TRANSPORT DYNAMICS IN ORDERED BILAYER ASSEMBLIES OF THE N-ALKANES ON PT(111)

The energetics of self-diffusion within ordered bilayer assemblies of linear hydrocarbons on Pt(111) (layer-to-layer) ha ve been characteriz ed using isothermal molecular beam-surface scattering in conjunction w ith temperature programmed desorption (TPD) and reflection-absorption infrared (RAIR) spectroscopies. The bilayers are prepared by layering a perdeuterated n-alkane on top of a perprotio n-alkane (or vice versa ). The exchange of molecules between the two layers is weakly activate d, less so than is either desorption of the multilayer from the substr ate (the monolayer is more strongly bound) or the various phase transi tions which lead to the loss of two-dimensional order in a correspondi ng densely-packed monolayer of the n-alkanes. The exchange process is further characterized by substantial size-asymmetry and isotope substi tution effects which result in a preference for the selective retentio n of the longer and (for identical chain lengths) the protio hydrocarb on at the surface regardless of the initial deposition order. Layer-to -layer exchange occurs by a displacive mechanism which follows simple mass action principles: increasing the coverage of the post-absorbed s pecies increases the extent of exchange. The difference in the activat ion energy for desorption (from the bilayer) and for exchange is simil ar to 1.5 kcal/mol for both a C-8 and C-10 perdeuterated n-alkane disp lacing an adsorbed (protio) chain of equal length. Thus, although the activation energy for self-diffusion increases with chain length, it i s always less than the activation energy for sublimation by a constant amount. The implications of these results for energy dissipation mech anisms and relaxation dynamics in organic thin films are discussed and analogies to the properties of the so-called plastic-crystalline stat e are developed.