Energy-dependent dielectric tensor axes in monoclinic α-3,4,9,10-perylene tetracarboxylic dianhydride

We have determined the complex dielectric tensor of single crystalline 3,4,9,10-perylene tetracarboxylic dianhydride (alpha-PTCDA) as a function of energy in the range between 1.4 and 5.0 eV. The results obtained reflect the monoclinic symmetry of the crystal: The principal axes of the real and the imaginary part of the tensor in general do not coincide and show chromatic dispersion. Monoclinic behavior allows rotation of the components epsilon(X) and epsilon(Z) in the plane perpendicular to the unique symmetry axis Upsilon The experimental results indicate that the energies of the optical transitions observed in the weak epsilon(X) component coincide with energies in which a resonance effect due to coupling with the stronger epsilon(Z) component occurs. These resonances appear at energies close to electronic excitations such as the optical gap, the transport gap and the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) peak-to-peak gap and their assignments are discussed based on theoretical calculations.