Structure and dynamics of monolayer films of n-alkane molecules adsorbed on graphite

摘要

Molecular dynamics (MD) simulations and quasielastic incoherent neutron scattering (QENS) experiments with an energy resolution of 60-90 #mu#eV (approx a time range of 70-50 ps) have previously been used to explain the qualitative difference in the behavior of butane and hexane films upon melting. The former melts abruptly whereas the melting transition in the latter is more gradual. There was found some quasielastic scattering in the hexane films even at temperatures as low as 10percent below the melting point. That is unusual in a crystalline phase, since it is the signature of diffusive motion. It was found to be related to the formation of gauche defects in some of the hexane molecules, where the resulting change in shape introduced some rotational diffusive modes. There was no quasielastic scattering in the butane films below the melting point since the melting is not driven by the formation of gauche defects but rather by a tilt of the molecules away from the surface, which also reduces the "footprint" of the molecules on the surface as a precursor to the melting transition.