SPECTROSCOPIC PROPERTIES OF 3,4,9,10 - PERYLENE -
- TETRACARBOXYLIC - DIANHYDRIDE (PTCDA) THIN FILMS

In the present work, we compare the spectroscopic properties of epitaxial thin films of PTCDA with dissolved PTCDA monomers. The vibrational bands appearing in linear absorption of dissolved molecules can be related quantitatively to the deformation of the molecule between the geometries of the electronic ground state and the relaxed excited state. As a matter of fact, this deformation determines also the resonant Raman spectra of the internal vibrations observed for PTCDA crystals and thin film samples, indicating that the internal geometric changes of the excited molecules are only weakly influenced by the surroundings. However, the optical properties in the crystalline phase are strongly influenced by a new phenomenon: Exciton transfer between neighbouring molecules. From microscopic calculations of the exciton transfer, we deduce a consistent set of model parameters for a quantitative interpretation of recent experimental data concerning the linear optical properties of PTCDA thin films. On this basis, we can compute the ${\bf k}$-space dispersion of the excitonic transitions, resulting in a minimum at the boundary of the Brillouin zone. These low-lying excited states are compared with recent photoluminescence data, where a surprisingly large Stokes shift was observed.
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