Molecular-dynamics simulation of structures and dynamics of n-butane adlayers on Pt(111)

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We have used molecular-dynamics simulations to study the structure and mobility of n-butane adlayers on Pt(111). Simulations were performed with a united-atom model of n-butane for adlayer coverages ranging from the submonolayer to the multilayer regime. At submonolayer coverages, n-butane molecules adsorb with their molecular plane parallel to the surface. Upon increasing coverage close to a monolayer, some of the molecules form a tilted structure with their long axes oriented away from the surface. The in-plane ordering of the molecules has been studied. At coverages near monolayer saturation, the molecules exhibit temperature-dependent ordering similar to that observed in experimental low-energy electron diffraction studies. At moderate submonolayer coverages, the diffusion activation energy is coverage-independent and the diffusion coefficients can be accurately described using a site-blocking model. The site blocking model is not accurate at low coverages where correlated flights become important, and at coverages close to a monolayer. In the multilayer regime, molecules in the top layer are an order of magnitude more mobile than those in the layer adjacent to the surface. Finally, we discuss the vibrational characteristics of the adsorbed butane molecules by analyzing their velocity autocorrelation functions.