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 ${\mathrm{C}}_2{\mathrm{H}}_2$ ( ${\mathrm{C}}_2{\mathrm{D}}_2$) 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.

• Received 16 March 1998

DOI:https://doi.org/10.1103/PhysRevLett.81.1263