ADSORPTION OF WATER-VAPOR ON THE MGO(100) SINGLE-CRYSTAL SURFACE

The adsorption of H2O on the MgO(100) surface prepared by cleaving a s ingle crystal in situ under ultrahigh vacuum has been studied. For the first time Low Energy Electron Diffraction (LEED) has revealed an ord ering of the H2O monolayer with a c(4x2) translational symmetry at 150 K, highly sensitive Polarization Fourier Transform Infrared Spectrosc opy (PIR) has shown a strong, very broad polarized absorption from sim ilar to 3050-3500 cm(-1) in the spectral range of the symmetric and an tisymmetric H2O stretching vibration v(1) and v(3). The peak absorptio n 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 mole cular plane is nearly parallel to the surface. This broad polarized ba nd is assigned to the ordered, nearly planar H2O monolayer with all OH groups participating in strong O-H ... O bonds between nearest neighb or H2O 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 absorptio ns at 3672, 3626 and 3513 cm(-1) with integrated absorptions in the ra nge from 4.10(-4) to 5.10(-3) cm(-1) and their disappearance above 240 K have been detected. They are ascribed to isolated H2O molecules on steps (3672 cm(-1)) and terraces, respectively. For D2O on MgO(100) al l corresponding absorptions have been measured.