  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.
