  The interaction between PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride)
molecules and solid rare gas samples is studied by means of fluorescence
emission spectroscopy. On the one hand, laser-excited PTCDA-doped large argon,
neon and para-hydrogen clusters in comparison with PTCDA embedded in helium
nanodroplets are spectroscopically characterized with respect to line
broadening and shifting. A fast non-radiative relaxation is observed before a
radiative decay in the electronic ground state takes place. On the other hand,
fluorescence emission studies of PTCDA embedded in bulk neon and argon matrices
results in much more complex spectral signatures characterized by a splitting
of the different emission lines. These can be assigned to the appearance of
site isomers of the surrounding matrix lattice structure.
