Berlin 2012 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 14: Adsorption on semiconductors, oxides and insulators II

O 14.7: Vortrag

Montag, 26. März 2012, 17:30–17:45, MA 041

Optical spectroscopy of PTCDA molecules on a KCl (100) surface in the limit of ultralow densities — •A. Paulheim, M. Müller, C. Marquardt, and M. Sokolowski — Institute for Chemistry, University of Bonn

We use fluorescence spectroscopy as a tool to study aggregation and structural site transitions of molecules in the limit of isolated entities on surface. For this purpose submonolayers of PTCDA were vacuum deposited on thin epitaxial KCl films grown on the Ag(100) surface. The structural and optical properties were investigated by SPA-LEED and by fluorescence(FL) and FL excitation spectroscopy, respectively. For the limit of isolated molecules (1% ML coverage), we observed an irreversible transition in the FL spectra, if the intensity of the exciting light is increased by an additional focussing optics. The FL spectra of this novel phase is blue shifted with respect to that of the original phase by 130 cm⁻¹ and the vibronic modes exhibit a smaller FWHM due to reduced inhomogeneous site broadening. In addition, we analysed the polarization of the emitted light and were able to determine the azimuthal orientation of the molecules with respect to the substrate. In both phases, the original and the novel phase, the molecules show the same orientation. So far, the origin of the blue shift is unknown. A possible reason may be photo-induced diffusion of the molecules to surface step sites. This demonstrates that photo-induced heating in combination with FL spectroscopy can be used as a technique for studying the diffusion of molecules on surfaces and the optical properties of molecules depending on the respective adsorption sites.