Single-Molecule Investigation of Energy Dynamics in a Coupled Plasmon-Exciton System

We investigate the near-field interaction between an isolated free-base phthalocyanine molecule and a plasmon localized in the gap between an NaCl-covered Ag(111) surface and the tip apex of a scanning tunneling microscope. When the tip is located in the close proximity of the molecule, asymmetric dips emerge in the broad luminescence spectrum of the plasmon generated by the tunneling current. The origin of the dips is explained by energy transfer between the plasmon and molecular excitons and a quantum mechanical interference effect, where molecular vibrations provide additional degrees of freedom in the dynamic process.