We propose an action spectroscopy for single-molecule motion induced by vibrational excitation with a scanning tunneling microscope (STM). Calculations of the inelastic tunneling current for excitation of the C-O stretch mode of the CO molecule on metal surfaces are combined with a theory which describes how the energy in the vibrational mode is transferred to a reaction coordinate mode to overcome the activation barrier. The calculated rate for CO hopping on Pd (110) as a function of the bias voltage agrees with the experimental result. It is proposed that the second derivative of the reaction rate with respect to the bias voltage is related to the vibrational density of states, which usually cannot be directly observed in STM inelastic electron tunneling spectroscopy when a molecule motion is induced by vibrational excitation.
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