Molecular adsorption on ionic substrates: Characterization of terrace and step sites Original Research Article

Semi-classical interaction potentials are used to determine the adsorption sites of isolated CO2 molecules adsorbed on NaCl(100) and MgO(100) surfaces containing steps. The goal is to discuss and compare the signatures of the adsorption on terrace and step sites through three general properties: adsorption characteristics (site symmetry, equilibrium configuration, isosteric heat), surface diffusion ability and infrared response of the admolecules. We show that the molecular adsorption on steps leads to significantly enhanced adsorption energy values when compared to terrace sites. But the change in the interaction due to the presence of the step appears to remain localized to the unit cell containing the step. The adsorption on both step and terrace is sufficiently strong to hinder diffusion at low temperature. As a result, the adsorption at step appears only as an efficient process for diffusing molecules at higher temperature (T ⩾ 100 K). Although the infrared signals associated with admolecules adsorbed on terrace and step sites are generally weak and not clearly resolved in experiments, we can nevertheless explain the site influence and discriminate the corresponding peaks from those associated with long range ordered islands. Application to the calculation of the frequency, intensity and linewidth of the v3 vibrational mode for 13CO2 adsorbed on NaCl shows a fair agreement with the available experimental data.