THE MONOLAYER CO ADSORBED ON MGO(100) DETECTED BY POLARIZATION INFRARED-SPECTROSCOPY

The adsorption of carbon monoxide on the MgO(100) surface, prepared in situ by cleaving a single crystal in ultra high vacuum, has been studied at temperatures between 32 and 56 K using polarization Fourier transform infrared spectroscopy. Above 45 K a single IR absorption at 2150.5 cm(-1) is detected with p-polarization only which indicates a dynamic CO dipole perpendicular to the surface. Below 45 K the spectra of the monolayer are completely different. Using p-polarization the main IR absorption is detected at 2151.2 cm(-1). Two additional features are observed at 2137.2 and at 2132.5 cm(-1), only the latter is also observed with s-polarization. The spectrum indicates an ordered monolayer with two energetically different adsorption sites per unit cell. A c(4 X 2) symmetry is inferred in concert with helium atom scattering [Gerlach et al., Surf. Sci. 331-333 (1995) 1490]; from LEED [Audibert et al., Surf. Sci. 273(1992) L467; Panella et al., J. Phys. Paris 4 (1994) 905] a (4 X 2) symmetry was derived. The p-polarized absorption at 2151.2 cm(-1) is assigned to a dynamic CO dipole perpendicular to the surface, the doublet is attributed to the in-phase and the out-of-phase vibrational exciton of two tilted CO molecules at energetically equivalent sites. Their projections on the surface are oriented antiparallel, the dipole of the in-phase exciton at 2137.2 cm(-1) is perpendicular to the surface and has no azimuthal component, the dipole of the out-of-phase exciton at 2132.5 cm(-1) is parallel to the surface. A weak prevalence of one domain is inferred from the spectra.