Hydrogen bonding, H-D exchange, and molecular mixing in water films on ${\mathrm{TiO}}_2(110)$ have been studied using water electron-stimulated desorption. For $T<70\mathrm{K}$, films with different water isotopes adsorbed on the ${\mathrm{Ti}}^{4+}$ and bridging oxygen rows can be prepared. For $T>70\mathrm{K}$, H-D and molecular exchange between these “layers” occur with a distribution of activation energies and is surprisingly efficient. The results demonstrate that all the water molecules that are directly bound to ${\mathrm{TiO}}_2(110)$—i.e., in the first and second monolayers—are also hydrogen bonded to each other.

• Received 23 May 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.196103