Paper SS1-MoA9
Hydrogen Bonding, H/D Exchange and Molecular Mobility in Thin Water Films on TiO₂(110)

Monday, October 15, 2007, 4:40 pm, Room 608

We use the electron-stimulated desorption (ESD) of water from films of D₂O, H₂¹⁶O and H₂¹⁸O to investigate hydrogen bonding, H/D exchange and molecular mixing in water films on TiO₂ (110) for coverages ≤ 2 ML. By depositing water on the Ti⁴⁺ adsorption sites (H₂O_Ti) at 190 K using one water isotope and water on the bridge bonded oxygen sites (H₂O_BBO) at T ≤ 70 K using a different isotope, films with no appreciable mixing of the isotopes between the layers can be prepared. When H₂O_BBO is deposited at T > 70 K, partial or complete mixing of the layers occurs depending on the temperature and time. H/D exchange between the first and second water layers occurs at ~15 K lower temperatures than H₂¹⁶O/H₂¹⁸O exchange. Isothermal experiments demonstrate that the mixing occurs with a distribution of activation energies centered on 0.29 ± 0.07 eV (0.26 ± 0.07 eV) for H₂¹⁶O/H₂¹⁸O (H/D) exchange. While these distributions of activation energies are relatively narrow, they have a profound affect on the kinetics. The results demonstrate that H₂O_BBO is hydrogen bonded to H₂O_Ti. Since the lateral distance (0.325 nm) for atop adsorption at these sites is too large for hydrogen bonding, one (or both) of the adsorbates must be laterally displaced toward the other in agreement with theoretical predictions.