Structures of a long-chain n-alkane, n-C44H90, on a Au(111) surface: An infrared reflection absorption spectroscopic study

Vacuum-deposited films of a long-chain n-alkane, n-C44H90, tetratetracontan e (TTC), on a Au(111) surface were investigated by infrared reflection abso rption spectroscopy. We found four different structures for TTC on the Au s urface. One is a "flat-on" structure formed by molecules in an all-trans co nformation with the molecular plane composed of carbon atoms parallel to th e surface. This structure was found in the first monolayer physisorbed on t he surface. The second structure includes the gauche conformation, which sh ows a change in the orientation of the methylene groups with an increase in the thickness of TTC. The third structure is the crystalline state formed by all-trans molecules, which was found in the third and thicker layers wit h the molecular long axis parallel to the surface, These three structures w ere found for films deposited and measured at room temperature, With an inc rease in temperature after deposition at room temperature, the first and se cond structures were easily converted to another structure. This fourth str ucture consists of TTC molecules in the vicinity Of the surface with their molecular axes parallel to the surface and their molecular planes tilted fr om the surface. The molecules in this structure interact with the Au(111) s urface through some of their methylene groups, while the molecules in the f lat-on structure contact the surface through all of their methylene groups, The appearance of these two structures is controlled by the balance of two kinds of interaction: methylene group-Au surface interaction and the inter action among TTC molecules. In the second layer, the formation of TTC molec ules including a gauche conformation is ascribed to the geometrical incompa tibility between flat-on TTC and crystalline TTC.