EVOLUTION OF LATERAL ORDER AND MOLECULAR REORIENTATION IN THE BENZOATE/Cu(110) SYSTEM

Coverage-dependent adsorption studies of benzoic acid on a Cu(110) single crystal surface resulted in a sequence of novel, large periodicity structures containing differently oriented benzoate species, as shown by LEED, STM and FTIR. At low coverage, only flat-lying benzoate species are present and faceting of step edges is observed. The favorable mobility of flat-lying species results in formation of large, single domain islands with a$\kern5pt 4\quad 3\choose -1\quad 5$periodicity containing four molecules per unit cell. Additional adsorption produces island growth, with single domains typically covering entire terraces and, simultaneously, addition of a fifth benzoate species per unit cell oriented perpendicular to the surface. This structure, denoted as the “α phase,” corresponds to a local coverage ofθ=0.21ML. Then, the periodicity changes to a$\kern5pt 4\quad 3\choose -1\quad 9$structure with single and paired rows of upright species, retaining the dominant direction and large single domain sizes, in addition to six flat-lying species. The second structure, referred to as the “β phase,” corresponds to a local coverage ofθ= 0.23ML. The saturation coverage,c(8×2)structure, atθ=0.25ML, grows in from step edges and contains only upright benzoate species. The results are interpreted in terms of the interplay between molecular orientation, site-specificity, diffusion and the intermolecular interactions favoring long range, two-dimensional order.