Orientation Switching of Single Molecules on Surface Excited by Tunneling Electrons and Ultrafast Laser Pulses.

We investigate the orientation switching of individual azobenzene molecules adsorbed on a Au(111) surface using a laser-assisted scanning tunneling microscope (STM). It is found that the rotational motion of the molecule can be regulated by both sample bias and laser wavelength. By measuring the switching rate and state occupation as a function of both bias voltage and photon energy, the threshold in sample bias and the minimal photon energy are derived. It has been revealed that the tip-induced local electrostatic potential remarkably contributes to the reduction in hopping barrier. We also find that the tunneling electrons and photons play distinct roles in controlling rotational dynamics of single azobenzene molecules on the surface, which are useful for understanding dynamic behaviors in similar molecular systems.