The melting of a two-dimensional lattice of flexible rod-shaped molecules: Structure and dynamics studied by He atom scattering

Monolayers of several saturated long-chain hydrocarbons (n-octane, perdeuterated n-octane, n-nonane and n-decane) adsorbed on a Cu(111) surface were studied by high-resolution elastic and inelastic He atom scattering. At low temperatures (<160 K) the alkane chains form an ordered, well-defined two-dimensional (3D) lattice with the molecular C-C-C planes being oriented parallel to the surface. At higher temperatures a phase transition to a disordered, liquid-like state with the transition temperature dependent on the length of the hydrocarbon chain is observed. These structural changes are accompanied by shifts of the frequencies of the external and internal vibrations of the adsorbed molecules, which indicate the presence of a large amount of conformational defects. In addition, the broadening of the quasielastic peak reveals a strong increase of the diffusion coefficient above the transition temperature.