Vibrational and structural properties of OH adsorbed on Pt(111)

OH species adsorbed on Pt(111) were studied in a combined investigation usi ng scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy (HREELS). OH was formed by two different reactions, by re action of H2O with O, and as an intermediate in the reaction of O with hydr ogen to H2O. In both cases, two ordered OH phases were observed, a (root 3x root 3)R30 degrees and a (3x3) structure, for which models are proposed. B oth structures have OH coverages of 2/3, and their formation is driven by h ydrogen bond formation between the adparticles; the OH adsorption site is m ost likely on top. OH molecules at defects in the adlayer, in particular at island edges, are spectroscopically distinguishable and contribute signifi cantly to the vibrational spectra in disordered OH layers. This is importan t for the water formation reaction, where the OH islands are small. The dis crepancies between previous HREELS studies on OH can be explained by the di fferent degree of order under the various formation conditions. (C) 1999 Am erican Institute of Physics. [S0021- 9606(99)70148-0].