Infrared vibration spectroscopy of molecular adsorbates on tungsten using reflection inelastic electron scattering

The observation of adsorbate vibrational energies in the range, 30 ?, hv_vib ? 1000 meV, by electron-energy-loss spectroscopy, provides detailed information on the geometry of atomic and molecular complexes. The “surface normal dipole selection rule”, is discussed and illustrated with results obtained for CO and C₂H₂ adsorption on the principal low-index faces of tungsten, viz.: W(100), W(110) and W(111) using a high-resolution electron reflection spectrometer. Specifically, the behaviour of chemisorbedd diatomic carbon monoxide and polyatomic acetylene is compared as a function of coverage and surface crystallography. Comparison is made with the spectral information obtained by reflection infrared spectroscopy and recent ultraviolet photoelectron spectroscopy studies of the chemisorption binding energies. The energy loss spectra are discussed in terms of current adsorbate models and the possible formation of “distorted rehybridized surface molecular complexes” based on molecular orbital theories of organometallic compounds.