Energy-Resolved Probing and Rectification Spectroscopy of Single Molecules

The main goal of the studies presented in this dissertation is to continue the endeavor in improving of energy-probing capability of STM for single molecule spectroscopy. Here, we extend energy spectrum into THz region for STM vibrational spectroscopy by combining an 8-Kelvin scanning tunneling STM with a continuous wave THz source. Development of THz rectification technique allowed probing of vibrational excitations with simultaneous chemical sensitivity, THz field sensitivity and <10 MHz energy resolution. On the other hand, by incorporating tunable NIR lasers into an STM junction, we demonstrate the energy-resolved probing of vibrational overtone resonant excitation through the vibration-mediated conformational transition of single pyrrolidine molecules. Photon equilibrium action spectroscopy was performed to observe wavelength dependent resonance excitation features with sub-meV energy resolution. These features were shown to be sensitive to the molecular local adsorption environment while being insensitive to the junction near field profile induced by junction plasmon excitation or the optical setup. Moreover, the light induced equilibrium change in the reversible conformational transition of pyrrolidine leads to generation of photocurrent. Using optical rectification spectroscopy, we investigated the influence of laser power and tunneling setpoint on the photocurrent. The photocurrent was shown to be governed by the competition between photon and electron contribution to the molecular reversible transition, which can be tuned through laser power, bias and tip-molecular interaction. Through the optical rectification study of pyrrolidine, we demonstrate its potential application as a tunable single-molecule photo-rectifier device when trapped between two electrodes