ELSEVIER Surface Science 389 (1997) 88-102 surface science Molecular-dynamics simulation of structures and dynamics of n-butane adlayers on Pt ( 111 ) J.S. Raut, D.S. Sholl, K.A. Fichthorn * Departments of Chemical Engineering and Physics, The Pennsylvania State University, UniversityPark, PA 16802, USA Received 10 January 1997; accepted for publication 5 May 1997 Abstract 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. © 1997 Elsevier Science B.V. Keywords: Alkenes; Computer simulations; Equilibrium thermodynamics and statistical mechanics; Molecular dynamics; Physical adsorption; Platinum; Single crystal surfaces; Solid-gas interfaces; Surface diffusion; Surface thermodynamics 1. Introduction The structure and mobility of alkane adlayers on metal surfaces are fundamental to a variety of technologies involving; for example, heterogeneous catalysis, coating, adhesion and lubrication [1-3]. Achieving a detailed microscopic understanding of alkane films still remains a challenging problem. Studies of alkane films on surfaces have primarily focused on static, structural features of these films [%17]. In recent years various aspects of these films, particularly those related to their two-dimen- * Corresponding author. Fax: (+ 1) 814 865.7846. 0039-6028/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0039-6028(97)00361-0 sional phase transition behavior have been exam- ined experimentally using techniques such as low- energy electron diffraction (LEED) [7], infrared (IR) spectroscopy [ 15], neutron diffraction [ 17] and helium-scattering [14]. A few simulation studies have also explored the dynamics associated with the melting of two-dimensional alkane films [16,17]. Molecular dynamics (MD) simulations of single alkane molecules on metal surfaces have revealed that these molecules are frequently delo- calized and diffuse by long flights which traverse multiple binding sites, even at moderate temper- atures [4-6]. At low temperatures, the diffusion of isolated molecular adsorbates such as alkanes is considerably more complex than the diffusion of