Sodium-induced ordering of the benzoate species on Si(100)x1: a combined HREELS, XPS and NEXAFS study

The structure of benzoate on Na/Si(100)-2 x 1 has been studied by high resolution electron energy loss spectroscopy, X-ray photoelectron spectroscopy and near edge X-ray adsorption fine structure spectroscopy. At room temperature, benzoic acid (C6H5-COOH) chemisorbs on Na/Si(100)-2 x 1 through a cleavage of the OH bond in the carboxylic group. The benzoate molecules formed are bonded exclusively to the sodium atoms in a bidentate coordination, in which the oxygen atoms are equivalent. At room temperature, benzoate saturation on Na/Si(100)-2 x 1 is reached at a coverage of one benzoate species for each Na atom or silicon dimer. At this coverage, the molecules are tilted in polar direction by 62 degrees+/-4 degrees to the surface plane and azimuthally rotated by 41 degrees+/-4 degrees with respect to the [01 (1) under bar] surface azimuth. We propose an adsorbate structure, in which the benzoate molecules are oriented parallel to each other in densely packed rows. (C) 2000 Elsevier Science B.V. All rights reserved.