Regensburg 2013 – scientific programme

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O: Fachverband Oberflächenphysik

O 35: Poster Session I (Metal, semiconductor and oxide substrates: structure and adsorbates; Graphene)

O 35.53: Poster

Tuesday, March 12, 2013, 18:15–21:45, Poster B1

Optical spectroscopy of PTCDA molecules on a KCl (100) surface in the limit of low coverages: site selection and diffusion — Alexander Paulheim, Mathias Müller, Christian Marquardt, and •Moritz Sokolowski — Institute for Physical Chemistry, University of Bonn, Wegelerstraße 12, 53115 Bonn, Germany

We use optical spectroscopy as a tool to study aggregation and structural site transitions of molecules in the limit of isolated entities on a surface. For this purpose we performed optical spectroscopy on the model molecule perylene-3,4,9,10-tetracarboxyl acid dianhydride (PTCDA) for very low coverages (below 1% of a monolayer) on thin (100) oriented KCl films, epitaxially grown on a Ag(100) surface. The structural and optical properties were investigated by SPA-LEED and by fluorescence and FL excitation spectroscopy, respectively. Two different states of PTCDA molecules can be distinguished: an initial state which is observed directly after deposition onto the cold (20 K) sample, and a final state which is found after thermal annealing or intensive optical excitation. The spectrum of the final state is blue-shifted with respect to that of the initial state by 130 cm⁻¹ and exhibits a smaller FWHM due to reduced inhomogeneous site broadening. We explain this by a thermally or optically induced diffusion of the PTCDA molecules from initial terrace sites to energetically favored step edge sites as seen by Karacuban et al. [1] with STM. Polarization dependent spectroscopy reveals the same azimuthal orientation of the molecules on both adsorption sites.
H. Karacuban et al., Nanotechnology 2011, 22, 295305.