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 co mmonly determined using infrared reflection absorption spectroscopy (IRRAS) . Whereas the acquisition of the IR spectrum itself is rather straightforwa rd, the determination of orientational parameters (tilt angle, twist angle) is complicated by the requirement to use the bulk reference spectrum of th e 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 knowl edge of the complex refractive index, but the intensities of the IR bands a re 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 intensitie s requires a second IR-active vibration with a differently oriented transit ion dipole moment (TDM) to eliminate the relative concentration. Both metho ds rely on the assumption that the individual TDMs have the same strength i n the bulk as well as at the surface. We will demonstrate that this assumpt ion is frequently not true. All these difficulties are demonstrated by the determination of molecular orientation of octadecanethiol and p-terphenylth iol on a gold surface.