bands. Overtone and combination progressions are well resolved in the average SERRS spectra of all three PTCD molecules.