Structures of the Ordered Water Monolayer on MgO(001)

Global ab initio structure optimizations combined with statistical thermodynamics and experimental studies reveal atomic structures of the ordered water monolayer on the MgO(001) surface. Calculations based on density functional theory predict the existence of two stable surface structures: a c(4x2) structure containing ten water molecules per unit cell stable at low temperature and a p(3 x 2) structure containing six water molecules per unit cell stable at high temperature. Both structures feature four surface hydroxyl groups resulting from the dissociation of two water molecules per surface cell. The calculated properties of the two structures are in agreement with a multitude of experimental data, including infrared reflection absorption, sum frequency generation and X-ray photoelectron spectroscopy. Comparison of calculated and experimental vibrational spectra allows for assignment of the observed vibrational modes.