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PTCDA (3,4,9,10-perylene-tetracarboxylic dianhydride) is a planar molecule having well-known opto-electronic properties. In the presented experiment it is used as a probe for the structure of argon and hydrogen clusters. Large argon clusters are known to be solid, only very small clusters are fluid. In case of hydrogen, the structure, dependent on the size of the clusters, is still unclear. The study of the electronic and vibrational spectrum of PTCDA attached to Ar and H2 clusters is expected to give valuable information about their structures. In our experiments PTCDA molecules have been embedded in clusters (He, Ar or H2) which were formed via supersonic expansion of pressurized gas from a cold nozzle. Laser Induced Fluorescence spectra of these molecules show vibrationally resolved spectra (sharper for He droplets when compared to Ar and H2 clusters). Furthermore, for a deeper insight of the observed features, helium nanodroplets were doped with both PTCDA molecules and small Ar or H2 clusters. We studied the electronic spectrum in dependence of the doping order and cluster size. Doping helium droplets with PTCDA first and secondly with the clusters leads to a complete loss of the vibrational resolution: PTCDA is surrounded by a solid argon or hydrogen cluster. When forming the clusters before adding the PTCDA, vibrational resolution is also lost for small clusters but above a certain size, the vibrational structure becomes visible again. In case of hydrogen clusters, this behavior might be interpreted as a liquid-solid transition in the size range of around 300. Further increasing the size leads to the appearance of new optical features in the spectrum, pointing to structural modifications of the solid clusters. |
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