Molecule-step interactions in the aggregation of admolecules on ionic substrates

The aggregation of CO2 molecules physisorbed on ionic substrates is studied on the basis of interaction potential calculations performed at 0 K. The molecules adsorbed on MgO (100) have a configuration which is consistent with the (2t̊2 × t̊2)R45° monolayer structure, even at the first steps of aggregation; the presence of steps reinforces this tendency. On NaCl(100), the ordering process is quite different since the CO2 molecules behave as a lattice gas and the steps do not favor a (2 × 1) monolayer geometry; a critical number of molecules (N ≈ 60) in an aggregate is required to stabilize this geometry, in agreement with the experimental data. Bare steps are strong traps for the linear ordering of admolecules and coated steps behave as aggregation lines for upstairs and downstairs CO2 molecules.