Acetylene structure and dynamics on Pd(111)

Acetylene molecules adsorbed onto a Pd(111) crystal surface were imaged by scanning tunneling microscopy (STM) below 70 K. Single C2H2 molecules appeared as a combination of a protrusion and a depression 3.5 Angstrom apart. This peculiar shape is due to the molecular pi orbital, which is oriented at a shallow angle out of the surface. Six orientations of the molecule with respect to the substrate were observed, indicating two different threefold hollow binding sites each with three possible rotational states. At 44 K, thermally activated rotation of the molecules between the three equivalent states on the same threefold hollow site occurs on the time scale of seconds. Diffusion of molecules between adjacent threefold sites began at similar to 70 K. The findings agree with the structure of the adsorption site determined by total-energy calculations and with the STM image calculated using the electron scattering quantum chemistry method.