Adsorption of Water Vapor on the MgO(100) Single Crystal Surface

The adsorption of H 2 O on the MgO(100) surface prepared by cleaving a single crystal in situ under ultrahigh vacuum has been studied. For the first time Low Energy Electron Diffraction (LEED) has revealed an ordering of the H 2 O monolayer with a c(4×2) translational symmetry at 150 K, highly sensitive Polarization Fourier Transform Infrared Spectroscopy (PIR) has shown a strong, very broad polarized absorption from ∼3050–3500 cm −1 in the spectral range of the symmetric and antisymmetric H 2 O stretching vibration v 1 and v 3 . The peak absorption is less than 0.1%, the integrated absorption 0.07 cm −1 and 0.11 cm −1 for p‐ and s‐polarization, respectively, indicating that the molecular plane is nearly parallel to the surface. This broad polarized band is assigned to the ordered, nearly planar H 2 O monolayer with all OH groups participating in strong O–H ⃛O bonds between nearest neighbor H 2 O molecules. In the ordered monolayer no dangling OH groups have been found. Additionally a sharp, p‐polarized absorption at 3680 cm −1 has been observed, attributed to dangling OH bonds of the adsorbate on steps. At temperatures above 190 K three sharp, polarized absorptions at 3672, 3626 and 3513 cm −1 with integrated absorptions in the range from 4·10 −4 to 5·10 −3 cm −1 and their disappearance above 240 K have been detected. They are ascribed to isolated H 2 O molecules on steps (3672 cm −1 ) and terraces, respectively. For D 2 O on MgO(100) all corresponding absorptions have been measured.