If the production of a technical device requires the deposition of large molecules on a substrate it might be important to know if the orientation of the molecules in the layer is random or if certain directions dominate. In the case of orientation a layer of molecules will eventually exhibit anisotropic optical properties. In such cases the molecular orientation can be determined by optical analysis. Here we show how infrared spectra of such a system can be used to determine orientation angles of the molecules.

We assume that the anisotropy of the sample is of such a kind that the optical constants in the z-direction (perpendicular to the macroscopic plane of the thin film) differ from those in the x- and y-direction (which are the same). The following graph shows two such situations: One of complete orientation where the orientation of all molecules are exactly the same, and one of only partial orientation:

In both cases the optical response in z-direction is different from that in the x- and y-directions. Applying electric fields in different directions and recording the response of the molecules one can reconstruct the average tilt angle of the molecules with respect to the z-axis.

The following steps are discussed in the following:

•Setup of optical constants of the substrate and the anisotropic molecular layer

•Definition of the layer stack and the spectra that are to be simulated

•Controlling the optical constants of the molecules by the tilt angle as master parameter

•Determination of the tilt angle from experimental data