Structure of Physisorbed Overlayers of Dipolar Molecules: A Combined Study by Atomic-Beam Scattering and Molecular Dynamics

Atomic-beam diffraction and molecular-dynamics calculations have been combined to study the structures of overlayers of the dipolar molecules CH₃F and CH₃Cl physisorbed on bare and xenon-plated graphite crystals. On the former substrate, CH₃F adopts a commensurate triangular lattice with one molecule per unit cell, whereas on the latter the unit cell doubles. On both substrates CH₃Cl is incommensurate with antiferroelectrically ordered molecules arranged in a uniaxially distorted structure.