Theoretically modelling the water bilayer on the Al(111) surface using cluster calculations

Cluster calculations involving two water molecules are used to theoretically investigate the formation of the water bilayer on the Al(111) surface. In previous calculations we found a single water to molecularly adsorb at an on-top site vertically above an Al atom. We also computed vibrational frequencies which were consistent with data obtained at low water coverage by electron energy loss spectra (EELS). The EELS experiment initially has a peak at 3720 cm(-1) which becomes augmented and eventually dominated by a second peak at 3450 cm(-1) as the water coverage on the Al(111) surface is increased. In the present calculations we optimized the geometry of two water molecules on Al-10 and Al-15 clusters. The resulting structures appear to be prototypes for the formation of a water bilayer on the Al(111) surface: one of the water molecules again adsorbs on-top of an At atom, and then the second water hydrogen bonds to the first water molecule, at a height corresponding to the second layer of a water bilayer, vertically above an Al atom neighboring the Al atom bonded to the first layer water. The computed vibrational frequencies for the chemisorbed water dimer have a number of features in common with the EELS results obtained at higher water coverage. Especially striking is that we compute a similar to 300 cm(-1) blue shift in the H-bonded OH stretching mode which matches the splitting observed for the high frequency modes observed in EELS at higher water coverage. These dimer calculations add further support to our previous suggestions that the 3720 cm(-1) vibration is due to a symmetric OH stretch in non H-bonded water and that water only molecularly adsorbs on Al(111) at low temperatures. Again we allow for adsorbate induced surface relaxation effects in;he cluster calculations.