We investigate the dynamics of low-coverage ethanethiolate on Cu(111) using helium spin-echo spectroscopy. Above 210 K, the measurements are dominated by translational hopping with an activation energy of only $86\pm 5\mathrm{meV}$. At lower temperatures (150–210 K) a further process becomes apparent which has the signature of confined motion. We demonstrate the experimental results are consistent with scattering from an anchored rotor, enabling identification of sixfold jump rotation of the ethyl tail group around a static sulfur adsorption site, with a rotational activation energy of $18\pm 8\mathrm{meV}$. Our approach represents a new form of rotational spectroscopy which can be used to study rotational surface diffusion.

• Received 25 March 2011

DOI:https://doi.org/10.1103/PhysRevLett.106.256101