Determination of molecular orientation in self-assembled monolayers using IR absorption intensities:: The importance of grinding effects

The orientation of organic chain molecules grafted to a metal surface is commonly determined using infrared reflection absorption spectroscopy (IRRAS). Whereas the acquisition of the IR spectrum itself is rather straightforward, the determination of orientational parameters (tilt angle, twist angle) is complicated by the requirement to use the bulk reference spectrum of the substance taken to form the self-assembled monolayer (SAM). In the past, mainly two methods have been used to extract orientational information from IR data. The application of the absolute method requires the precise knowledge of the complex refractive index, but the intensities of the IR bands are influenced by Mie scattering of light in the pellet which depends on the particle size and shape and therefore on the grinding time and conditions. On the other hand, the application of the method using relative intensities requires a second IR-active vibration with a differently oriented transition dipole moment (TDM) to eliminate the relative concentration. Both methods rely on the assumption that the individual TDMs have the same strength in the bulk as well as at the surface. We will demonstrate that this assumption is frequently not true. All these difficulties are demonstrated by the determination of molecular orientation of octadecanethiol and p-terphenylthiol on a gold surface.