ORIENTATION AND ABSOLUTE COVERAGE OF BENZENE, ANILINE, AND PHENOL ON AG(110) DETERMINED BY NEXAFS AND XPS

The orientation, absolute coverage, and core-electron binding energies of benzene, aniline, and phenol have been measured with near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). At 0.13 +/- 0.02 monolayer (ML) coverage, the plane of the benzene ring orients with a maximum tilt angle with respect to the plane of the surface of 18-degrees. The aromatic ring of aniline tilts at 39 +/- 5-degrees at 0.27 +/- 0.07 ML coverage, and the phenol aromatic ring tilts 40 +/- 5-degrees from the plane of the surface at a coverage of 0.35 +/- 0.06 ML. The saturation coverage of benzene on the Ag(110) surface is 0.31 +/- 0.05 ML. After annealing a multilayer of phenol to 225 K the coverage of phenol remaining on the surface is 0.35 +/- 0.06 ML. Aniline is present at 0.75 +/- 0.15 ML at saturation coverage, suggesting the presence of a second layer; temperature-programmed desorption shows states indicative of a second layer also. The C(1s) XPS binding energy of benzene monolayer is 284.7 +/- 0.3 eV. As expected, two C(1s) XPS binding energies were measured both for aniline at 284.4 +/- 0.3 eV and 285.5 +/- 0.3 eV and for phenol at 284.4 +/- 0.3 eV and 285.9 +/- 0.3 eV. The splitting due to different carbons of 1.1 eV observed for aniline agrees well with the gas phase splitting of 1.2 eV, and the 1.6 eV splitting observed for phenol is identical to the splitting measured for gas phase phenol. A clear splitting of the pi* resonances is also observed in the NEXAFS spectra of aniline and phenol and has been assigned to transitions from the two C(1s) levels into the three-pi* molecular orbitals.