MOLECULAR ADSORPTION ON IONIC SUBSTRATES - CHARACTERIZATION OF TERRACE AND STEP SITES

Semi-classical interaction potentials are used to determine the adsorp tion sites of isolated CO2 molecules adsorbed on NaCl(100) and MgO(100 ) surfaces containing steps. The goal is to discuss and compare the si gnatures of the adsorption on terrace and step sites through three gen eral properties: adsorption characteristics (site symmetry, equilibriu m configuration, isosteric heat), surface diffusion ability and infrar ed 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 t he presence of the step appears to remain localized to the unit cell c ontaining the step. The adsorption on both step and terrace is suffici ently 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 greater than or equal to 100 K). Al though the infrared signals associated with admolecules adsorbed on te rrace and step sites are generally weak and not clearly resolved in ex periments, we can nevertheless explain the site influence and discrimi nate the corresponding peaks from those associated with long range ord ered islands. Application to the calculation of the frequency, intensi ty and linewidth of the nu(3) vibrational mode for (CO2)-C-13 adsorbed on NaCl shows a fair agreement with the available experimental data.