Lateral motion of adsorbate induced by vibrational mode excitation with inelastic tunneling electron

We show a lateral hopping of adsorbates of CO molecules on Pd(1 1 0) induced by the tunneling electron of scanning tunneling microscopy. When tunneling electron with bias voltage of a few tenth of volt is dosed into an isolated adsorbate whose lateral motion is originally frozen at liquid He temperature, hopping of adsorbates to an equivalent site is observed. This phenomenon shows a clear threshold in the bias voltage, and most interestingly the threshold voltage corresponds to the excitation energy of the high-lying vibration mode of the molecule (C-O stretching mode for CO case). We consider a model in which the low-frequency vibrational modes (T- and R-modes), directly related to the lateral hopping mechanism, are excited in accordance with the excitation of the high-lying mode through anharmonic coupling.