THE ADSORPTION GEOMETRY OF BENZOIC ACID ONNi(110)

The adsorption geometry of benzoic acid on Ni(110) was investigated by X-ray photoelectron spectroscopy, angle-resolved near edge X-ray absorption fine structure spectroscopy at the C1s and O1s excitation edges, and thermal desorption spectroscopy. At a substrate temperature of 360 K and saturation coverage the adsorbate forms a benzoate with molecules adsorbed on the surface via two equivalent oxygen atoms. For this dense adsorbate layer the molecular plane was found to be azimuthally rotated by about± 30°with respect to the$[1{\underline{1}}0]$surface azimuth and tilted by≈ 30°with respect to the surface normal. At about half the saturation coverage some rearrangement of molecules occurs in such a way that their planes become directed along the [001] surface direction, which is different from the adsorption geometry observed after annealing the saturated layer. At even smaller coverage the benzoate molecules appear to adsorb with their molecular planes nearly parallel to the surface. Semiempirical quantum-mechanical cluster calculations indicate that the carboxylate oxygens favor the highly coordinated sites in the$[1{\underline{1}}0]$-oriented troughs of the Ni(110) surface. Possible adsorption models are discussed.