Coupling of Vibrational Excitation to the Rotational Motion of a Single Adsorbed Molecule

The reversible rotation of a single isolated acetylene molecule between two diagonal sites on the Cu(100) surface at 8 K was induced and monitored with tunneling electrons from a scanning tunneling microscope (STM). Excitation of the C---H (C---D) stretch mode of C₂H₂ ( C₂D₂) at 358 meV (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) stretch 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 inelastic tunneling and coupling probabilities.