Boundary Layer Dynamics in Mixed Hydrocarbon Thin Films on Metal Surfaces

Abstract

Hydrocarbon bilayers on Pt(111) are an ideal model system for studying the dynamics of molecules in organic thin films. These adsorbates form a well-ordered and well-defined molecular assembly. While maintaining a crystalline-like structure, molecules in this assembly are able to diffuse distances comparable to the lengths of the molecules. Utilizing spectroscopic methods, these dynamic processes are evidenced and quantified. Due to the selectivity of reflection absorption infrared spectroscopy, both identification of chemical species and determination of the position and orientation of each component in the assembly can be accomplished. Vibrational spectroscopy of these adsorbates and the origin of specific vibrational features are discussed in detail. Using isotopic labeling of one component in the bilayer allows the dynamics of each component to be differentiated, demonstrating the mobility of the molecules as a function of temperature. Temperature programmed desorption spectroscopy provides quantitation of the exchange process and isotope effects observed in these systems. For equal chain length, preferential segregation of the protio species to the surface is observed, this is believed to be due to the differences in zero point energies for the adsorbates and the more exothermic adsorption process for the protio species. A variation of temperature programmed desorption spectroscopy, molecular beam isothermal scattering, allows development of a kinetic model for exchange. This barrier for exchange is found to be 1.5 kcal/mol less that the activation energies for desorption, independent of chain length. In mixed component bilayers, preferential Pt(111) surface segregation is observed for longer chain, protio, and linear hydrocarbons. The dynamic effects observed in the spectroscopic investigation of these assemblies is incorporated into a modified desorption model for linear alkanes on Pt(111).