THE ADSORPTION OF CO2 AND N2O ON THE MGO(001) SINGLE-CRYSTAL SURFACE - A COMPARATIVE PIRSS AND LEED STUDY

The adsorption of the isoelectronic molecules CO2 and, for the first t ime, N2O on the MgO(001) surface has been studied at 80 K using high-r esolution polarization infrared surface spectroscopy (PIRSS) and low e nergy electron diffraction (LEED). High-quality surfaces were prepared by cleaving a single crystal in situ under UHV. At 80 K for the monol ayer N2O-MgO(001) a (2 root 2 x root 2)R45 degrees superstructure with one glide plane parallel to the long side of the rectangular unit cel l was found by LEED. The corresponding infrared spectra of the asymmet ric stretching vibration nu(3) are characterized by three sharp, polar ization dependent absorptions. Two of them are attributed to a correla tion field splitting in the ordered layer. Line B at 2217.0 cm(-1) is assigned to the antisymmetric collective vibration of the monolayer. I ts dynamical dipole moment is oriented parallel to the surface and per pendicular to the projection of the dynamical dipole of line A at 2229 .6 cm(-1). Therefore the symmetric collective vibration is attributed to line A, its dynamical dipole moment being inclined relative to the surface. The origin of the third absorption (C) at 2216.1 cm(-1) is no t yet understood. The same superstructure is observed for the monolaye r CO2-MgO(001). The polarized infrared spectra in the nu(3) spectral r ange are similar for both adsorbates, two lines being due to a correla tion held splitting.