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Acetylene molecules adsorbed onto a Pd͑111͒ crystal surface were imaged by scanning tunneling microscopy ͑STM͒ below 70 K. Single C 2 H 2 molecules appeared as a combination of a protrusion and a depression 3.5 Å apart. This peculiar shape is due to the molecular 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 ϳ70 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. ͓S0163-1829͑98͒50320-7͔Atomic resolution imaging of atoms and molecules on catalyst surfaces to determine their structure and to follow their reaction pathways is one goal of surface science. Variable temperature scanning tunneling microscopy ͑STM͒ has the potential to accomplish this goal by providing a direct view of the structure and dynamics of the molecular adsorbates. In principle, the rate of each step in the reaction can be brought into the time scale of STM imaging ͑seconds to minutes͒ by adjusting the temperature of the surface. In addition, atomic manipulation of reactants with the tip opens the possibility of inducing and exploring specific reactions pathways. In our laboratory, we are performing these studies with the catalytic reaction of acetylene cyclotrimerization to form benzene on Pd͑111͒. Here we present results that show in unprecedented detail the structure of the adsorbed acetylene molecule and its conformational changes as it moves over the surface of Pd͑111͒.A considerable amount of information on this reaction has emerged from studies performed over the past 15 years. 1-3 The reaction occurs under both high pressure on dispersed catalysts 4 and on single crystals in ultrahigh vacuum ͑UHV͒. It is highly structure sensitive and selective on the Pd͑111͒ surface, with up to 35% of the acetylene converted to benzene in UHV. Low-energy electron diffraction studies indicate that acetylene orders in a ()ϫ))R30°lattice. 5,6 Kesmodel et al. have acquired vibrational spectra that indicate that the molecule is rehybridized upon adsorption to approximately sp 2.5 . 7,8 They proposed a model in which the molecule is associated with three metal atoms and lies with its C-C bond parallel to the surface. 8 Photoelectron spectroscopy studies 6 indicate that the orbital degeneracy is broken by the adsorption symmetry. Sellers 9 performed theoretical calculations of the molecule on clusters at high-symmetry binding sites. The results fit best with the vibrational spectrum for the molecule bound at a threefold hollow site with a C-C bond length of 1.41 Å, a C-H length of 1.09 Å, and a C-C-H ang...

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