H2O AND THE CO+H2O CO-ADSORBATE ON THE AL(111) SURFACE AT LOW-TEMPERATURE

H2O at submonolayer coverages and the CO + H2O co-adsorbate on Al(111) were investigated at low temperatures (20 K) by means of high-resolution electron energy-loss spectroscopy (HREELS), UV photoelectron spectroscopy (UPS), and thermal desorption spectroscopy (TDS). At a coverage of 0.05 monolayer (ML) a H2O monomer was observed with one strong loss at 37 meV (300 cm-1) due to a combination of frustrated translations and rotations and a weak peak at 194 meV (1565 cm-1) due to the scissoring vibration. At higher coverages the hydrogen-bonded ice spectrum develops which is known from studies at higher substrate temperatures. A coadsorbate of 5/6 ML CO and 1/6 ML H2O is characterized by one very strong loss at 51 meV (410 cm-1) and zero intensity of the CO stretching mode at 265 meV (2140 cm-1) in the dipole scattering mode. This is explained by assuming an in-plane orientation of the CO molecules induced by lateral interaction between single H2O molecules with a two-dimensional shell of CO. In this configuration the dipole component of CO normal to the surface is zero so that the CO stretching mode cannot be excited in the dipole scattering mode.