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Molecular Motion on Semiconductor Surface via Tip-enhanced Multiple Excitation

In a low-temperature study with a scanning tunneling microscope (STM), the irreducible lateral motion of a CO molecule adsorbed on a Si(001) surface showed a hyperlinear dependence on the tunneling current. This dependence implies that the adsorbate displacement is caused by multiple excitations of adsorbate vibration modes, a situation thus far observed only at metal surfaces. The local vibronic temperature at the atomic scale on the surface heated by ohmic inelastic scattering of tunneling electrons indicates that there is an activation barrier of 0.11 eV for the irreversible motion of CO, in agreement with the adiabatic potential obtained from first-principles calculation. The highly efficient local heating is caused by a mid-gap state at the surface induced by the electric field of the STM tip.