Structural models and thermal desorption energetics for multilayer assemblies of the n-alkanes on Pt(111)

The structural and thermodynamic properties of linear hydrocarbon multilayer assemblies supported on a Pt(111) single-crystalline substrate are investigated with temperature-programmed desorption and infrared reflection absorption spectroscopies. The multilayer assemblies are comprised of all-trans chains preferentially aligned with their ccc places parallel to that of the surface. Coverage-dependent vibrational spectra reveal that the deposition process, up to the first monolayer total coverage, proceeds first via an island growth mechanism. This result demonstrates that an important contribution is made by (chain-length-dependent) attractive lateral interactions to the binding energies of the monolayer films. The subsequent layers deposited in a multilayer appear to contain a modest concentration of conformational/orientational defects (and perhaps layer misplacements as well). The energetics of the desorption processes for these hydrocarbon assemblies are rationalized on the basis of a conserved value of a prototypical segmental heat of adsorption. The kinetic model embodying this description invokes a layer-by-layer desorption and invokes a central role for lateral interactions and diffusion in maintaining the kinetic competence of a general mechanism of monolayer evaporation from two-dimensional island domains.