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

TL;DR

This study demonstrates two-photon induced hot electron transfer to a single molecule in an STM, revealing ultrafast dynamics and atomic-scale spatial variations, advancing understanding of nanoscale photo-induced charge transfer.

Contribution

It introduces a novel method to observe two-photon excited electron transfer at the single-molecule level using STM with femtosecond laser pulses.

Findings

Electron transfer rate depends quadratically on laser power.

Polarization measurements confirm two-photon excitation.

Ultrafast charge transfer dynamics observed with two-pulse correlation.

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.