ORIENTATION AND PERIODICITY IN THE C(4X8) AND P(2X1) STRUCTURES OF 3-THIOPHENE CARBOXYLIC-ACID ON CU(110)

The chemisorption of 3-thiophene carboxylic acid on Cu(110) between 30 0 and 350 K has hen investigated by high resolution electron energy lo ss spectroscopy (HREELS), scanning tunnelling microscopy (STM) and low energy electron diffraction (LEED). Ab initio molecular orbital calcu lations of the molecular ion aided in vibrational frequency assignment s, interpretation of STM images and estimation of intra- and inter-mol ecular interactions influencing formation of the c(4 x 8) and p(2 x 1) structures. HREELS shows that at low coverage, the molecule lays flat with its rr orbitals interacting with the surface. Increasing the cov erage induces the molecules to reorient perpendicular to the surface a nd form a c(4 x 8) intermediate structure. Impact scattering in HREELS demonstrates that the molecules are preferentially aligned with the t hiophene ring in the [110] azimuth. STM images suggest that the uprigh t carboxylate species form rows of four adjacent molecules face-to-fac e along the [001] direction separated by four lattice constants in [11 0]. Subsequent rows are shifted by two lattice constants along [110], resulting in an overall c(4 x 8) periodicity and a coverage of 0.25 ML . With increasing coverage, the c(4 x 8) structure changes to a p(2 x 1) structure. A model with the carboxylates bound in short bridge site s two lattice constants apart along [110] with a local coverage of 0.5 ML is proposed. Steric repulsion in the p(2 x 1) structure results in rotation of the thiophene ring by an estimated 30 degrees away from t he [110] direction, consistent with impact scattering HREELS measureme nts. Calculated dipole-dipole repulsion between the carboxylate groups is large compared to any dipole-dipole attraction which could result from anti-parallel alignment of the static dipole moments of the thiop hene rings.