FACE SPECIFICITY AND THE ROLE OF METAL ADATOMS IN MOLECULAR-REORIENTATION AT SURFACES

Using reflection absorption infrared spectroscopy (RALRS), the coverag e-dependent reorientation of the benzoate species on the (110) and (11 1) faces of copper is compared and contrasted. Whereas on Cu(110) benz oate reorients from a flat-lying to an upright orientation with increa sing coverage, on Cu(lll), at all coverages, benzoate is aligned norma l to the surface. The formation of periodic, flat-lying copper-benzoat e structures has been attributed to the availability of metal adatoms, which differs dramatically between the (111) and (110) faces. We disc uss the face specificity of molecular orientation by comparing calcula ted formation energies of adatom vacancies from ledges and kink sites on (100), (110) and (111) faces. Further support for this model is giv en by the evaporation of sodium, either by pre- or post-dosing, onto l ow-coverage benzoate/Cu(111), which induces benzoate to convert from a perpendicular to a parallel orientation. Likewise, coevaporation of C u while dosing benzoic acid onto the Cu(111) surface also results in a majority of flat-lying benzoate species. Finally, for adsorption on t he p(2 x 1)O/Cu(110) reconstruction, benzoate occurs only as the uprig ht species, which is consistent with reducing the copper mobility and availability on the (110) face. We therefore suggest the possible role of metal adatoms as a new mechanism in controlling adsorbate orientat ion and therefore face specificity in surface reactions. (C) 1998 Else vier Science B.V. All rights reserved.