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Abstract

The technique of scanned-energy photoelectron diffraction has been used to obtain detailed
information on the adsorption site and internal structure of propyne (CH₃CCH) and 3,3,3-trifluoropropyne (CF₃CCH) on Cu(111). Both molecules are bound to the surface via the
acetylenic unit in a site analogous to that previously found for ethyne on Cu(111). In particular, the acetylenic unit is parallel to the surface in a cross-bridging position such that one of the C atoms is above a fcc hollow site while the other is above a hcp hollow site, giving a C–C bond length of 1.47 Å. The methyl/trifluoromethyl group is strongly tilted away from the surface and is attached with equal probability to the C atoms in the fcc and hcp hollow sites. The molecular plane is perpendicular to the surface. The fluoro group of CF₃CCH is preferentially oriented in the cis conformation. From the LEED pattern and the calculated coverage of 0.25 ML, a possible overlayer arrangement for propyne and 3,3,3-trifluoropropyne on Cu(111) has been proposed. For this overlayer the orientation of the F₃ group can be explained by intermolecular steric repulsion.