CONFORMATIONS AND DIFFUSION OF N-PENTANE ADSORBED ON A METALLIC SURFACE

The diffusion of n-pentane molecules on metallic surfaces is modeled b y molecular dynamics simulations. On atomically smooth, face-centered- cubic (111) crystal surfaces, adsorbed n-pentanes have several conform ational adsorption states, each of which diffuses at a different rate. Diffusion barriers are distinct from intramolecular conformational is omerization barriers. Weak friction is observed. Hopping diffusion is observed near the diffusion threshold. As temperature is raised, corre lated barrier crossing and recrossing increases. Adsorbed n-pentanes s lide, transverse to their axes, without changing their orientation wit h respect to the surface. Conformational changes occur on a longer tim e scale than diffusional barrier crossing and are restricted by the su rface. At temperatures far above the diffusion threshold, partial mole cular desorption occurs and conformational isomerization is less restr icted. The molecules slide, roll, and tumble continuously across the s urface.