Single-Molecule Resonance Raman Spectroscopy

Abstract

Near-field Raman spectroscopy has attracted significant attention as a tool for performing chemical analysis with submolecular spatial resolution. The mechanism in atomic and molecular scale Raman spectroscopy, however, is not well understood due to the strong perturbation of the molecule during measurements in previous studies. In particular, the resonance effect that greatly enhances Raman scattering has not been clarified on a single molecule scale. Here we demonstrate single-molecule resonance Raman imaging with a scanning tunneling microscope (STM) at submolecular resolution of copper naphthalocyanine molecules. In our experiment, an ultra-thin insulating NaCl film is used as the substrate to keep the molecule in an undisturbed condition for performing a precise analysis. We have succeeded in resonance Raman spectroscopy by tuning the excitation wavelength to the intrinsic electronic transitions of the molecule. This makes it possible to acquire accurate Raman spectra with a short measurement time at single molecule sensitivities. The resonance Raman maps show three different spatial distribution patterns depending on the symmetry of the vibration modes. These results are explained by considering the interaction between the electromagnetic field of the plasmon in the tip-substrate gap and the intrinsic molecular symmetry, thereby a selection rule for the plasmon-enhanced resonant Raman effect is established.