COUPLING OF VIBRATIONAL-EXCITATION TO THE ROTATIONAL MOTION OF A SINGLE ADSORBED MOLECULE

The reversible rotation of a single isolated acetylene molecule betwee n two diagonal sites on the Cu(100) surface at 8 K was induced and mon itored with tunneling electrons from a scanning tunneling microscope ( STM). Excitation of the C-H (C-D) stretch mode of C2H2 (C2D2) at 358 m eV (266 meV) led to a 10-fold (60-fold) increase in the rotation rate. This increase is attributed to energy transfer from the C-H (C-D) str etch mode to the hindered rotational motion of the molecule. Inelastic electron tunneling spectroscopy with the STM provides the energies of the stretch modes and allows a quantitative determination of the inel astic tunneling and coupling probabilities.