CO2 ADSORPTION ON THE MGO(100) SINGLE-CRYSTAL SURFACE DETECTED BY POLARIZATION FTIR SPECTROSCOPY AND SPA-LEED

The adsorption of CO2 on the MgO(100) single crystal surface at 82 K h as been studied using polarization Fourier transform infrared spectros copy and spot profile analysis of low energy electron diffraction. The crystals were cleaved in situ under UHV, yielding clean, extended MgO (100) terraces with minimum defect density. At full monolayer coverage , a sharp, polarized multiplet of the nu(3) infrared absorption was de tected. As inferred from isotope absorptions, the multiplet splitting is caused by a correlation field, which shows exciton motion and long range ordering in the adsorbate monolayer, with at least two translati onally inequivalent CO2 molecules at equivalent adsorption sites. Due to a predominant orientation of atomic steps on the cleavage plane a p referential orientation of adsorbate domains with two-fold symmetry wa s observed and their azimuthal orientation along the (001) and (010) c rystal directions measured by polarization spectroscopy at normal inci dence. Both the azimuthal and the polar orientation of the dynamic dip oles of the adsorbate exciton modes were determined. The tilt angle of the aligned adsorbate molecules with respect to the surface was deduc ed to be 27 degrees +/- 10 degrees. LEED studies revealed a commensura te (2 root 2 x root 2) R45 degrees structure of the CO2 monolayer. Com bining the FTIR and LEED results, an adsorbate structure of pg symmetr y containing two molecules per unit cell and a glide plane parallel to the short side of the rectangle is deduced.