Quasielastic neutron scattering and molecular dynamics simulation studies of the melting transition in butane and hexane monolayers adsorbed on graphite

摘要: ~Received 14 May 1997; accepted 24 June 1997! Quasielastic neutron scattering experiments and molecular dynamics ~MD! simulations have been used to investigate diffusive motion near the melting transition of monolayers flexible rod-shaped molecules. The were conducted on butane hexane adsorbed an exfoliated graphite substrate. For butane, quasielastic broader than experimental energy resolution width 70 meV appears abruptly at monolayer point Tm 5116 K, whereas, for monolayer, it 20 K below ( 5170 K). To facilitate comparison with experiment, spectra calculated from MD analyzed using same models fitting algorithms as spectra. This combination techniques gives a microscopic picture process in these two which is consistent earlier diffraction experiments. Butane melts liquid phase where molecules trans conformation translationally diffuse while rotating about their center mass. In case show that appearance coincides transformation some gauche conformations. Furthermore, if are prevented forming simulation, incoherent function contains no component . Modeling both simulated favors plastic there nearly isotropic rotational diffusion mass, but translational diffusion. elastic observed above coexistence solid clusters fluid phase, predicted by simulations. T/Tm>1.3, vanishes simulation indicates presence alone. qualitative similarities between lend support previously proposed ‘‘footprint reduction’’ mechanism flexible, © 1997 American Institute Physics.@S0021-9606~97!50237-6#