Two-photon Induced Hot Electron Transfer to a Single Molecule in a Scanning Tunneling Microscope

Abstract: The junction of a scanning tunneling microscope (STM) operating in the tunneling regime was irradiated with femtosecond laser pulses. A photo-excited hot electron in the STM tip resonantly tunnels into an excited state of a single molecule on the surface, converting it from the neutral to the anion. The electron transfer rate depends quadratically on the incident laser power, suggesting a two-photon excitation process. This nonlinear optical process is further confirmed by the polarization measurement. Spatial dependence of the electron transfer rate exhibits atomic-scale variations. A two-pulse correlation experiment reveals the ultrafast dynamic nature of photo-induced charging process in the STM junction. Results from these experiments are important for understanding photo-induced interfacial charge transfer in many nanoscale inorganic-organic structures.