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 time, N2O on the MgO(001) surface has been studied at 80 K using high-resolution polarization infrared surface spectroscopy (PIRSS) and low energy electron diffraction (LEED). High-quality surfaces were prepared by cleaving a single crystal in situ under UHV. At 80 K for the monolayer 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 cell was found by LEED. The corresponding infrared spectra of the asymmetric stretching vibration nu(3) are characterized by three sharp, polarization dependent absorptions. Two of them are attributed to a correlation field splitting in the ordered layer. Line B at 2217.0 cm(-1) is assigned to the antisymmetric collective vibration of the monolayer. Its dynamical dipole moment is oriented parallel to the surface and perpendicular 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 not yet understood. The same superstructure is observed for the monolayer CO2-MgO(001). The polarized infrared spectra in the nu(3) spectral range are similar for both adsorbates, two lines being due to a correlation held splitting.