Theory of Inelastic Tunneling Current-Driven Motions of Single Adsorbates (Review Article)

The theory of inelastic electron tunneling spectroscopy (IETS) and motions of single adsorbed atoms and molecules on metal surfaces induced by vibrational excitation with a scanning tunneling microscope (STM) is reviewed. The theory of STM-IETS is described using the adsorbate-induced resonance model. Elementary processes of how an adsorbate overcomes the potential barrier along the reaction coordinate (RC) by inelastic tunneling current are described with a focus on direct excitation of the RC mode by coherent and incoherent vibrational ladder climbing and an indirect one through anharmonic coupling to a mode excited by tunneling electrons. Action spectroscopy of single molecule motions is also discussed. The latter allows a direct access to the vibrational density of states, which can not be otherwise observed in the STM-IETS because of a competition between the elastic and inelastic tunneling currents.