Regensburg 2002 – wissenschaftliches Programm

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SYOH: Organische Halbleiter

SYOH 7: Poster

SYOH 7.6: Poster

Montag, 11. März 2002, 18:00–21:00, C

Characterization of low energy vibrational modes of PTCDA-layers using Raman spectroscopy — •T. Muck¹, M. Schneider², E. Umbach², V. Wagner¹, and J. Geurts¹ — ¹Experimentelle Physik III — ²Experimentelle Physik II, Universität Würzburg, Am Hubland, D-97074 Würzburg

Analyzing vibrational phonon modes is crucial for the understanding of the crystalline properties of organic materials due to molecular interaction.

For this purpose we performed Raman spectroscopy on thin films of PTCDA on Ag(111) at 20 K. We observe a wide range of phonon modes which can not be explained by the classic model of six Raman-allowed phonons (librons) for two molecules per unit cell. To clarify the origin of these additional modes we present a model of coupling between inter- and intramolecular interactions using first principle calculations (Gaussian98) and atom-atom-potential method. Therefore we assume that PTCDA is a pure van-der-Waals-crystal, neglecting Coulomb forces (quadrupole moments). These simulations do not only provide the eigenvectors but also the symmetry of the coupled vibrations resulting in 14 Raman-active modes below 200 cm⁻¹. Furthermore, from the obtained dispersion curves disorder effects can be taken into account as well.