  Oligomers of the organic semiconductor PTCDA are studied by means of helium
nanodroplet isolation (HENDI) spectroscopy. In contrast to the monomer
absorption spectrum, which exhibits clearly separated, very sharp absorption
lines, it is found that the oligomer spectrum consists of three main peaks
having an apparent width orders of magnitude larger than the width of the
monomer lines. Using a simple theoretical model for the oligomer, in which a
Frenkel exciton couples to internal vibrational modes of the monomers, these
experimental findings are nicely reproduced. The three peaks present in the
oligomer spectrum can already be obtained taking only one effective vibrational
mode of the PTCDA molecule into account. The inclusion of more vibrational
modes leads to quasi continuous spectra, resembling the broad oligomer spectra.
