Senior Research Scientist, Surface Chemistry Laboratory, RIKEN
The study of single molecules provides deep insights into the nature of bonding and underlying quantum mechanics concerning the control of chemical reactions. The scanning tunnelling microscope (STM) is a versatile and powerful tool for investigating and controlling the chemistry of individual molecules on solid surfaces. The most important feature of the STM is that the tunnelling electrons are involved in its operation. The coupling of tunnelling electrons with the electronic and vibrational states of the target molecule allows us to realize mode-selective and state-selective chemistry of the individual molecules.
I will address two main issues with our experimental and theoretical efforts on investigating the interaction of electrons with vibrational and electronic states of a single molecule on surfaces. The first part is assigned to the excitation of vibrational modes to selectively induce particular dynamic motion and chemical reaction of a single molecule on metal surfaces. The microscopic mechanism of vibrationally induced molecular motions and the selection rules for the single-molecule vibrational spectroscopy are also discussed. A precise control of molecular motions and chemical reaction by exciting vibrational modes with an STM will also be explained (Figure(a)). The second part focuses on the novel reaction dynamics of a single molecule on an ultrathin insulating film surface. The ultrathin insulating film decouples the molecular orbitals from the metallic substrate, to prevent electronic quenching of the excited molecule, resulting in a much longer lifetime of excited states, which enable the exploration of novel reaction dynamics of molecules. Here, I show selective control of reaction pathways of a single water molecule on an MgO ultrathin film grown on a silver single crystal surface (Figure(b)).
Figure(a) - A schematic figure of vibrational excitation of a single molecule by injecting tunnelling electrons from the STM tip, and the STM images measured before and after single-molecule reaction by vibrational excitation [Phys. Rev. Lett. 89 (2002) 126104].
Figure(b) - Electron injection from the STM into a target molecule on the surface of 2-ML MgO film grown on Ag (100), and the dissociation pathways of a single water molecule [Nature Materials 9 (2010) 442].