  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.
