Vibrational Excitation of a Single Benzene Molecule Adsorbed on Cu(110) Studied by Scanning Tunneling Microscope Light Emission Spectroscopy

We have investigated the vibrational excitation of a single benzene molecule adsorbed on Cu(110) using scanning tunneling microscope light emission (STM-LE) spectroscopy. The STM-LE spectrum of the benzene molecule has a broad light emission peak and is similar to that of the Cu substrate, indicating that the STM-LE is radiated via the excitation of localized surface plasmons (LSP) confined in the tip-sample gap. However, careful analysis revealed that the STM-LE spectrum of the benzene molecule exhibits a steplike structure appearing at an energy near the quantum cutoff. Isotope-labeled experiments revealed that the fine structure is associated with the vibrational excitation of the benzene molecule, that is, the CH out-of-place bending mode. We concluded that the vibrational excitation of benzene observed in the present study is not caused by the tunneling electrons through the inelastic tunneling process but by the Raman process due to the strong electric field generated just under the STM tip.