Diffusion Mechanisms of Molecules on Structured Surfaces
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DIFFUSION MECHANISMS OF MOLECULES ON STRUCTURED SURFACES
MARVIN SILVERBERG Department of Chemical & Biological Sciences, Oregon Graduate Institute of Science & Technology, 19600 N.W. von Neumann Drive, Beaverton, OR 97006-1999
ABSTRACT
The diffusion of ethane and n-pentane on metallic surfaces is studied by molecular dynamics simulations. On atomically smooth, faced centered cubic (111) crystal surfaces, n-pentane has several conformational states, each of which diffuses at a different rate. Diffusion barriers are distinct from intramolecular conformational isomerization barriers. Weak friction is observed. Hopping diffusion occurs near the diffusion threshold. As the temperature is raised, correlated barrier crossing and recrossing increases. n-Pentane slides, transverse to its axis, without changing its orientation with respect to the surface. Conformational changes occur on a longer time scale than diffusional barrier crossing and are restricted by the surface. Far above the diffusion threshold temperature, partial molecular desorption occurs and conformational isomerization is less restricted. The molecules slide, roll and tumble continuously across the surface. A similar pattern of temperature-dependent change in diffusion mechanisms occurs in ethane but without conformational changes. INTRODUCTION
Determining the mechanisms of diffusion of large molecules adsorbed on metal surfaces is a basic issue in surface chemical kinetics. Recently, alkane diffusion on metallic surfaces has become a subject of intense experimental and theoretical interest and measurements have been performed using laser-induced thermal desorption. Diffusion coefficients have been obtained for alkanes of various chain length and for pentane isomers.' A common theoretical approach is to describe the diffusive motion of adsorbed atoms and molecules by kinetic hopping models.2 In such theories, adsorbed particles are regarded as sequentially executing uncorrelated hops among well-defined preferred adsorption sites of minimum potential energy. The absence of correlations among sequential hops of the particles results from rapid dissipation of translational energy or strong friction. Weak friction regimes can be distinguished from the above by a significant distribution of hopping lengths. Furthermore, correlated hopping among sites can occur since the translational energy is not necessarily dissipated in a single hopping event to a near neighbor site. For the systems examined here, the existence of weak or strong friction is directly determined by molecular dynamics simulations. In addition, we study the effects of conformational isomers on surface diffusion mechanisms and the variation of diffusion mechanisms with temperature. It was found that different conformations of the molecule have distinctly different diffusive properties. The simulations stress accuracy in the potentials representing molecular backbone degrees of freedomn. The surface is accurately treated as a Leonard Jones solid. Mat. Res. Soc. Symp. Proc. Vol. 290. 01993 Mate
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