Monolayer and multilayer films of nitrobenzene on Au(111) surfaces: Bonding and geometry

We have investigated the interaction of nitrobenzene (C₆H₅NO₂) with a Au(111) surface. Nitrobenzene adsorbs molecularly and reversibly on Au(111) at 90 K. Three peaks occur in temperature programmed desorption at 290, 210 and 195 K due to desorption from the monolayer, bilayer and multilayer (as measured at low coverage for each phase). We report the infrared reflection-absorption spectra (IRAS) of nitrobenzene at submonolayer, monolayer and multilayer coverages. In the monolayer, nitrobenzene adsorbs with its molecular plane parallel to the surface and only two, out-of-plane molecular vibrations are observed in IRAS. In thicker films grown at 90 K, nitrobenzene molecules in the next several layers have an overall preferential orientation toward being parallel with the Au(111) surface plane.