Long-range periodicity in c(8x2) benzoate/Cu(110): a combined STM, LEED and HREELS study

The adsorption of benzoic acid on Cu(110) between 300 and 350 K results in benzoate species oriented perpendicular to the surface with a c(8 x 2) periodicity at saturation coverage. STM images of the ordered structure and non-dipolar scattering in HREELS demonstrate alignment in the [110] azimuth. We interpret the tunnelling mechanism in STM, negative ion resonance scattering in HREELS, and assign low-frequency vibrational modes utilising ab initio molecular orbital calculations. We propose a model with four molecules per c(8 x 2) unit cell (theta=0.25 ML) arranged in two out-of-phase, zig-zag rows along the [001] direction with the carboxylates bound in short bridge sites alternately three and five lattice constants apart along the [110] direction. Adsorbate-induced dipole-active phonons in the frequency range 65-185 cm(-1) in HREELS support the adsorption model. (C) 1997 Elsevier Science B.V.