Low-Energy Modes of Molecules and Atoms Chemisorbed on a Metal Surface. CO and O on Pt(111) Studied by Infrared Spectroscopy

Doktorsavhandling, 1999

This thesis is devoted to the investigation of low-energy metal-adsorbate vibrational modes of CO molecules and O atoms chemisorbed on a Pt(111) surface using infrared spectroscopy (IRS). The infrared selection rule states that only vibrational modes having a non-vanishing component of its dynamical dipole moment perpendicular to the surface can be directly excited using IRS, known as dipole-allowed modes. However, there exist at least two mechanisms that indirectly can excite dipole-forbidden modes vibrating parallel to the surface. One is seen as a doublet structure due to anharmonic coupling to a combination of dipole-forbidden modes, a Fermi resonance. The second is observed as a dip, an increase in the surface reflectivity at the energy of a dipole-forbidden mode, an anti-absorption. The low-energy modes of CO chemisorbed in the ordered c(4x2) structure on Pt(111) were investigated at 100 K. The dipole-allowed Pt-CO stretch of both ontop- and bridge bonded CO were investigated. For the bridge-bonded species a doublet line shape was observed for one of the isotopes which we assigned to a Fermi resonance, a coupling between the Pt-CO stretch and a combination of dipole-forbidden modes, being the first Fermi resonance observed for a low-energy mode. The temperature dependence of the doublet was very strong, it vanished at 50 K and became one single peak which gained in intensity with a factor of two which supports the Fermi resonance interpretation if the combination mode involved thermally excited low-energy modes. No anti-absorption was observed for the frustrated rotation of CO ontop on Pt(111). The detailed IRS investigation of oxygen chemisorbed in the p(2x2) structure on the same surface was the first for an atomic adsorbate on a metal surface. The dipole-allowed Pt-O stretch of atomic oxygen in a hollow site was detected around 480 cm-1 and investigated between 50 and 300 K. Its line width exhibited a strong temperature dependence. A dip, an anti-absorption, was observed in the spectrum around 400 cm-1 which we assigned to the dipole-forbidden frustrated translation. The temperature dependence of the anti-absorption was weak. An adsorbate dependence was observed for the anti-absorption. The accompanying broadband reflectance change was two times larger for O/Pt(111) than for CO/Pt(111). The relative integrated absorptance of the dipole-allowed metal-adsorbate stretches was discussed.