The monolayer structure of CO2 adsorbed on the NaCl(100) single crystal surface: A tensor low-energy electron diffraction analysis

摘要

For the first time a detailed experimental analysis of all structural parameters of the p(2x1) monolayer phase of CO2 physisorbed at 75 K on a NaCl(100) single crystal substrate has been performed. The system was investigated by means of low-energy electron diffraction (LEED) at primary beam currents in the nA range. Intensities of six beams, including two of fractional order, were recorded at normal incidence in the energy range between 70 and 250 eV. These served as experimental data set for a TLEED-based I(V) analysis, in which the observed glide-plane symmetry as well as the linearity of the physisorbed molecules were fully considered. The substrate was represented by (a) an ideally bulk-terminated surface and (b) the relaxed surface which was reported before for the clean NaCl(100) surface. For both substrate models satisfactory agreement between experimental and theoretical I(V) curves was attained with Pendry R factors R-P of 0.23 and 0.22, respectively, and similar molecular best-fit structures. Thereafter, the LEED experiment supports the herringbone adsorbate lattice proposed by theoretical studies as well as earlier experiments, with adsorption sites close to the sodium cations. However, the vertical distance of the molecules to the surface is significantly smaller than predicted by potential calculations, the lower oxygens in the tilted molecules being (a) 2.44+/-0.05 Angstrom and (b) 2.46+/-0.04 Angstrom above the geometrical means of the topmost layer in the different surface models, respectively. The corresponding molecular tilt angles are 62+/-2degrees and 61+/-2degrees with respect to the surface normal, in good agreement with the value resulting from the polarization dependence of infrared spectra of the nu(3) monolayer vibration. (C) 2003 American Institute of Physics. [References: 35]