Structure determination of propyne and 3,3,3-trifluoropropyne on Cu(111)

The technique of scanned-energy photoelectron diffraction has been used to obtain detailed information on the adsorption site and internal structure o f propyne (CH3CCH) and 3,3,3-trifluoropropyne (CF3CCH) on Cu(111). Both mol ecules 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 acetyle nic 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 hc p hollow site, giving a C-C bond length of 1.47 Angstrom. The methyl/triflu oromethyl group is strongly tilted away from the surface and is attached wi th equal probability to the C atoms in the fcc and hcp hollow sites. The mo lecular plane is perpendicular to the surface. The fluoro group of CF3CCH i s preferentially oriented in the cis conformation. From the LEED pattern an d 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 o verlayer the orientation of the F-3 group can be explained by intermolecula r steric repulsion. (C) 2000 American Institute of Physics. [S0021-9606(00) 70517-4].