Optical properties of pentacene thin films and single crystals

The optical properties of pentacene single crystals and thin films are systematically investigated by variable angle spectroscopic ellipsometry. In combination with x-ray diffraction and atomic force microscopy the method allows us to distinguish between the complex optical constants ($n$ and $k$) of the different polymorphs formed in thin films during the growth process. On the biaxial anisotropic single crystal it is possible to determine the optical constants along the respective crystallographic orientations. Temperature-dependent measurements in the range from $T=5\mathrm{K}\text{to}350\mathrm{K}$ confirm the assignment of the optical spectra. The results are in good agreement with ab initio calculations of the optical properties of solid pentacene by Tiago et al. [Phys. Rev. B. 67, 115212 (2003)].