Molecular alignment in α-CuPc films probed by reflection anisotropy spectroscopy

• α-CuPc films on Si were probed by reflection anisotropy spectroscopy.
• In-plane optical anisotropy was investigated by azimuthal rotation of the sample.
• Orientation of the Q band transition dipole moments was determined.
• Our results support the crystallographic model of α-CuPc with one molecule/unit cell.

Reflection anisotropy spectroscopy (RAS) is generally recognized to be very sensitive to small in-plane anisotropies in thin films, but RA spectra modeling is scarce in literature. Here we propose a simple optical layer model to extract the intrinsic in-plane optical anisotropy of CuPc films from the measured RA spectra recorded as a function of azimuthal sample angle and using the knowledge of the uniaxial optical anisotropy of the film determined from ellipsometry. In addition, the orientation of the transition dipole moments of the first absorption band is determined and the results support the crystallographic model in which the CuPc molecules are aligned parallel in two neighboring stacks in the α-phase.

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