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, tetratetracontane (TTC), on a Au(111) surface were investigated by infrared reflection absorption spectroscopy. We found four different structures for TTC on the Au surface. One is a flat-on structure formed by molecules in an all-trans conformation with the molecular plane composed of carbon atoms parallel to the surface. This structure was found in the first monolayer physisorbed on the surface. The second structure includes the gauche conformation, which shows 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 with the molecular long axis parallel to the surface, These three structures were found for films deposited and measured at room temperature, With an increase in temperature after deposition at room temperature, the first and second structures were easily converted to another structure. This fourth structure consists of TTC molecules in the vicinity Of the surface with their molecular axes parallel to the surface and their molecular planes tilted from the surface. The molecules in this structure interact with the Au(111) surface through some of their methylene groups, while the molecules in the flat-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 interaction among TTC molecules. In the second layer, the formation of TTC molecules including a gauche conformation is ascribed to the geometrical incompatibility between flat-on TTC and crystalline TTC.